Potential temperature is a theoretical value used in meteorology or weather forecasting, and in oceanography or study of 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 air cools at higher altitudes, and oceans at greater depths, which makes direct comparison of different air or water masses difficult.
An equation used to define potential temperature in air is known as Poisson's equation. Standard pressure of 29.97 inches of mercury (1000 millibars) is used in a calculation to convert the actual temperature. This equation is named for Simeon Denis Poisson, a French mathematician and physicist who developed it. The calculation assumes no heat or mass is added or removed during the pressure conversion, an assumption called adiabatic pressure change.
Meteorologists look at air masses as they move around the earth, and attempt to determine what effects will occur over time. Air cools as it rises and heats as it falls, so comparing actual temperatures at different points can result in errors in the weather forecasts. Potential temperature assumes all of the 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 looking at a single air mass. As air masses circulate, they may encounter mountains or changing terrain. If an air mass rises and cools, the real temperature of the air will be lower. Potential temperature ignores this fact, and looks at the air mass at the standard pressure to determine if the characteristics of the air mass are changing.
Lapse rate is the term for the change in temperature that occurs as altitude increases. The standard lapse rate 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, create atmospheric conditions where the lapse rate cannot be used for temperature estimates. Potential temperature can be used to standardize these air masses at a single pressure, allowing comparisons to be made.
One important consideration when using this calculation is the dew point of the air mass. The parcel of air being considered must be unsaturated air, or air that is not at its dew point. This is important because the calculation assumes no mass or energy enters or leaves the air sample. Air that is saturated can create rain, which is a loss of mass that will make this calculation unusable.