The Skew-T Log-P diagram is a vital tool in meteorology, used to analyze upper air observations obtained from radiosondes. By plotting temperature, dew point, and wind data, the Skew-T provides a wealth of information about the atmosphere’s thermodynamic state. This guide provides an in-depth explanation of the Skew-T Log-P diagram, including its components, how to interpret it, and its significance in weather forecasting.
The Skew-T Log-P diagram consists of six fundamental sets of lines, each representing a specific atmospheric variable. Understanding these lines is crucial for accurate interpretation of the diagram.
Temperature Lines
Temperature lines are angled at 45 degrees, with temperature values increasing from the upper left to the lower right corner of the chart. The “Skew-T” name comes from this 45-degree tilt. Originally, temperature lines were vertical, but the modification in 1947 improved analysis.
Pressure Lines
Pressure lines are horizontal, spaced further apart towards the top of the chart (100 millibars) and closer together at the bottom (1050 millibars). This spacing reflects the logarithmic decrease in atmospheric density with increasing altitude.
Atmospheric pressure decreases logarithmically with altitude. The “Log-P” portion of the Skew-T Log-P diagram refers to the plotting of pressure levels as the logarithm of the pressure.
Dry Adiabats
These slightly curved lines increase in value (°C) from the lower left to the upper right. Dry adiabats represent the rate at which unsaturated air cools as it rises.
As unsaturated air rises, it expands and cools at a rate of 9.8°C per 1,000 meters (5.5°F/1,000 feet) until it reaches 100% relative humidity and becomes saturated. This cooling rate is known as the “dry adiabatic lapse rate,” and the dry adiabats on the Skew-T diagram depict this rate.
Moist (or Saturated) Adiabats
These curved lines increase in value (°C) from left to right. Moist adiabats represent the rate at which saturated air cools as it rises.
When air reaches 100% relative humidity, further cooling causes water vapor to condense, releasing heat. This heat release alters the cooling rate, and the moist adiabats on the Skew-T diagram represent this adjusted rate. Near the surface, saturated air cools at approximately 4°C per 1,000 meters (2.2°F/1,000 feet). This rate decreases with altitude due to diminishing water vapor content.
In the upper troposphere, dry and moist adiabats converge as the cooling rate approaches the dry adiabatic lapse rate of 9.8°C/1,000 meters (5.5°F/1,000 feet). Very cold air contains little water vapor, as cooling causes condensation or deposition, reducing the amount of gaseous water vapor.
Conversely, warm air can hold significant amounts of water vapor. Warming increases evaporation and sublimation, adding water vapor to the atmosphere. The condensation of water vapor in warm, moist air in the tropics fuels phenomena such as tropical cyclones and thunderstorms.
Mixing Ratio Lines
In meteorology, the mixing ratio is the mass of water vapor compared to the mass of dry air, expressed in grams per kilogram (g/kg). The Skew-T diagram allows for the determination of both the ordinary mixing ratio and the saturation mixing ratio.
The mixing ratio at a specific level is determined by locating where the dew point temperature line intersects the mixing ratio line. This indicates the actual amount of water vapor present in the air at that level.
The saturation mixing ratio, representing the maximum amount of water vapor the air can hold at a given level, is found where the temperature line intersects the mixing ratio line.
Wind Staff (Wind Barbs)
Wind staffs, or wind barbs, indicate wind speed and direction at different pressure levels. These are plotted based on the tracked position of the radiosonde as it drifts with the wind. Each barb represents a specific wind speed increment, and the orientation of the barb indicates the wind direction.
Analyzing wind barbs on a Skew-T diagram provides valuable information about wind shear and atmospheric stability.
The Skew-T Log-P diagram is a fundamental tool for meteorologists. It allows for a comprehensive understanding of the vertical temperature and moisture profile of the atmosphere, and provides information on the potential for convection, cloud development, and severe weather. By carefully analyzing the various lines and plotted data, meteorologists can make informed forecasts and assessments of current atmospheric conditions. Mastery of the Skew-T Log-P diagram is essential for anyone involved in weather forecasting and atmospheric research.
