NASA Surface meteorology and Solar Energy - Definitions
Copy taken from the NASA SSE site, http://eosweb.larc.nasa.gov/sse/,  May 13, 2005


Parameters for Solar Cooking:
Monthly Averaged Insolation Incident On A Horizontal Surface
10-year Average
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Each monthly averaged value is evaluated as the numerical average of 3-hourly values for the given month.
Units
kWh/m2/day
Note
also referred to as global horizontal radiation
Reference
SSE Methodology, Section 4, for detailed discussion of the methodology for deriving the SSE horizontal surface insolation from satellite observations.

Monthly Averaged Midday Insolation Incident On A Horizontal Surface
10-year Average
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Each monthly averaged value is evaluated as the numerical average of the 3-hourly values, one per day, at the time (GMT) closest to local solar noon. The time (GMT) used is within 1.5 hours of local solar noon.
Units
kW/m2
Reference
SSE Methodology, Section 4, for detailed discussion of the methodology for deriving the SSE horizontal surface insolation from satellite observations.

Monthly Averaged Clear Sky Insolation Incident On A Horizontal Surface
10-year Average
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth when the cloud cover is less than 10%, averaged for that month over the 10-year period (July 1983 - June 1993).
Units
kWh/m2/day
Reference
SSE Methodology, Section 4, for detailed discussion of the methodology for deriving horizontal surface insolation from satellite observations.

Monthly Averaged Clear Sky Days
10-year Average
The monthly average of the number of days having an average cloud cover less than 10% during a given month, averaged for that month over the 10-year period (July 1983 - June 1993).
Units
days


Parameters for Sizing and Pointing of Solar Panels and for Solar Thermal Applications:
Monthly Averaged Insolation Incident On A Horizontal Surface
10-year Average
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Each monthly averaged value is evaluated as the numerical average of 3-hourly values for the given month.
Minimum And Maximum Difference From Monthly Averaged Insolation
The minimum and maximum values for a given month indicate the percent difference between the year that has the least (minimum) or most (maximum) monthly averaged insolation and the 10-year monthly averaged insolation.
Units
Note
also referred to as global horizontal radiation
Reference
SSE Methodology, Section 4, for detailed discussion of the methodology for deriving surface insolation from satellite observations.

Monthly Averaged Diffuse Radiation Incident On A Horizontal Surface / Erbs et al. Method
10-year Average
The monthly average amount of solar radiation incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). The horizontal diffuse radiation was evaluated using the Erbs et al. method described in SSE Methodology Section 5.1.1.
Minimum and Maximum Diffuse Radiation
The minimum and maximum values for a given month indicate the least (minimum) and most (maximum) monthly average diffuse radiation for any one year in the 10-year period.
Average Clearness Index (K)
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth divided by the monthly average incoming top-of-atmosphere insolation for a given month, averaged for that month over the 10-year period (July 1983 - June 1993); (i.e. clearness index is the fraction of insolation at the top of the atmosphere which reaches the surface of the earth).
Minimum and Maximum Clearness Index (K)
The minimum and maximum values for a given month indicate the least (minimum) and most (maximum) monthly average clearness index for any one year in the 10-year period.
Units
Note
no data is available when the clearness index is < 0.3 or > 0.8
Reference
SSE Methodology, Section 5.1.1

Monthly Averaged Diffuse Radiation Incident On A Horizontal Surface / Page Method
10-year Average
The monthly average amount of solar radiation incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). The horizontal diffuse radiation was evaluated using the Extended Page Method described in SSE Methodology Section 5.1.2.
Minimum and Maximum Diffuse Radiation
The minimum and maximum values for a given month indicate the least (minimum) and most (maximum) monthly average diffuse radiation for any one year in the 10-year period.
Units
kWh/m2/day
Reference
SSE Methodology, Section 5.1.2

Monthly Averaged Direct Normal Radiation / RETScreen-type Method
10-year Average
The monthly average amount of solar radiation incident on a surface oriented normal to the solar radiation for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). The direct normal radiation is evaluated using the RETScreen method discussed in SSE Methodology Section 5.3.1.
Minimum And Maximum Difference From Monthly Averaged Direct Normal Radiation
The minimum and maximum values for a given month indicate the percent difference between the year that has the least (minimum) or most (maximum) monthly averaged direct normal radiation and the 10-year monthly averaged value.
Units
Reference
SSE Methodology, Section 5.3.1

