What Is a Skew-T?

A Skew-T or a SkewT-LogP is a diagram showing the vertical profile of temperature, dew point temperature, and winds in the atmosphere. The name comes from the fact that pressure is plotted on a log scale, which rapidly decreases upward along the chart, and temperature is plotted at a 45 degree angle to pressure, hence “Skew-T.” Temperature is plotted at an angle because it allows for some thermodynamic variables to be calculated more easily. For example, Convective Available Potential Energy (or CAPE) can only be plotted with this skewed temperature axis. It is important to understand that temperature increases along an axis from the top left corner to the bottom right corner of a Skew-T as opposed to just from left to right.

Mixing Ratio

Two environmental variables are plotted in the Skew-T chart, which represent the environmental air temperature and the environmental dew point temperature with height. The dew point temperature represents the temperature at which moisture would begin to condense out of the air if the air temperature were the same as the dew point temperature. In effect, the dew point temperature represents a physical measurement of how much moisture is in the air; the higher the dew point temperature, the more water vapor is in the air, and vice versa. That amount of water vapor can also be expressed as specific humidity, which represents a ratio of how much the vapor weighs in relation to the total weight of the air. Lines of constant, specific humidity are plotted on Skew-Ts as dashed green lines, and they represent lines of a constant mixing ratio of water vapor to dry air. Dew point temperatures which follow these lines of constant mixing ratio represent a layer of the atmosphere which has the same amount of water vapor or moisture throughout the layer.

Lapse Rates

Two more important lines, which can be found on blank Skew-Ts, are the dry adiabatic lapse rate and moist adiabatic lapse rate. These lapse rates represent standard temperature decreases with height for 100% dry and 100% moist air, respectively. The dry adiabatic lapse rate is about 9.8 degrees Celsius per kilometer, while the moist adiabatic lapse rate is about 5 degrees Celsius per kilometer. Thus, dryer air would be expected to cool more rapidly with height than moist air. In other words, the water vapor in moist air helps keep air warmer than it would be otherwise. The lines of constant dry adiabatic lapse rate typically extend from the bottom right to the top left of the Skew-T, while the lines of constant moist adiabatic lapse rate start out roughly straight up at lower levels (higher pressures) and eventually bend towards the dry adiabatic lapse rate at higher levels (lower pressures) as this air high up in the atmosphere (where the air pressure is very low) cannot hold much water vapor.