How high is that cumulus cloud?

I hope you find this to be a fun explainer! Here’s the scenario, you look out the window and see a fluffy cumulus cloud. Let’s say it is the “cumulus congestus” cloud variety, which means that it is taller than is wide. Your friend next to you says, “I wonder how high that cloud base is”? Armed with readily available local weather information and fundamentals about the atmosphere, you can be sure to impress your friend with a good approximation for that cloud base! In meteorology terminology, the altitude a cloud first develops is called the “lifted condensation level” or LCL.

For a quick background, a visible cloud occurs when atmospheric water vapor (a gas) condenses into liquid water droplets. One mechanism to force water vapor surrounding you to condense is to cool it to its dewpoint temperature, which is a common weather measurement to assess the amount of water vapor content in the air mass.

Hang on, just a little more background to go before impressing your friend with a quick back-of-the-envelope calculation for finding the LCL. If you are seeing cumulus clouds develop overhead, you likely have convective heating taking place. Convection means the air mass outside your window is being carried upward into the atmosphere by surface heating. This is when key concepts tie together!

Parcels of rising warmer air leaving the surface (perhaps visualize hot air balloons lifting off) start to encounter lower atmospheric pressure further up. Less pressure means these rising parcels can expand and, therefore, cool as molecules contained within have more space and make less collisions to generate heat. Here’s our cooling mechanism! It turns out, an unsaturated atmosphere tends to cool at a rate of 5.5 degrees Fahrenheit per 1,000 feet going up in the tropospheric layer. This is also called the “dry adiabatic lapse rate”.

Given an air temperature (T) of 75 degrees and a dewpoint (Td) of 55 degrees at a nearby weather station, the cloud base, or LCL, would be estimated to be:

(75 – 55)/5.5 = ~3,600 feet above the ground!

Keep in mind, the cloud would have had a lower LCL altitude if the spread between air temperature and dewpoint were smaller.

Photo Credit: Jonny William Malloy