Lapse Rates

Lapse Rate is a term that refers to the rate at which temperature varies in the vertical. There are various types of Lapse Rates.

Environmental Lapse Rate

The environmental lapse rate is the rate at which temperature changes in the vertical in the troposphere, as observed by an upwards moving radiosonde. This varies greatly from day to day. When this lapse rate is averaged out for all places and times, it is called the Standard (or Average) Lapse Rate, which is around 3.0F/1000 ft. It's important to keep in mind that this lapse rate is determined by a vertically moving radiosonde. The air itself is not moving up or down.

Dry Adiabatic Lapse Rate

The lapse rate that occurs in a vertically moving air parcel in which no condensation is occurringl. The temperature change is related to the expansional cooling (compressional warming) that occurs when the air moves upward (downward). It is entirely deterined by the pressure distribution in the atmosphere in question. For earth's atmosphere, in the troposphere, for example, the pressure is 200 mb at the top and 1000 mb at the bottom. Thus, the dry adiabatic lapse rate is constant, 5.5F/1000 ft (1C/100m). This is known as the dry adiabatic lapse rate because no heat is added or subtracted from the moving air parcel (adiabatic) and no moisture is condensing (dry).

Wet Adiabatic Lapse Rate

The lapse rate that occurs in a vertically moving air parcel in which condensation is occurring. For example, although an upwards moving air parcel will always experience expansional cooling as a dominant effect, a certain amount of heating offsets that cooling due to latent heat release associated with condensation. This latent heat release is dependent upon temperature and pressure, so the wet adiabatic rate is not a constant. It averages around 3.5F/1000 ft in the lower troposphere.