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Potential temperature is a theoretical value used in meteorology and oceanography to standardize air and water masses at a standard pressure. It assumes that all air masses are at the same pressure and the character of the air mass does not change as it moves. The calculation uses a Poisson equation and assumes adiabatic pressure change. The standard gradient in stable air is about 3.5 degrees F per 1000 feet of altitude. The calculation is unusable for saturated air.
Potential temperature is a theoretical value used in meteorology or weather forecasting and oceanography or the study of the oceans. This value, called theta in meteorology, is the temperature an air mass would have if it were brought to a standard pressure. The importance of using a standard temperature is that the air cools at higher altitudes and the oceans at greater depths, which makes direct comparisons between different air or water masses difficult.
An equation used to define the potential temperature in air is known as a Poisson equation. The standard pressure of 29.97 inches of mercury (1000 millibars) is used in a calculation to convert to actual temperature. This equation is named after Simeon Denis Poisson, a French mathematician and physicist who developed it. The calculation assumes that no heat or mass is added or removed during the pressure conversion, an assumption called adiabatic pressure change.
Meteorologists observe air masses as they move around the earth and attempt to determine what effects will occur over time. Air cools as it rises and warms as it falls, so comparing actual temperatures at different points can cause errors in weather forecasts. Potential temperature assumes that all air masses are at the same pressure and the character or composition of the air mass does not change as it moves.
This effect is also important for observing a single air mass. As air masses circulate, they may encounter mountains or change terrain. If an air mass rises and cools, the actual air temperature will be lower. Potential temperature ignores this fact and looks at the air mass at standard pressure to determine if the characteristics of the air mass are changing.
Timelapse is the term for the temperature change that occurs as altitude increases. The standard gradient in stable air can be estimated at about 3.5 degrees F (about 2 degrees C) per 1000 feet (300 meters) of altitude. Unstable air, such as low-pressure areas with storms or cold and warm fronts, creates atmospheric conditions where the lapse rate cannot be used for temperature estimates. Potential temperature can be used to standardize these air masses to a single pressure, allowing comparisons to be made.
An important consideration when using this calculation is the dew point of the air mass. The air parcel considered must be unsaturated or non-dew point air. This is important because the calculation assumes that no mass or energy enters or leaves the air sample. Saturated air can create rain, which is a mass loss that will render this calculation unusable.
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