Monthly Averaged Direct Normal Radiation / Page Method
10-year Average
The monthly average amount of solar radiation incident on a surface oriented normal to the solar radiation for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). The direct normal radiation is evaluated using the Extended Page/Empirical Staylor Hourly Method described in SSE Methodology Section 5.3.2.
Minimum And Maximum Difference From Monthly Averaged Direct Normal Radiation
The minimum and maximum values for a given month indicate the percent difference between the year that has the least (minimum) or most (maximum) monthly averaged direct normal radiation and the 10-year monthly averaged value.
Units
Reference
SSE Methodology, Section 5.3.2

Monthly Averaged Insolation Incident On A Horizontal Surface At Indicated GMT Times
Average@GMT
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for 3-hour intervals of GMT during the given month, averaged for that month over the 10-year period (July 1983 - June 1993).
Units
kW/m2
Reference
SSE Methodology, Section 4, for detailed discussion of the methodology for deriving the SSE horizontal surface insolation from satellite observations.

Monthly Averaged Insolation Clearness Index (0 to 1.0)
10-year Average
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth divided by the monthly average incoming top-of-atmosphere insolation for a given month, averaged for that month over the 10-year period (July 1983 - June 1993); (i.e. clearness index is the fraction of insolation at the top of the atmosphere which reaches the surface of the earth).
Minimum and Maximum
The minimum and maximum values for a given month indicate the least (minimum) and most (maximum) monthly average clearness index for any one year in the 10-year period.
Units
dimensionless

Monthly Averaged Insolation Normalized Clearness Index (0 to 1.0)
10-year Average
The monthly average zenith angle-independent expression of the insolation clearness index for a given month, averaged for that month over the 10-year period (July 1983 - June 1993); (i.e. clearness index is the fraction of insolation at the top of the atmosphere which reaches the surface of the earth).
K' = K / (1.031 * exp(-1.4 / (0.9 + 9.4 / m )) + 0.1)
where:
K' = normalized clearness index
K = clearness index
m = air mass from Kasten's pyrheliometric formula
Units
dimensionless

Monthly Averaged Clear Sky Insolation Incident On A Horizontal Surface
10-year Average
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth when the cloud cover is less than 10%, averaged for that month over the 10-year period (July 1983 - June 1993).
Units
kWh/m2/day

Monthly Averaged Clear Sky Insolation Clearness Index (0 to 1.0)
10-year Average
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth when the cloud cover is less than 10% divided by the monthly average incoming top-of-atmosphere insolation for a given month, averaged for that month over the 10-year period (July 1983 - June 1993); (i.e. clearness index is the fraction of insolation at the top of the atmosphere which reaches the surface of the earth).
Units
dimensionless

Monthly Averaged Clear Sky Insolation Normalized Clearness Index (0 to 1.0)
10-year Average
The monthly averaged zenith angle-independent expression of the clear sky insolation clearness index for a given month, averaged for that month over the 10-year period (July 1983 - June 1993); (i.e. clearness index is the fraction of insolation at the top of the atmosphere which reaches the surface of the earth).
Units
dimensionless


Solar Geometry:
Monthly Averaged Solar Noon
Average
The time when the sun is due south in the northern hemisphere or due north in the southern hemisphere. The determination of monthly averaged solar noon for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).
Units
GMT time
Note
during polar winter the sun may be below the horizon at solar noon

Monthly Averaged Daylight
Average
The number of hours between sunrise and sunset. The determination of monthly averaged daylight for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).
Units
hours
Note
polar daylight is 0 in winter and 24 in summer

Monthly Averaged Daylight Average Of Hourly Cosine Solar Zenith Angles
Average
The average cosine of the angle between the sun and directly overhead during daylight hours. The determination of monthly averaged daylight average of hourly cosine solar zenith angles for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).
Average cos(ΘZ) = {f cos-1 (-f/g) + g[1 - (f/g)2]} / cos-1(-f/g)
where:
f = sin(latitude) * sin(solar declination)
g = cos(latitude) * cos(solar declination)
Units
dimensionless
Reference
SSE Methodology, Section 5.1.2, Ref. 23: Gupta et al., 2001, The Langley Parameterized Shortwave Algorithm (LPSA) for Surface Radiation Budget Studies

