I was recently asked by a student “how high are the clouds?” I usually look outside and take a guess or if marginal and someone else is already flying I will ask them to report it back, but what if neither option is viable, maybe you are a new pilot or no one else is flying and you don’t have access to a weather station for your field that calculates it for you?
The first thing to do is to look outside if it doesn’t look flyable it probably isn’t, go and have a tea or coffee and check again later! but for a more technical answer see below:
First, find the outside temperature in degrees C.
Next, find the current dew point.
Subtract the dew point from the temperature and divide the answer by 2.5, and finally multiply that by 1,000 to give the approximate cloud base in feet.
e.g. Temperature = 14c, Dew point = 12c. 14-12= 2
2/2.5 = 0.8 multiplied by 1,000 gives the cloud base of 800ft AGL
To find it with reference to MSL just add the field elevation to the above.
But what if you don’t know the dew point?
This is often given in a METAR but if you are flying from a small strip you may not have this information relevant to your location.
First, find the relative humidity (RH), this is given by most weather stations and is also shown on the Met Office weather forecast.
The Lawrence method of approximating dew point temperature (Td) is based on the observation that for humidity levels greater than 50%, Td decreases by about 1°C for every 5% decrease in RH
The following formula Td = T – ((100 – RH)/5) gives the approximate dew point temperature Td, where T is the current temperature in C.
You can now either use the above formula with the calculated dew point to work out the approximate cloud base or continue below.
Putting it into practice to estimate today’s cloud base
When calculating the cloud base from relative humidities greater than 50% we can simplify it to:
Cloud base = (((100 – RH)/5)/2.5 )* 1,000
e.g using the forecast information shown on the right we have an approximate 880ft cloud base
(((100 – 89)/5)/2.5)*1,000 = 880
Breaking that down:
100 – 89 = 11 so we now have ((11/5)/2.5)*1000
11/5 = 2.2 so now we have (2.2/2.5)*1000
2.2/2.5 = 0.88 leaving us with 0.88 *1000
0.88 * 1000 = 880 which is the approximate cloud base in feet.
Now you know how it is derived it can be simplified slightly thus:
((100 – 89)/12.5)*1,000 = 880
As a side note, this type of question can come up in your Met exams!
