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 You will need Java Script loaded on your computer or device.

You should see a screen like this below.
There are various types of Skew-T data bases. Op40, the one automatically ticked at the top, is the latest version of what we want. (Bak40 is an ealier version that is being phased out.)
  • In the box at the bottom that says "Site(s)" type cyow or yow to get the current diagram for Ottawa.
  • Click on the [SIMPLE java plots] button. Use the [Java plots] button for a more complex diagram after you have done a bit of reading on how to interpret Skew-T diagrams - more about that below.
You may get a pop up box asking "Do you want to run this application". Just click "Run".

Interpreting the [SIMPLE java plots] diagram.
Clouds form when the dewpoint temperature and the air temperature are almost the same. So in the diagram below 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:
The METAR did not however indicated that the cloud layer was quite thin as indicated on the Skew-T diagram.

Wind speed is illustrated on the right with wind barbs in red and a graphic wind profile in blue. Scrolling your mouse up and down the diagram will highlight individual wind barbs at various alitudes.

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. I don't think they are very good. Try these 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 CYOWToronto CYYZMirabel CYMXHamilton CYHM
Kingston CYGKQuebec CYQBNorth Bay CYYBSyracuse KSYR
Trenton CYTRLondon CYXUBuffalo KBUFBurlington 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.

Cole Pavey