Monthly Averaged Cosine Solar Zenith Angle At Mid-Time Between Sunrise And Solar Noon
Average
Approximate monthy average ratio of horizontal surface solar beam radiation to direct normal radiation. The determination of monthly averaged cosine solar zenith angle at mid-time between sunrise and solar noon for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).
cos(ΘZMT) = f + g[(g - f) / 2g]
where:
f = sin(latitude) * sin(solar declination)
g = cos(latitude) * cos(solar declination)
Units
dimensionless
Reference
SSE Methodology, Section 5.3.2

Monthly Averaged Declination
Average
The angular distance of the sun north (positive) or south (negative) of the equator. Declination varies through the year from 23.45 north to 23.45 south and reaches the minimum/maximum at the southern/northern summer solstices. The determination of monthly averaged declination for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).
Units
degrees

Monthly Averaged Sunset Hour Angle
Average
The angle that the earth has rotated between the time of solar noon and sunset. Note that the earth rotates 15 with respect to the sun each hour. The determination of monthly averaged sunset hour angle for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).
Units
degrees

Monthly Averaged Maximum Solar Angle Relative To The Horizon
Average
The maximum vertical angle of the sun above the horizon. The determination of monthly averaged maximum solar angle relative to the horizon for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).
Units
degrees

Monthly Averaged Hourly Solar Angles Relative To The Horizon
Average
The vertical angle of the sun above the horizon. The determination of monthly averaged hourly solar angles relative to the horizon for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).
Units
degrees
Note
This information is provided to assist the user in interpreting diurnal variations in both insolation and clouds.

Monthly Averaged Hourly Solar Azimuth Angles
Average
The arc of the horizon measured clockwise from True North, to the point where a vertical circle through the sun intersects the horizon. The determination of monthly averaged hourly solar azimuth angles for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).
Units
degrees
Note
These angles are provided as a function of GMT to assist the user in interpreting diurnal variations of insolation and clouds in this data set. If either mornings or afternoons are habitually cloudy, it may be useful to point the solar panels slightly to the east or west instead of directly south.


Parameters for Tilted Solar Panels:
Monthly Averaged Radiation Incident On An Equator-Pointed Tilted Surface / RETScreen Method
SSE HRZ
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).
Clearness Index (K)
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth divided by the monthly average incoming top-of-atmosphere insolation for a given month, averaged for that month over the 10-year period.
Erbs DIF
The monthly average amount of solar radiation for a given month incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk, averaged for that month over the 10-year period.
RET DNR
The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for a given month, averaged for that month over the 10-year period.
Tilt 0, Latitude-15, Latitude, Latitude+15, 90
The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Note that the differences between the Tilt 0 values and the SSE HRZ values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using the RETScreen Isotopic Diffuse Method discussed in SSE Methodology, Section 6.1.1.
OPT
The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.
OPT ANG
The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.
Units
Reference
SSE Methodology, Section 6.1.1

Monthly Averaged Radiation Incident On An Equator-Pointed Tilted Surface / Perez/Page Method
SSE HRZ
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).
Clearness Index (K)
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth divided by the monthly average incoming top-of-atmosphere insolation for a given month, averaged for that month over the 10-year period.
PAGE DIF
The monthly average amount of solar radiation incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for a given month, averaged for that month over the 10-year period.
PAGE DNR
The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for a given month, averaged for that month over the 10-year period.
Tilt 0, Latitude-15, Latitude, Latitude+15, 90
The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Note that the differences between the Tilt 0 values and the SSE HRZ values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using the Perez Non-Isotropic/Extended Page Horizontal Diffuse Method discussed in SSE Methodology Section 6.1.2.
OPT
The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.
OPT ANG
The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.
Units
Reference
SSE Methodology, Section 6.1.2

