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Skew-T log-P Diagram
[edit]The Skew-T log-P diagram is a crucial meteorological tool used for plotting and analyzing vertical profiles of atmospheric temperature, moisture, and wind, providing a comprehensive overview of upper air conditions. The diagram's name originates from its distinctive feature: temperature lines that are skewed at a 45° angle, and pressure isobars spaced logarithmically.[1][2]
Data Source
[edit]Data for these vertical atmospheric profiles, known as sounding plots, are derived from sources including radiosondes, dropsondes, aircraft, numerical weather prediction (NWP) model outputs, and satellite sounders. These profiles are used to assess a variety of meteorological conditions, most notably atmospheric stability.[1] Radiosonde data over the world can be also found on the webpage of Wyoming University's Department of Atmospheric Sciences open to public.[3]
How To Read
[edit]The Skew-T log-P diagram comprises six sets of axes: temperature, pressure, dry adiabats, moist adiabats, mixing ratio, and wind staffs. Each set serves a specific purpose, aiding meteorologists in understanding atmospheric processes such as the rate of cooling of rising air and the amount of water vapor present at different altitudes.[2]
Temperature
[edit]On the Skew-T log-P diagram, temperature lines are angled at 45°, with values increasing from the upper left to the lower right. Originally, these lines were vertical, but in 1947, they were tilted to improve analysis. This adjustment led to the term "Skew-T," highlighting the skewed orientation of the temperature lines.[2]
Pressure
[edit]Pressure lines on the Skew-T log-P diagram are drawn horizontally, with distances increasing from the bottom (1050 millibars) to the top (100 millibars) due to decreasing atmospheric density with elevation. This setup reflects the logarithmic decrease in atmospheric pressure with height, known as the "Log-P" portion of the diagram . Isobars are nearly horizontal, spaced logarithmically every 50 hPa, and standard atmosphere heights are marked on the right side in feet and meters for reference.[1][2]
Dry Adiabats
[edit]On a Skew-T diagram, dry adiabats are represented by slightly curved, solid lines that slope from the lower right to the upper left, indicating lines of constant potential temperature. These lines illustrate the rate at which unsaturated air, with a relative humidity less than 100%, cools as it rises adiabatically—without exchanging heat with its surroundings. Specifically, the dry adiabatic lapse rate is approximately 9.8°C per 1,000 meters (5.5°F per 1,000 feet), reflecting how the temperature of a parcel of air decreases as it expands and ascends. Each dry adiabat intersects the 1000-hPa isobar at a temperature that is labeled alongside the isotherms, providing a visual reference for the temperature change in rising or descending dry air parcels.[1][2]
Moist Adiabats
[edit]On a Skew-T diagram, moist adiabats, also known as saturation adiabats or saturation pseudo-adiabats, are represented by slightly curved lines that increase in value from left to right. These lines illustrate the rate at which saturated air—air at 100% relative humidity—cools as it rises. As saturated air ascends, it cools at a rate of about 4°C per 1,000 meters (2.2°F/1,000 feet) near the surface, with the cooling rate decreasing at higher altitudes due to the reduction in water vapor. This cooling process is influenced by the release of latent heat during condensation, which affects the temperature change of the rising air parcel.[1][2]
The moist adiabats on the Skew-T diagram become nearly parallel to the dry adiabats in the upper troposphere, where the cooling rate approximates that of dry air at 9.8°C per 1,000 meters (5.5°F per 1,000 feet). This parallelism occurs because, in very cold air, most water vapor has condensed or deposited, leaving little moisture to impact the cooling rate significantly. Conversely, in warmer, more humid conditions, the release of heat from condensation plays a crucial role in phenomena such as tropical cyclones and thunderstorms. Each saturation adiabat is labeled with the Celsius temperature where it intersects the 1000 hPa isobar, and its slope and spacing vary with height and temperature, reflecting changes in moisture and pressure.[1][2]
Mixing Ratio
[edit]In meteorology, the mixing ratio represents the mass of water vapor relative to the mass of dry air, expressed in grams per kilogram. On a Skew-T diagram, the mixing ratio at any given level can be determined by where the dew point temperature line intersects with the mixing ratio lines. Additionally, the saturation mixing ratio, which indicates the maximum amount of water vapor the air can hold at a given temperature and pressure, is found where the temperature line crosses the saturation mixing ratio line.[1][2]
Saturation mixing ratio lines, or humidity mixing ratio lines, on the Skew-T diagram are slightly curved, dashed lines that slope from the lower left to the upper right. These lines denote constant values of water vapor capacity, representing the number of grams of water required to saturate one kilogram of dry air at specific temperatures and pressures. The lines are labeled with values ranging from 0.1 to 40.0 grams per kilogram at the bottom of the diagram. Due to the non-linear variation of vapor capacity with temperature, the intervals between these labels are not uniform.[1][2]
Wind Staff
[edit]On a Skew-T diagram, wind speed and direction are plotted using conventional wind plot models on dedicated wind staffs located on the right side of the diagram. These staffs are used to track the wind's speed and direction at different pressure levels based on the radiosonde’s movement. Wind speed is indicated in knots with barbs and flags: each flag represents 50 knots, each full barb represents 10 knots, and each half barb represents 5 knots. For instance, three full barbs and one half barb denote a wind speed of 35 knots, while one flag, one full barb, and one half barb indicate a wind speed of 65 knots.[1]
The direction of the wind is shown by the orientation of the barbs and flags, which are positioned at the end of the staff. For example, if the flags and barbs are on the left side of a horizontal staff and point upwards, this indicates that the wind is coming from the west. Each wind staff on the Skew-T diagram corresponds to a specific height or pressure level, with the plotted winds depicted in black along a separate axis, illustrating the wind conditions at that level.[1]