TSkew-T/Log-P Diagram

This help is from the University of California-San Diego. Colors and plot content may differ from the Arctic and Antarctic plots provided at this site.

The skew-T (skewed temperature/logarithmic pressure) diagram is an instantaneous picture of the temperature, humidity, and winds in the atmosphere above a particular point on the earth's surface in a column from the surface to about 16 kilometers (km) (10 miles) above mean seal level. The horizontal axis is temperature in Celsius (C) (water freezes at 0C and boils at 100C). The vertical axis is atmospheric pressure in millibars (mb), which decreases with altitude. 1000 mb is just above the earth's surface for stations near sea level (and may be well below the earth's surface at mountain stations), 700 mb is about 3 km (10,000 feet above mean sea level (MSL)), and 500 mb is about 5.5 km (18,000 MSL). The measurements depicted in the skew-T diagram are collected in a "sounding" of the air in the column (using a sensor package carried aloft by a balloon).

The solid blue line is the temperature from the sounding. The temperature is measured on the chart by the red grid that slants (is "skewed") right with height. This slant is set so a temperature that cools at a normal rate with increasing elevation will show as an almost vertical line. The dashed blue line is the dewpoint temperature, a measure of humidity. Where the dewpoint line is far from the temperature line, the air is dry; where close, wet. The closer the lines, the higher the humidity. Where the two lines overlap, the air is saturated (humidity is 100%). When the lines are close or overlapping, clouds are likely. If a saturated region is sufficiently thick and close to the ground, it is usually raining or snowing. In this diagram, there appears to be a thick cloud layer from the surface to about 3.5 km (12,000 MSL). The solid green lines that slant up to the left are "dry adiabats" which represent how the temperature of dry air would cool if it were pushed upward ("lifted"), or warm if pushed down. The dashed green lines are "wet adiabats" which show how the temperature of a saturated air parcel cools as it moves upward (temperature can not warm by moving down a wet adiabat as the air would immediately "dry" to less than saturated conditions).

The wind barbs (found along the right edge of the diagram) show the direction the wind is coming from (the shaft of the barb) and the wind speed. In this case (which is from Salem, Oregon on October 22, 1996), the winds are from the south at the surface, from the southwest in the lower atmosphere, then veer (the direction changes clockwise) westerly to northwesterly with increasing elevation. The lines attached to the tail of the shaft indicate the speed. A short line indicates 5 knots (nautical miles per hour, abbreviated as kts) or 5.75 mph, a long line is 10 kts (11.5 mph), and a filled triangle is 50 kts (57.5 mph). If one triangle, two long lines and one short line were on a wind barb, it would indicate 75 kts (50+10+10+5) or about 86 mph.