Thursday, July 18, 2013

What Is A Skew-T & Why Use It?

The Skew-T/Log-P Diagram, combined with its brother the Hodograph, is the best tool for severe weather forecasting. SPC Mesoanalysis is great, as are many other tools, but a severe weather forecaster cannot get by without looking at actual real soundings plotted on one of these diagrams.

Yet these diagrams are also a bit difficult to interpret. I frequently hear college freshmen and non-majors in our intro-level Meteorology course bemoaning the use of Skew-T diagrams for this very reason.

But really, they are not a force to be feared. They are one to be loved, and with my help, by the end of this article you too will learn to love them.

The Skew-T/Log-P is a thermodynamic diagram, meaning that it deals with temperature and moisture and how that relates to energy. It is specifically named for the facts that temperatures lines are skewed, and pressure lines decrease at logarithmic intervals. Below is a blank diagram, from the Plymouth State Weather Center’s website. This blank diagram, while a wonder itself, is actually quite useless without real data plotted on it, which we’ll get to later.

Atmospheric pressure is plotted at the left, decreasing with height just as it does in the real atmosphere. There are five different types of line on the blank chart above, and the first of these to talk about is the isobars. Isobars are lines of equal pressure, and they are completely horizontal across the diagram, as seen highlighted with blue lines below.

Notice how much bigger the difference between 300 mb and 200 mb is than the difference between 1000 mb and 900 mb, for example.

The next line to talk about is the isotherm, or line of same temperature. Isotherms are commonly drawn at an interval of 10 degrees Celsius, and on this chart they are also labeled in Kelvin. On this particular chart, temperature ranges from -50 C (223 K) to 40 C (313 K).

I have highlighted the isotherms in red. Notice that, as I mentioned before, they are skewed. This is done to keep the diagram compact. The upper levels of the atmosphere are so much colder than the surface that if the isotherms were straight vertical lines, the temperature (when plotted) would fly off to the left too quickly.

It is important to note that when a temperature line of real data is plotted, it can look like it is changing with height in ways that it really is not. You need to pay close attention to whether the temperature is crossing a line from colder to warmer, from warmer to colder, or whether it’s running parallel to an isotherm.

In the graphic below, pretend that four hypothetical temperature curves are plotted.

Only in Number 4 is temperature actually increasing with height. We would call Number 3 “isothermal,” meaning that it is not changing at all with height. Despite being completely vertical, the temperature in Number 1 is actually decreasing with height.

If a real temperature curve was plotted, we would now know enough to figure out what the temperature is at different levels of the atmosphere. You could identify the 700 mb temperature by finding where the actual temperature curve (which, again, is not yet plotted) intersects the 700 mb isobar, and then follow your finger down (or draw a line) parallel to the isotherms until you reach the bottom of the chart.

The next important line on the blank diagram is the Dry Adiabatic Lapse Rate, or DALR, for short. The DALR is an atmospheric constant: 9.8 degrees C/km. It is the rate at which a parcel of air completely devoid of moisture would cool as it rises through the atmosphere.

These lines are highlighted with orange color below.

If the temperature curve is parallel to one of these lines, we know that the parcels of air must be very dry, and that the Lapse Rate at that layer is essentially 9.8 C/km.

Fourth is the Moist Adiabatic Lapse Rate, or MALR. Unlike the DALR, the MALR is not a constant. It depends on how much moisture is present. In the diagram below, MALR lines are drawn in green. Notice that as they approach the top of the atmosphere, where the air is very cold and hence there is very little moisture, the MALR essentially becomes the same as the DALR.

The last important line is the Mixing Ratio Line, shown below in yellow. Notice that these lines also increase exponentially.

Mixing Ratio is a measure of moisture in the air, in units of grams per kilogram. So a Mixing Ratio of 10.0 indicates that in a kilogram of air, 10.0 grams of that air will be water vapor. Unlike Relative Humidity, Mixing Ratio is an absolute measure of moisture in the air. You can tell what the Dew Point is at a given pressure level by finding where the Dew Point Curve (again, not yet plotted) intersects the isobar, and then following a Mixing Ratio Line to the surface, and then looking at where the falls between isotherms.

Just a quick note on moisture variables: Dew Point and Mixing Ratio, while not the same, share a constant relationship, such that (for instance) a Mixing Ratio of 15.6 g/kg is always the same as saying a Dew Point of 70 F.

I will be back Monday with Part 2 on using Skew-T diagrams, which will take all of this tedium and put it to good use.