If the air comes back to where it started, the atmosphere is stable. If the air continues to rise the atmosphere is unstable. In the figure above the air in the parcel has ended up colder and denser than the surrounding air.

Warm air rises primarily due its lower density as compared to cooler air. As the temperature increases, the density of the air decreases. As the denser air reaches the earth’s surface it spreads and undercuts the less dense air which, in turn, forces the less dense air into motion causing it to rise.

Air is considered unstable, in the lowest layers of an air mass when the air is warmer and or more humid than the surrounding air. When this occurs the air will rise, as that air parcel is warmer than the air surrounding it. In an unstable environment, the weather can change suddenly and can be violent.

What are characteristics of an unstable air mass?

Air that has a tendency to sink is known as a stable air. If the air parcel’s temperature is greater than the temperature of the surrounding air mass, the air parcel is less dense and tends to rise. Rising air, as we have already learned, is known as unstable.

When the atmosphere is unstable, the decrease in temperature with height exceeds 5.51F per 1,000 feet. Once a parcel of air starts to rise, it will continue to rise until it cools to the temperature of the surrounding air.

Low Visibility Because stable air masses are free of disturbances, they often allow particles such as dust, smoke or other haze to linger in the air without being shifted or blown around. The result is poor visibility for people on the ground and, especially, people flying in aircraft.

Weather is strongly affected by how stable or unstable the atmosphere is. Stable air means that the weather is likely to be calm. It may rain or snow slowly and steadily, it may be sunny, but the weather will not change quickly.

Fog

If the environmental lapse rate is 4°F per 1000 ft, it is less than the dry adiabatic rate of 5.5°F per 1000 ft, and the atmosphere is considered stable.

This process, referred to as free convection (or natural convection), occurs when the environmental lapse rate (the rate of change of an atmospheric variable, such as temperature or density, with increasing altitude) of temperature decreases at a rate greater than 1 °C per 100 metres (approximately 1 °F per 150 feet).

The adiabatic lapse rate is the rate at which the temperature of an air parcel changes in response to the compression or expansion associated with elevation change, under the assumption that the process is adiabatic, i.e., no heat exchange occurs between the given air parcel and its surroundings.

When rising air parcels are saturated, they cool at a slower rate because of the release of latent heat. Unlike the DALR, the saturated adiabatic lapse rate (SALR) is not constant, but we often use 6°C/km as an average value.

The dry adiabatic lapse rate is a near constant of 9.8 C/km, however, the wet adiabatic lapse rate is much less of a constant. The wet adiabatic lapse rate varies from about 4 C/km to nearly 9.8 C/km.

The Saturated Adiabatic Lapse Rate (SALR) is therefore the rate at which saturated air cools with height and is, at low levels and latitudes, 1.5°C34.7 °F 274.65 K 494.37 °R per thousand feet.

The temperature decreases with the dry adiabatic lapse rate, until it hits the dew point, where water vapor in the air begins to condense. Above that altitude, the adiabatic lapse rate decreases to the moist adiabatic lapse rate as the air continues to rise.

The rate of decrease of temperature with height of a parcel undergoing a pseudoadiabatic process.