Monthly Averaged Radiation Incident On An Equator-Pointed Tilted Surface / Perez/Erbs et al. Method
SSE HRZ
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).
Clearness Index (K)
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth divided by the monthly average incoming top-of-atmosphere insolation for a given month, averaged for that month over the 10-year period.
Erbs DIF
The monthly average amount of solar radiation for a given month incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk, averaged for that month over the 10-year period.
RET DNR
The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for a given month, averaged for that month over the 10-year period.
Tilt 0, Latitude-15, Latitude, Latitude+15, 90
The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Note that the differences between the Tilt 0 values and the SSE HRZ values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using the Perez Non-Isotropic/Erbs et al. Horizontal Diffuse Method discussed in SSE Methodology Section 6.1.3.
OPT
The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.
OPT ANG
The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.
Units
Reference
SSE Methodology, Section 6.1.3

Monthly Averaged Equivalent Sun Hours Radiation Incident On An Equator-pointed Tilted Surface / RETScreen Method
SSE MIN
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the least, or minimum, monthly averaged value in the 10-year period (July 1983 - June 1993).
Clearness Index (K)
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the least, or minimum, monthly averaged value over the 10-year period divided by the monthly average incoming top-of-atmosphere insolation for that month.
Erbs DIF
The monthly average amount of solar radiation for a given month incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for the year that has the most, or maximum, monthly averaged value over the 10-year period.
RET DNR
The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for the year that has the least, or minimum, monthly averaged value in the 10-year period.
Tilt 0, Latitude-15, Latitude, Latitude+15, 90
The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for the year that has the least, or minimum, monthly averaged value in the 10-year period. Note that the differences between the Tilt 0 values and the SSE MIN values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using SSE MIN in the RETScreen Isotopic Diffuse Method discussed in SSE Methodology Section 6.1.1.
OPT
The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.
OPT ANG
The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.
Units

Monthly Averaged Equivalent Sun Hours Radiation Incident On An Equator-pointed Tilted Surface / Perez/Page Method
SSE MIN
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the least, or minimum, monthly averaged value in the 10-year period (July 1983 - June 1993).
Clearness Index (K)
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the least, or minimum, monthly averaged value in the 10-year period divided by the monthly average incoming top-of-atmosphere insolation for that month.
PAGE DIF
The monthly average amount of solar radiation for a given month incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for the year that has the most, or maximum, monthly averaged value in the 10-year period.
PAGE DNR
The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for the year that has the least, or minimum, monthly averaged value in the 10-year period.
Tilt 0, Latitude-15, Latitude, Latitude+15, 90
The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for the year that has the least, or minimum, monthly averaged value in the 10-year period. Note that the differences between the Tilt 0 values and the SSE MIN values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using SSE MIN in the Perez Non-Isotropic/Extended Page Horizontal Diffuse Method discussed in SSE Methodology Section 6.1.2.
OPT
The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.
OPT ANG
The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.
Units

Monthly Averaged Equivalent Sun Hours Radiation Incident On An Equator-pointed Tilted Surface / Perez/Erbs et al. Method
SSE MIN
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the least, or minimum, monthly averaged value in the 10-year period (July 1983 - June 1993).
Clearness Index (K)
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the least, or minimum, monthly averaged value in the 10-year period divided by the monthly average incoming top-of-atmosphere insolation for that month.
Erbs DIF
The monthly average amount of solar radiation for a given month incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for the year that has the most, or maximum, monthly averaged value in the 10-year period.
RET DNR
The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for the year that has the least, or minimum, monthly averaged value in the 10-year period.
Tilt 0, Latitude-15, Latitude, Latitude+15, 90
The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for the year that has the least, or minimum, monthly averaged value in the 10-year period. Note that the differences between the Tilt 0 values and the SSE MIN values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using SSE MIN in the Perez Non-Isotropic/Erbs et al. Horizontal Diffuse Method discussed in SSE Methodology Section 6.1.3.
OPT
The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.
OPT ANG
The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.
Units

