Skew T Log P
1. What is a skew T diagram?
2. Where does the data come from?
3. How do we use it?
The lapse rate is the way the temperature changes as we go up and down in altitude. There are a variety of ways to look at that.
One way is to look at the Environmental Lapse Rate,
The ELR is where a thermometer measures the temperature as you go up or down in altitude and then figure the rate of change as you change altitude. That is measured every day, twice a day from about 1,300 places around the world by sending up a balloon that has a thermometer in it.
Another lapse is ISA, International Standard Atmosphere. This is the average of how the temperature changes as we go up and down in altitude. That average is based on actual measurements then averaging them as 2° per 1,000 feet.
Starting at sea level, the data is defined as 15°. That is the baseline for ISA. So at 5°, the density altitude is above field elevation. If the temperature is colder than 5°, the density altitude is below field elevation.
ISA is defined by ICAO, International Civil Aviation Organization, at the United Nations.
The temperature is not a constant 2° per 1,000 up to about 11,000 meters, which is about 36,000 feet, which is the top of the troposphere.
The troposphere is where the lower boundary of the stratosphere.
Troposphere is where the weather temperature decreases as you go up in altitude.
In the stratosphere for a while the temperature stays constant.
ISA says it stays constant at -56.5°C until you get up to 65,000 or 20,000 meters where it starts getting warmer again.
In your handbooks all the performance data is based on ISA.
Dry adiabatic lapse rate as long as that parcel of air is warmer than the environment around it, it will rise.
It cools and expand and will continue to rise until it cools to the temperature surrounding it, it is no longer buoyant. The rate at which this parcel of air cools is 3 per 1,000. Moist adiabatic lapse rate is when this parcel of air has moisture. When it cools to the point where it equals the dew point, you get a cloud. If the condensed air is still warmer than the environment it’s still gonna want to go up. And it’s still cooling. What happen to the ability of the air to hold moisture as you cool it? It goes down. Cold air can not hold as much moisture as warm air, so if we continue to cool this parcel of air that has now condensed to a point of saturation, the moisture has to come out in some form.
So when you convert from vapor to water, latent heat comes out. That’s why the moist adiabatic lapse rate is about 2 per 1,000 feet. Because we’re heating the atmosphere because we are taking water out.
Now once that parcel of air gets to the same temperature as the environment, it’s gonna stop going up.
Weather balloon have radiosondes in them and they go up at 0Z and 12Z from 69 stations in the U.S.. and about 1,300 world wide.
So you can see we don’t have a very rich data set. From the radiosondes, soundings, we get temperature, dew point, pressure and the location so we can also calculate the wind.
Now we can plug that info into a numerical model. We can then use that model of the data at other points other than the 69 points in the U.S. or NAM, North American Model.
How do we plot the data? We care about pressure and temperature. So we put these soundings from the balloons on a vertical profile of the atmosphere. From that we can compile how the dry adiabatic lapse rate and the moist adiabatic lapse rate compare to what the atmosphere really is. From this data we can figure out cloud height, where the icing is and where turbulence is likely to be.
So on a chart called the Skew T log P but we just call it the skew t, we just plot this temperature across the bottom scale and the pressure across the left side. If we do that we get this funny looking line.
Back in 1947 some weather dude said. “Oh wait a minute. What if we skew the temperature 45°.” So now instead of the temperature being vertical, it’s skewed. Now we get a chart that’s much easier to read.
The is all the Skewed T means. It’s simply a plot of temperature vs pressure equaling to alpha 2.
Bill then gave a lot of examples and asked the audience various questions based on the different charts he put up.