Lapse Rates and Smoke

Lapse rate is the decrease of temperature with height. Lapse rates help determine whether smoke will rise from a fire or sink back to the surface and are used to estimate atmospheric stability. When air is heated it expands, becomes less dense and more buoyant. This causes it to rise. A parcel of air that is heated at the ground surface by fire or solar radiation becomes warmer than its surroundings, causing it to lift off the surface. As it rises, it encounters lower pressure that causes further expansion. The more air expands, the cooler it becomes. If a parcel of air becomes cooler than its surroundings, it will sink.

Cooling by expansion without an exchange of heat at the parcel boundaries is called adiabatic cooling. In dry air, rising air parcels typically cool at a rate of about 5.5 °F per 1,000 feet (~ 10 °C/km). This is called the dry adiabatic lapse rate (DALR). For example, on a clear day if a heated parcel of air begins at sea level with a temperature of 70 °F (~21 °C), it will cool dry-adiabatically as it rises, reaching a temperature of 53.5 °F (~12 °C) at 3,000 feet (~915 m).

Rising moist air (relative humidity greater than about 70%) is said to undergo a saturation-adiabatic process. The saturated adiabatic lapse rate (SALR) or moist adiabatic lapse rate is a function of temperature and water content. This is because as moist air cools its water vapor condenses, giving off latent heat in the condensation process and causing a saturated parcel to cool more slowly than a dry parcel. Near the ground in mid-latitudes the SALR can be approximated at a rate of about 3 °F per 1,000 feet (~ 5.5 °C/km). For example, on a humid or rainy day, a heated parcel with a 70 °F (~21 °C) initial temperature at sea level, will reach a temperature of 61 °F (~16 °C) at 3,000 feet (~915 m).

Lapse rates are determined by comparing temperatures between different elevations. The temperature from a ridge-top weather station can be subtracted from the temperature at a nearby valley-located weather station to calculate lapse rate. More commonly, radiosonde observations (raobs) are used to determine lapse rates. These balloon-mounted instruments measure temperature, wind, pressure, and humidity at several elevations from the ground surface to thousands of feet. Raobs are available from weather services or at several sites on the Internet twice each day: at 0000 Universal Time Coordinated (UTC) and 1200 UTC.

There are several ways of plotting raob data. Typically a pseudo-adiabatic chart is used. This chart shows measured values of temperature vs. pressure over lines of DALR and SALR. The first figure illustrates how the above examples would appear on a standard pseudo-adiabatic chart. More recently, skew-T/log-P diagrams (skew-T for short) have become popular. Instead of plotting temperature and pressure on linear, orthogonal axes, skew-T diagrams plot the log of pressure and skew the temperature axis by 45°. The skew-T/log-P view of raob data allows features of the atmosphere to be more obvious than when plotted on a standard pseudo-adiabatic chart. The second figure illustrates the above examples on a skew-T diagram. On both standard pseudo-adiabatic charts and skew-T diagrams, elevation in meters or feet (corresponding to the pressure of a standard atmosphere) may be shown and wind direction and speed with height is represented parallel to or along the right-hand vertical axis. Many other features also may be included.

See: Lapse rates in the Fire Weather section for more information.