Monthly Averaged Peak Sun Hours Radiation Incident On An Equator-pointed Tilted Surface / RETScreen Method
SSE MAX
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the most, or maximum, monthly averaged value in the 10-year period (July 1983 - June 1993).
Clearness Index (K)
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the most, or maximum, monthly averaged value in the 10-year period divided by the monthly average incoming top-of-atmosphere insolation for that month.
Erbs DIF
The monthly average amount of solar radiation for a given month incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for the year that has the least, or minimum, monthly averaged value in the 10-year period.
RET DNR
The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for the year that has the most, or maximum, monthly averaged value in the 10-year period.
Tilt 0, Latitude-15, Latitude, Latitude+15, 90
The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for the year that has the most, or maximum, monthly averaged value in the 10-year period. Note that the differences between the Tilt 0 values and the SSE MAX values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using SSE MAX in the RETScreen Isotopic Diffuse Method discussed in SSE Methodology Section 6.1.1.
OPT
The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.
OPT ANG
The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.
Units

Monthly Averaged Peak Sun Hours Radiation Incident On An Equator-pointed Tilted Surface / Perez/Page Method
SSE MAX
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the most, or maximum, monthly averaged value in the 10-year period (July 1983 - June 1993).
Clearness Index (K)
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the most, or maximum, monthly averaged value in the 10-year period divided by the monthly average incoming top-of-atmosphere insolation for that month.
PAGE DIF
The monthly average amount of solar radiation for a given month incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for the year that has the least, or minimum, monthly averaged value in the 10-year period.
PAGE DNR
The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for the year that has the most, or maximum, monthly averaged value in the 10-year period.
Tilt 0, Latitude-15, Latitude, Latitude+15, 90
The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for the year that has the most, or maximum, monthly averaged value in the 10-year period. Note that the differences between the Tilt 0 values and the SSE MAX values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using SSE MAX in the Perez Non-Isotropic/Extended Page Horizontal Diffuse Method discussed in SSE Methodology Section 6.1.2.
OPT
The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.
OPT ANG
The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.
Units

Monthly Averaged Peak Sun Hours Radiation Incident On An Equator-pointed Tilted Surface / Perez/Erbs et al. Method
SSE MAX
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the most, or maximum, monthly averaged value in the 10-year period (July 1983 - June 1993).
Clearness Index (K)
The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the most, or maximum, monthly averaged value in the 10-year period divided by the monthly average incoming top-of-atmosphere insolation for that month.
Erbs DIF
The monthly average amount of solar radiation for a given month incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for the year that has the least, or minimum, monthly averaged value in the 10-year period.
RET DNR
The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for the year that has the most, or maximum, monthly averaged value in the 10-year period.
Tilt 0, Latitude-15, Latitude, Latitude+15, 90
The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for the year that has the most, or maximum, monthly averaged value in the 10-year period. Note that the differences between the Tilt 0 values and the SSE MAX values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using SSE MAX in the Perez Non-Isotropic/Erbs et al. Horizontal Diffuse Method discussed in SSE Methodology Section 6.1.3.
OPT
The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.
OPT ANG
The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.
Units


Parameters for Sizing Battery or other Energy-storage Systems:
Various industry organizations use different methods to size either battery or other types of backup systems. One international organization has required that all stand-alone medical equipment that it purchases must operate for 6 BLACK or NO-SUN days in parts of the tropics. The methods used require different solar insolation parameters. Three types of parameters are provided in the SSE data set. They are:
  1. Minimum available insolation over a consecutive-day period (1, 3, 7, 14, or 21 days) within a particular month over the 10-year period (July 1983 - June 1993) as a % of the expected average kWh/m2 value over the same consecutive-day period (%)
  2. Solar radiation deficits below expected values incident on a horizontal surface over a consecutive-day period (kWh/m2)
  3. Equivalent number of NO-SUN or BLACK days that must be supplied by the storage backup system (days)

These parameters are desired because of the fact that unusually cloudy conditions occurring over a number of consecutive days continually draw reserve power from batteries or some other storage device for solar systems not connected to an electrical power grid. Storage devices must be designed to withstand continuous below-average conditions in various regions of the globe. Determination of these parameters for a geographical region from ground-based observations is complex because cloud conditions are usually different every day during the below average period. The satellite-based observations in the SSE release 5 are ideally suited for determining a regional climatological average for these parameters. January conditions in Richmond, VA (37.583 N, 77.5 W) are used below to better illustrate the use of the parameters in the design of backup storage systems.

