# Clear Sky Calculator

> Estimates solar radiation intensity for sensor recalibration. (iPhone/iPad app by Apogee Instruments.)

- Source: https://appshunter.io/ios/app/clear-sky-calculator/id974693728 (this page in markdown: same URL + `.md`)
- Developer: [Apogee Instruments](https://appshunter.io/developer/955609109)
- Category: Business, Utilities
- Price: Free
- Rating: 5.00/5 from 2 App Store ratings · 1 written reviews indexed
- Age rating: 4+
- Requires: iOS 12.0 · 6 MB
- Languages: American English
- Released: 2015-04-04
- Data updated: 2026-06-04
- Monetization: free
- User reviews in markdown: https://appshunter.io/ios/app/clear-sky-calculator/id974693728/reviews.md

## What is Clear Sky Calculator?

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 (http://www.kimberly.uidaho.edu/water/asceewri/index.html). 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 http://www.apogeeinstruments.com/using-the-clear-sky-calculator/ )

A more detailed discussion of Clear Sky Calculator accuracy is given on the webpage (http://clearskycalculator.com/model_accuracy.htm), 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.

## Key features

- Estimates solar radiation intensity
- Determines sensor recalibration needs
- Global location and time support
- Uses ASCE Standardized Reference Evapotranspiration Equation
- Requires minimal input data

## Recent user reviews (1 of 1)

All indexed reviews: https://appshunter.io/ios/app/clear-sky-calculator/id974693728/reviews.md

### 5/5 — Lots of uses

*2016-01-02*

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.

## Frequently asked questions about Clear Sky Calculator

### What is the Clear Sky Calculator used for?

The Clear Sky Calculator is used to estimate the intensity of solar radiation incident on a horizontal surface. This estimation helps users determine if their radiation sensors, such as pyranometers and quantum sensors, need recalibration.

### How accurate is the Clear Sky Calculator?

When used near solar noon over multiple clear, unpolluted days during spring and summer months, the accuracy is estimated to be ± 4% globally. Specific examples show measured radiation closely tracking modeled values.

### What information do I need to use the Clear Sky Calculator?

The calculator requires site elevation, latitude, longitude, reference longitude, air temperature, and relative humidity measurements or estimates. These inputs are generally easy to obtain.

### Does the Clear Sky Calculator work on all devices?

The Clear Sky Calculator is available for iPhone, iPad, and iPod. It is designed to be accessible on Apple's mobile operating systems.

### How often is the Clear Sky Calculator updated?

The latest version, 2.0.5, was last updated on September 28, 2021. While not updated frequently, the core functionality remains stable for its intended purpose.

### Is the Clear Sky Calculator free to use?

Yes, the Clear Sky Calculator is completely free to download and use. There are no in-app purchases or subscription fees associated with it.

### What is the age rating for the Clear Sky Calculator?

The Clear Sky Calculator has an age rating of 4+. This indicates it is suitable for users of all ages, with no content restrictions.

### What is the developer of the Clear Sky Calculator?

The Clear Sky Calculator is developed by Apogee Instruments, a company specializing in scientific instrumentation.

## Version history (last 5 releases)

### 2.0.5 — 2021-09-28

Added ePAR option

### 2.0.4 — 2021-09-25

Added ePAR Option

### 2.0.3 — 2019-03-23

Fixed bug where the app failed to obtain weather information.

### 2.0.2 — 2018-09-18

Updated launch screen images.

### 2.0.1 — 2018-08-25

Fixed issue with Longitude and Timezone maps

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All apps by Apogee Instruments: https://appshunter.io/developer/955609109

## Related topics

[pyranometer](https://appshunter.io/ios/topics/pyranometer) · [apogee instruments](https://appshunter.io/ios/topics/apogee-instruments)

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*Data collected daily from the US App Store and indexed by [AppsHunter](https://appshunter.io/). User reviews are verbatim App Store reviews. Ratings, prices and chart positions refresh continuously; this snapshot is from 2026-06-04.*