Thursday, March 15, 2018

Bright and Dark Cumulus Clouds

I noticed that some of the cumulus clouds today were producing light drizzles. Some of the cumulus were darker than others and this made me wonder if they had a different temperature than the brighter clouds. The visible and thermal images below indicate there may be a small amount of temperature difference. The brighter white portion of the background cloud is slightly darker in thermal infrared and therefore slightly higher.

Visible light image

A thermal image of the same cloud

Cloud Altitude Experiment #2

Cumulus clouds filled the sky over NearSys Station. Since I hadn't tried to measure the Altitude of these clouds using both the thermal imager and new weather station, I made an attempt after I got home.

The thermal imager indicated the ground temperature was 33 degrees and the cloud temperature was 4 degrees. Assuming a dry adiabatic lapse rate of 5.4 degrees per thousand feet, the base altitude of the cumulus clouds over NearSys Station was determined to be 5,400 feet.

The weather station indicates the air temperature was 57 degrees and the dew point was 29 degrees. Assuming a dry adiabatic lapse rate of the air temperature until it reached the dew point indicated that the altitude of the clouds was 5,200 feet.

That's a pretty good agreement. And the altitude makes sense because cumulus clouds are typically between 1,500 and 10,000 feet AGL.

A visible image of a cumulus cloud over NearSys Station.

A thermal image of the same cloud (the picture is one degree colder that I initial saw).

Visibility for NearSys Station, 15 March 2018

The UAVSonde recorded images of the horizon at around 6:30 PM MDT. The visibility is at least 50 miles.

Looking west

Looking north

Looking east

Looking south

UAVSonde Data for NearSys Station, 15 March 2018

I flew the UAVSonde to an altitude of 400 feet while collecting temperature, relative humidity, and visibility data.

Here are the data
Ground Altitude
Temperature: 55.4*F
Relative Humidity: 27.7%

400 Foot Altitude
Temperature: 49.6*F
Relative Humidity: 29.8%

The Amprobe TR200 recorded data once per second. In the chart below, the two attempts needed to collect data is apparent. Two attempts were required because the UAVSonde flashed a battery warning during the first ascent. So I had to land the UAVSonde and replace the battery. The UAVSonde remained on station (at 400 feet AGL) for four minutes. The chart below indicates the approximate mid point of the UAVSonde on station.

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,

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.

Twenty-four Hour Temperature and Relative Humidity for NearSys Station, 12 March 2018

I made my first 24 hour test of the Amprobe TR200 datalogger. The chart below is my first attempt to show how air temperature and relative humidity vary over a day. You'll see that they vary inversely, as the air temperature gets higher, the relative humidity gets lower. This will hold as long as no additional moisture is added to the local atmosphere.

The datalogger gets hot in the sun, so I'll need to add a sun shield in the near future.

All Sky Photometer Data for 12 March 2018

It became windy at NearSys Station at around noon. so windy in fact that the photometer was blow over. I wasn't until after I got home that I realized the photometer was measuring the intensity of light getting through the grass blades.