Release 5 SSE satellite-based data indicate that average daily insolation is 2.15 kWh/m2 for January 10-year average cloud conditions. Clear sky values average 3.10 kWh/m2. Over the 10-year period (July 1983 - June 1993), the lowest one-day insolation in January was 12.5% of the expected value (2.15 kWh/m2) or 0.269 kWh/m2. This means that a battery or some other device had to supply 1.88 kWh/m2 or 87% of the solar energy expected on that day. The fraction of expected daily energy that has to be provided by the storage device is sometimes referred to as an equivalent NO-SUN or BLACK day. From a storage device viewpoint, the number of NO-SUN days was 0.87 on that worse case day because 2.15 kWh/m2 of energy are expected on every sunny day.

The need for additional storage capacity increases as the number of below average consecutive days increase. Again using conditions in Richmond, VA, the worse case "minimum available insolation over a consecutive-day period" for a 7 consecutive-day period where the insolation was 24.8% of expected the 7-day value of 15.05 kWh/m2 (7 x 2.15 kWh/m2). This means that the storage device had to supply 11.3 kWh/m2 or 75.2% of the 7-day expected energy on a continuous basis with no period for recharge. That amounts to 5.26 (7*75.2%/100 = 5.26) NO-SUN days if 2.15 kWh/m2 are expected on each average sunny day. It should be noted that the worse 7-day period might have occurred in a different year than the worse one-day over the 10-year period of the SSE data.

As the consecutive-day time period gets longer, lower than average cloud fraction conditions can enter the situation, causing brighter days and some recharging of the storage device. These effects are evident in the Richmond, VA data. Using the same analyses as above, January values are as follows:

Worse Consecutive Days Minimum Available Insolation in Consecutive-day Period (%) Deficits below Expected Values (kWh/m2) Equivalent NO-SUN days (days)
1 day 12.5 1.88 0.87
3 day 16.7 5.37 2.49
7 day 24.8 11.30 5.26
14 day 46.8 16.00 7.44
21 day 67.8 14.50 6.75
Month 86.9 8.68 4.03

Note that the worst of the Worse Consecutive Days is the 14-day period. This suggests that a solar energy system battery or backup system should have approximately 7.5 NO-SUN days or 16.125 kWh/m2 (7.5 x 2.15) of capacity if January is a critical month for operation of the solar energy system. Satellite-based day-to-day cloud variability over a historical time period can be used to design solar energy storage or backup capacity if more reliable data are not available.


Parameters for Sizing Surplus-product Storage Systems:
Available Surplus Insolation Over A Consecutive-day Period

Available surplus insolation over a consecutive-day period (1, 3, 7, 14, or 21 days) within a particular month over the 10-year period (July 1983 - June 1993) as a % of the expected average kWh/m2 value over the same consecutive-day period (%)


Cloud Information:
Monthly Averaged Daylight Cloud Amount
10-year Average
Percent of cloud amount during daylight for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).
Units
percent
Note
a value of zero indicates clear skies and a value of 100 indicates overcast skies

Monthly Averaged Cloud Amount At Indicated GMT Times
Average@GMT
Percent of cloud amount during daylight at 3-hour intervals of GMT for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).
Units
percent
Note
a value of zero indicates clear skies and a value of 100 indicates overcast skies

Monthly Averaged Frequency Of Cloud Amount At Indicated GMT Times
< 10% @GMT
Percent of time the cloud amount is less than 10% (clear skies) at 3-hour intervals of GMT during daylight for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).
10 - 70% @GMT
Percent of time the cloud amount is between 10 - 70% (broken-cloud skies) at 3-hour intervals of GMT during daylight for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).
>= 70% @GMT
Percent of time the cloud amount is greater than or equal to 70% (near-overcast skies) at 3-hour intervals of GMT during daylight for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).
Units
percent


Meteorology (Temperature):
Monthly Averaged Air Temperature At 10 m Above The Surface Of The Earth
10-year Average
The monthly average air temperature for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Temperature values are for 10 meters above the surface of the earth. Each monthly averaged value is evaluated as the numerical average of 3-hourly values for the given month.
Units
degrees Celsius
Reference
SSE Methodology, Section 8.1

