Calculating cloud layer elevations for both bases and tops using Skew-T Log-P diagrams
Cloud base elevations are readily available using METARS and TAFS, but cloud tops are a different story. Fortunately there is a way to get quite accurate estimates using Skew-T diagrams that are available with hourly updates from various airports. Both current and predictive data is available.
First let’s talk about how to get the diagrams before discussing how to use them.
Skew -T diagrams can be found at http://rucsoundings.noaa.gov. You should see a screen like that shown below.
There are various types of Skew-T data bases. Op40, the one automatically selected at the top, is the latest version of what we want. (Bak40 is an earlier version that is being phased out.)
- In the Site(s) field near the bottom, type CYOW or YOW to get the current diagram for Ottawa.
- Click the SIMPLE java plots button. For a more complex diagram, and after you have done a bit of reading on how to interpret Skew-T diagrams (more about that below), click the Java plots button.
You may get a pop up box asking “Do you want to run this application?” Click Run.
Interpreting the “SIMPLE java plots” diagram
Clouds form when the dew point temperature and the air temperature are almost the same. So in the diagram on the left, there is an apparent cloud layer with a base at about 8,500 ft and top at about 10,000 ft.
Confirming this, the METAR for CYOW at this time was: OVERCAST at 9000 FT.
The METAR did not however indicated that the cloud layer was quite as thin as indicated on the Skew-T diagram.
Tutorials for interpreting simple and complex Skew-T diagrams
You will find two links to tutorials at the bottom of the CYOW Ottawa screen you just created. They are limited in the amount and quality of information they provide. Try the following instead:
For a list of airports for which these diagrams are available click on “Airports” in the blue area to the right of the Site(s) box. Here is a list of locations with Skew-T diagrams that might be of interest to SFFC members.
|Ottawa CYOW||Toronto CYYZ||Mirabel CYMX||Hamilton CYHM|
|Kingston CYGK||Quebec CYQB||North Bay CYYB||Syracuse KSYR|
|Trenton CYTR||London CYXU||Buffalo KBUF||Burlington KBTV|
Note that Trenton, London, Quebec and North Bay are not listed in the online list of primarily USA airports. There are no doubt a lot of other useable Canadian airports as well. Just type in the airport registration to find out if it has a Skew_T diagram.