The Clear Sky Calculator is used to determine the need for recalibration of radiation sensors. It is most accurate when used near solar noon in the summer months. The calculator estimates the intensity of solar radiation (either total global shortwave radiation, measured by pyranometers, or global photosynthetic photon flux density, measured by quantum sensors) incident on a horizontal surface at any time of the day, at any location in the world. The equations used to estimate clear sky solar radiation with the Clear Sky Calculator come from the clear sky solar radiation model used to calculate net radiation in the ASCE Standardized Reference Evapotranspiration Equation ( The only input requirements to the calculator are site elevation, latitude, longitude, reference longitude, air temperature and relative humidity measurements or estimates. These data are typically easy to obtain, making the Clear Sky Calculator a simple solar radiation reference that can be used to estimate pyranometer and quantum sensor accuracy and determine the need for recalibration.

When used near solar noon over multiple clear, unpolluted days during spring and summer months, accuracy of the Clear Sky Calculator is estimated to be ± 4 % in all climates and locations around the world. As an example, modeled incoming shortwave radiation (SWi) from the Clear Sky Calculator closely tracked measured SWi (data from a heated and ventilated Kipp & Zonen CM21 pyranometer) for a clear day (April 21, 2012) in Logan, Utah. The ratio of measured SWi to modeled SWi was between 1.00 and 1.05 (0 % and 5 %) from 9 AM to 6 PM (solar zenith angles less than 65∞). The average ratio from two hours before solar noon to two hours after solar noon was 1.02 ± 0.01 (2 ± 1 %). (see )

A more detailed discussion of Clear Sky Calculator accuracy is given on the webpage (, where the necessary accuracy of the required inputs is discussed.

Apogee strongly encourages our customers to use the Clear Sky Calculator as an effective way to monitor pyranometer and quantum sensor performance and determine the need for sensor recalibration. If a sensor is consistently different from the Clear Sky Calculator by more than a few percent, please contact us about recalibration.

What's New

Version 2.0.0

• New interface. Fully automatic!
• Automatic “Clear Sky” solar output for current location, time, and weather.
• Quickly see the max solar output for the day, and when it happens.
• New, easy to use sensor verification process.
• Map view to quickly verify location.
• Edit values manually. Autofill by US zip code, or by using a map.
• Easily view intermediate calculated variables.

Ratings and Reviews

Lots of uses


This product is excellent for applications in addtion to calibrating Apogee instruments.

It is valuable for evaluating small solar charging systems. If the rated output of the solar system is not being achieved, you can use this tool to help determine why. The solar charging system power was determined using an irradiance of 1000 w/m2. The irradaince, for your test location, can be determined by this application if you have a clear day.


Apogee Instruments, Inc.
5.7 MB
Requires iOS 7.0 or later. Compatible with iPhone, iPad, and iPod touch.
Age Rating
Rated 4+
© Copyright 2015 Apogee Instruments, Inc.


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