Average Daily Temperature Range
10-year Average
The average difference between the average daily minimum and average daily maximum for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Temperature values are for 10 meters above the surface of the earth.
Units
degrees Celsius
Note
the warmest month is marked with an asterisk (*)
Reference
SSE Methodology, Section 8.1

Monthly Averaged Cooling Degree Days Above 18 C
10-year Average
The monthly average of the accumulation of degrees when the daily mean temperature is above 18 degrees Celsius over the 10-year period (July 1983 - June 1993). Temperature values are for 10 meters above the surface of the earth.
Monthly Cooling Degree Days
For the days of a given month,
sum the quantity [((Tmin + Tmax) / 2) - 18]
when (Tmin + Tmax) / 2 > 18.
Units
degree days

Monthly Averaged Heating Degree Days Below 18 C
10-year Average
The monthly average of the accumulation of degrees when the daily mean temperature is below 18 degrees Celsius over the 10-year period (July 1983 - June 1993). Temperature values are for 10 meters above the surface of the earth.
Monthly Heating Degree Days
For the days of a given month,
sum the quantity [18 - (Tmin + Tmax) / 2]
when (Tmin + Tmax) / 2 < 18.
Units
degree days
Reference
SSE Methodology, Section 8.1

Monthly Averaged Arctic Heating Degree Days Below 10 C
10-year Average
The monthly average of the accumulation of degrees when the daily mean temperature is below 10 degrees Celsius over the 10-year period (July 1983 - June 1993). Temperature values are for 10 meters above the surface of the earth.
Monthly Heating Degree Days
For the days of a given month,
sum the quantity [10 - (Tmin + Tmax) / 2]
when (Tmin + Tmax) / 2 < 10.
Units
degree days

Monthly Averaged Arctic Heating Degree Days Below 0 C
10-year Average
The monthly average of the accumulation of degrees when the daily mean temperature is below 0 degrees Celsius over the 10-year period (July 1983 - June 1993). Temperature values are for 10 meters above the surface of the earth.
Monthly Heating Degree Days
For the days of a given month,
sum the quantity [0 - (Tmin + Tmax) / 2]
when (Tmin + Tmax) / 2 < 0.
Units
degree days

Monthly Averaged Earth Skin Temperature
10-year Average
The monthly average of the earth's surface temperature for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).
Units
degrees Celsius
Note
data over oceans is sea surface temperature

Average Minimum, Maximum and Amplitude Of The Daily Mean Earth Temperature
Minimum
The minimum of the daily mean earth's surface temperature for a given month, averaged over the 10-year period (July 1983 - June 1993).
Maximum
The maximum of the daily mean earth's surface temperature for a given month, averaged over the 10-year period (July 1983 - June 1993).
Amplitude
One half of the difference between the 10-year average minimum and 10-year average maximum.
Units
degrees Celsius

Monthly Averaged Frost days
10-year Average
The monthly average of the number of days for which the temperature falls below 0 degrees Celsius for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Temperature values are for 10 meters above the surface of the earth.
Units
days

Monthly Averaged Dew Point Temperature
10-year Average
The monthly average dew point temperature for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Dew point temperature values are for 10 meters above the surface of the earth.
Units
degrees Celsius

Monthly Averaged Air temperature At 10 m Above The Surface Of The Earth For Indicated GMT Times
Average@GMT
The monthly average air temperature for 3-hour intervals of GMT during a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Temperature values are for 10 meters above the surface of the earth.
Units
degrees Celsius


Meteorology (Wind):
Monthly Averaged Wind speed At 50 m Above The Surface Of The Earth
10-year Average
The monthly average wind speed for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Wind speed values are for 50 meters above the surface of the earth. Each monthly averaged value is evaluated as the numerical average of 3-hourly values for the given month.
Minimum And Maximum Difference From Monthly Averaged Wind Speed At 50 m
The minimum and maximum values for a given month indicate the percent difference between the year that has the least (minimum) or most (maximum) monthly averaged wind speed at 50 m and the 10-year monthly averaged wind speed at 50 m.
Units
Reference
SSE Methodology, Section 7

