Jump to content
Thunder?
Local
Radar
Pollen
  • Skew-T Diagrams


    johnholmes
     Share

    hi

    This is the first page with an introduction to the tutorials about skew-T

    post-847-1114527689_thumb.jpg

    below is page 2 with diagram 1, which shows the skew-T diagram that you can find on Net Wx.

    post-847-1114529852_thumb.jpg

    The next diagram will show the relationship with just two variables, the pressure heights and dry bulb temperatures.

    herewith diagram 2

    post-847-1246783882_thumb.jpg

    In the next diagram, diagram 3, we introduce the DALR, read the enclosed to find out what that means.

    post-847-1115126695_thumb.jpg

    In the next diagram - diagram 4 we show and explain the ELR(ENVIRONMENTAL LAPSE) rate, and also what an INVERSION is

    post-847-1114683538_thumb.jpg

    Now to explain those two terms:

    From this we can do a little experiment.

    With a temperature of 20C follow the DALR up its line until it hits the ELR(Environmental Lapse Rate). At this point it cannot rise any further as it has the same temperature as the surrounding air. In fact for some way above it the ELR temperature continues to increase not decrease. When this happens we say it is an INVERSION. These are important to a forecaster trying to decide whether showers or thunderstorms may develop. With this sort of sounding then the answer is no. These inversions occur at various levels in the upper atmosphere.

    So now to introduce yet another line on the master skew-T diagram. This is called the SALR(SATURATED ADIABATIC LAPSE RATE). So far we have assumed our parcel of air is dry. Now it becomes moist. There is no in between in the atmosphere, air is either dry or moist.

    Condensation takes place as the air rises at its saturated rate, therefore latent heat is given out, and the moist air thus cools off more slowly than when it was dry. It cools at approximately 1.4C per 1,000ft; in other words at about half the rate it cooled when dry.

    So in diagram 5 we have the Temperature lines in BLUE; Pressure lines in BLACK, DALR lines in RED, and the SALR lines in ORANGE

    post-847-0-94217800-1294746311_thumb.jpg

    On the surface, many of us take weather readings, or read from an automatic weather station. This gives us something called the dewpoint and the relative humidity. Again no need to understand the physics of it, just accept that the dewpoint gives a measure of how moist the air is. One fact you must hold on to is that the higher the temperature then the more water vapour it can hold. So if the dry bulb temperature is 20C and the dewpoint is 5.0C, the relative humidity, say at 1000mb, is 36.8%. If the temperature was 7C, the relative humidity would be 87%. To get a relative humidity that value with a temperature of 20C, would need a dewpoint of 17.8C. So you see the warmer it is the more water vapour air can hold. This accounts for why it is so much wetter in the Tropics than the Arctic!

    So we now need to introduce some means of measuring the humidity of the air in the atmosphere. This is done by something called the HUMIDITY MIXING RATIO(HMR) and is illustrated in the diagram below.

    diagram 6 below

    post-847-1115308492_thumb.jpg

    Yes I know its getting all very complicated but we only have one left to add. That is the dewpoint curve on the ELR. for that please see below.

    Diagram 7 below

    post-847-1115310847_thumb.jpg

    Right that is all the theory, now we can look at what the original skew-t shows with all the lines on, I'll put this skew-T for 06Z for the London area on and try and explain all the lines again. Once you have had time to absorb these last three diagrams we can then start to look at 'live' ascents/skew-T diagrams to find out what we can about cloud development from them. In time we should get a good 'convection' day, then we can put our new knowledge to the test, along with any other that we may each have and see if our forecast is correct!

    added Jan 11 2011

    I am deleting the two attachments below as they are not working.

    As the diagram was to show how to use the skew-t I will do another one explaining this, but it may be some time before I am able to do this.

    diagram 9 below

    post-847-1116083066_thumb.jpg

    below is diagram 10 with the explanation before it.

    This was a check to see if the GFS overall forecast would give convective cloud.

    To set the scene; GFS has high pressure to the west and low pressure over Denmark - if only mid January!.

    NW winds for most, 5-10mph generally but nearer 15 mph in the exposed northeast.

    GFS rainfall for 12Z suggests showery outbreaks, generally slight and for most parts, other than some areas of the south west and along parts of the south coast which should stay dry..

    Afternoon temperatures from only 8-10C in the east to 12-13 C in the west/sw and perhaps 15C in a few sheltered spots along the south coast. Net Wx temps for 15Z are much the same; these are perhaps a bit underdone for the east as many places saw around 12-14C this afternoon(Sunday). Dewpoints should be about the same sort of value as today(the airmass is not changing, other than just tending to warm up a little, especially in the west).

    So taking the 12Z skew-T predict for east Lothian, as being the most representative for the eastern side, what do we have.

    see below: diagram 10

    post-847-1116084740_thumb.jpg

    This just about concludes the explanation of Skew-T diagrams and how they can be used to predict whether, or not, convective cloud is expected. Further tutorials will be made as and when situations arise, for more insight into thunderstorm prediction,frontal cloud, how to use the winds alongside the skew-T, maximum and minimum temperatures, will there be fog overnight, will it snow or rain.

    thank you to everyone who has read/used them. Please continue to give me any feed back.

    John

     Share


    User Feedback

    Recommended Comments

    As a complete beginner to weather forecasting I found this artical very interesting. In the last paragraph it's suggested further tutorials will be made, have any more been made? if so, could someone post a link to them.

    Thanks

    Paul

    Link to comment
    Share on other sites



    Create an account or sign in to comment

    You need to be a member in order to leave a comment

    Create an account

    Sign up for a new account in our community. It's easy!

    Register a new account

    Sign in

    Already have an account? Sign in here.

    Sign In Now

×
×
  • Create New...