Meteorologists look at a lot of data each day to prepare a forecast. These data usually consist of various weather maps, radar and satellite imagery, and upper air maps. Other charts and diagrams are also used. One chart that is especially useful to meteorologists is known as a Skew-T diagram.
A Skew-T diagram, also known as a sounding, is plotted from data measured by weather balloons. National Weather Service observation sites usually release weather balloons (rawinsondes) twice a day. When the weather is expected to be severe, some sites may release them more often. Below is a blank Skew-T diagram.
On the horizontal axis of the diagram, the temperature is plotted; on the vertical axis, the pressure is plotted. Pressure decreases as you go up the vertical axis, just like it does in the atmosphere. The green dashed lines represent the mixing ratio, which is a ratio of the mass of water vapor to the mass of dry air (usually presented in units of grams per kilogram). The "Skew T" part of the name comes from the fact that the temperature lines are plotted skewed 45° to the right. Pressure is plotted on a logarithmic scale to approximate the way it decreases with height.
Example Skew-T with Plotted Data
There are several things to discuss on this diagram, but let's start with the basics. On the Skew-T itself, there are two thick lines. The blue line is the dew point profile; the red line is the temperature profile. Just to the right of the diagram are wind barbs plotted with increasing height (using the same convention used on a weather map). Suppose you want to know the temperature, dew point, and wind at 500 millibars (mb). First, find where the temperature profile intersects the 500 mb level and then follow the temperature line back to the horizontal axis. The same method applies when using the dew point profile. In this sounding at 500 mb, the temperature is -10°C, the dew point is around -29°C and the wind is from the west at 5 knots.
Other facts can be inferred from this diagram as well. The further apart the temperature and dew point profiles are, the drier the air is; the closer they are, the moister the air. The slope of the profiles, especially for temperature, can be used as an indication of the stability of the atmosphere. The more the temperature profile leans to the left, the greater the instability, which can lead to the development of thunderstorms.
So why would we use these diagrams? Depending on the time of year, soundings can be used to forecast a wide variety of phenomena. In the warm season, I might use a sounding to determine whether thunderstorms may develop. If so, I can also use it to determine the potential for a storm to produce hail, downbursts, or tornadoes. In the cold season, the temperature profile is crucial in determining the type of precipitation that may occur. When forecasting snow, both the temperature and dew point profiles can be significant. Snowflakes develop between -12°C and -18°C with relative humidity of at least 80%. Sleet and freezing rain can occur if the surface is below freezing and there is a layer of warm air above freezing just above the surface. The depth of this warmer air determines whether precipitation falls as sleet or freezing rain; a deep warm layer would favor freezing rain whereas a shallow warm layer may favor sleet.
While soundings can be a useful tool to forecasters, it is essential to understand their limitations. Though soundings are considered a vertical profile of the atmosphere, they're not truly a vertical profile. The balloon can drift downstream during its ascent, recording temperatures many miles from where it was launched. Since it can take tens of minutes for the balloon to make a complete ascent, the profiles obtained are not instantaneous. One major limitation is the time, and space scarcity as only 100 locations release weather balloons in the US.
Even with the limitations listed above, soundings are a useful tool to meteorologists for assessing the vertical structure of the atmosphere. Using traditional weather maps in conjunction with soundings can help paint a complete picture of what is occurring in the atmosphere, which can lead to more accurate forecasts.
This is the first blog in a three part series discussing Skew-T diagrams. Part 2 will explain backing and veering winds, as well as how they can affect the temperature profile. We'll end the series with a look at instability and how it relates to thunderstorm development. So stay tuned!