Skew T : Masterclass 1 of 4Added : Thursday at 14:09 Right, by now you've hopefully had a look at the SKEW-T charts on Metcheck. If you haven't, then take a look at them under the WEATHER > ADVANCED area on the site and you'll see something like this :-
OK, the main thing we learn today are how the chart is laid out and what the lines mean.
The first thing you should notice are the numbers running up the left hand side of the image from 100 to 1000. These are the levels in the atmosphere in millibars and go from the ground (around 1000mb) up to the top of the troposphere which is around 100mb.
Next, if you look on the right of the image you'll notice wind arrows. These tell us the direction and strength of the wind which is expected at each level in the atmosphere. This is pretty important, as tells us whether the wind is twisting with height or whether it is simply getting stronger but in the same direction.
Next, take a look at the bottom of the chart (above the dates and times) and you'll see a scale from -40c to 50c. These are (surprise, surprise) the temperatures.
Plotted on the chart we have 4 thick lines.
The black one is the 0c, or freezing temperature. Don't worry too much about this one at the moment, but it means that if any of the lines are to the right of this then the temperature is above freezing and if it's to the left then it's below.
The red line is the temperature of the air. So, the chart shows how the temperature of the air falls as you head up through the atmosphere (most times, but we will come to inversions in part 3).
The blue line is the dew point of the air. Now, while you might think "What's the point in telling me the dew point?" Well, if the temperature of the air and the dew point are the same then the air is saturated which means.... cloud! If you look at the chart above you can see that from 700mb to around 500mb above you that the air is saturated and therefore it probably cloudy at this level.
If you look at the one below then you'll see they don't even come close to each other!
Finally, the orange line. This is really important in forecasting things like thunderstorms and showers as it tells us the lapse rate of the air.
The lapse rate is the rate which we would expect a rising parcel of air to cool and expand as it rises in the atmosphere. This is pretty much a constant value, a bit like when you turn your cooker off it will decrease in heat at a specific rate and the atmosphere is the same, especially when the air is dry.
The eagle eyed of you will notice that the orange line buckles a short way from the bottom of the graph. The point at which this buckles is called the LCL height or the Lifted Condensation Level. This is where the air is now saturated and will continue to cool but at a slower rate, after all the air is saturated and wet things cool down slower than dry things (it's why a pan of hot water will cool down slower than a dry pan).
So, that's your homework for tonight. Now you know what the lines are go check the Skew-T for your location and see how they change as you select different times.
Tomorrow we will take a look at some examples of these lines and show you what they mean.
METEOROLOGIST : MARSH
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