Monthly Averaged Percent Of Time The Wind Speed At 50 m Above The Surface Of The Earth Is Within The Indicated Range
Wind Speed Ranges
The monthly average percent of time that wind speed is within the indicated range for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Wind speed values are for 50 meters above the surface of the earth.
Units
percent

Monthly Averaged Wind Speed At 50 m Above The Surface Of The Earth For Indicated GMT Times
Average@GMT
The monthly average wind speed for 3-hour intervals of GMT during a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Wind speed values are for 50 meters above the surface of the earth.
Units
meters per second (m/s)

Monthly Averaged Wind Direction At 50 m Above The Surface Of The Earth
10-year Average
The monthly average wind direction for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Wind direction values are for 50 meters above the surface of the earth.
Units
degrees
Notes

Monthly Averaged Wind Direction At 50 m Above The Surface Of The Earth For Indicated GMT Times
Average@GMT
The monthly average wind direction for 3-hour intervals of GMT during a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Wind direction values are for 50 meters above the surface of the earth.
Units
degrees
Notes

Monthly Averaged Wind Speed At 10 m Above The Surface Of The Earth For Terrain Similar To Airports
10-year Average
The monthly average wind speed for a given month, averaged for that month over the 10-year period (July 1983 - June 1993) where the wind speed was evaluated at 10 m above the surface of the earth assuming the underlying terrain is similar to that typical of airports (e.g. "airport" flat rough grass category taken from Gipe; SSE Methodology, Section 7, Ref. 13).
Units
meters per second (m/s)
Reference
SSE Methodology, Section 7

Difference Between The Average Wind Speed At 10 m Above The Surface Of The Earth And The Average Wind speed At 50 m Above The Surface Of The Earth
10-year Average
The percent difference between the 10-year monthly average of the wind speed at 10 m above the surface of the earth and the 10-year monthly average of the wind speed at 50 m above the surface of the earth.

Percent difference at 10 m = ((Wind Speed At 10 m - Wind Speed At 50 m) / Wind Speed At 50 m) * 100
Units
percent

Monthly Averaged Wind Speed Adjusted For Height And Vegetation Type
10-year Average
The monthly average wind speed for a given month, averaged for that month over the 10-year period (July 1983 - June 1993) where the wind speed is evaluated using the Gipe Power Law. The wind speed at 50 m above the surface of the earth and the surface roughness exponent based on the chosen vegetation type are used in the equation:
Gipe Power Law equation
Units
meters per second (m/s)
Reference
SSE Methodology, Section 7, Ref. 31: Gipe, 1999, Wind Energy Basics


Meteorology (Other):
Monthly Averaged Relative Humidity
10-year Average
The monthly average of relative humidity for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). The relative humidity is calculated from the air temperature and specific humidity at 10 meters above the surface of the earth and the surface pressure. The estimated uncertainty is 9%.
Units percent
Reference
SSE Methodology, Section 8.3

Monthly Averaged Humidity Ratio
10-year Average
The monthly average of the humidity ratio for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Humidity ratio values are for 10 meters above the surface of the earth.
Units
kg/kg
Note
also referred to as Specific Humidity

Monthly Averaged Atmospheric Pressure
10-year Average
The monthly average of atmospheric pressure at the surface of the earth for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).
Units
kPa
Reference
SSE Methodology, Section 8.2

Monthly Averaged Total Column Precipitable Water
10-year Average
The monthly average of the amount of moisture in the air from 20 mb to the surface for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).
Units
cm

Monthly Averaged Precipitation
10-year Average
The average daily rain rate based upon the total monthly averaged amount of rain for the given month divided by the number of days in the month, averaged for that month over the 10-year period (July 1983 - June 1993).
Units
mm/day
Reference
SSE Methodology, Section 8.4


Supporting Information:
Monthly Averaged Top-of-atmosphere Insolation
10-year Average
The monthly average amount of the total solar radiation incident on the top-of-atmosphere for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).
Units
kWh/m2/day
Note
also referred to as extraterrestrial radiation (ETR)

Monthly Averaged Surface Albedo
10-year Average
The monthly average ratio of the solar energy reflected by the surface of the earth to monthly average solar energy incident on the surface of the earth for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). (i.e. Fraction of insolation reflected by the surface of the earth.)
Units
dimensionless