Tuesday, March 13, 2018

Determining the Altitude of Clouds

I've wanted to run this test for a while now and finally got around to it.

By measuring the dew point and the air temperature at the ground, I should be able to determine the approximate altitude of the cloud base. Here's why this should work.

On dry air, the air temperature normally decreases by 5.4 degrees F per 1,000 feet. When the air temperature reaches the dew point, the relative humidity reaches 100% and condensation occurs. Now that condensation can occur sooner if the air contains condensation nuclei for water to condense on, so the altitude is not exact.

Before running this experiment, I built a sling psychrometer to measure the air temperature and dew point. You can find the information to do the same thing at, https://www.flinnsci.com/build-your-own-sling-psychrometer/dc10301/

I also upgraded my home weather station to one that reports temperature, dew point, relative humidity, and air pressure.

I also have a thermal imager that remotely measures ground and cloud temperature.

My test this afternoon found the following information.
Thermal Imager:
Ground temperature  of 40* and cloud temperature of 6*
Weather Station:
Air temperature of 63* and dew point of 34*
Sling Psychrometer:
Dry bulb temperature of 61* and wet bulb if 52*

The temperature difference for the thermal imager indicates a cloud base at 6,300 feet
The temperature difference for the weather station indicates a cloud base at 5,400 feet
The temperature difference for the sling psychrometer indicates a cloud base at 1,700 feet.

The weather station and thermal imager give nearly the same altitude while the psychrometer diverges significantly. So either the psychrometer is not working for me or I still need to learn how to use it properly.

The cloud cover above NearSys Station is stratus, with a little mammatus. Stratus is a low cloud and found at about 6,000 feet. So I suspect the thermal imager and weather station will be pretty good at determining the cloud altitude. Especially if I can get similar results with cirrus clouds.

The cloud cover currently over NearSys Station.

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