Question of Stability?

It's a Question of Stability?

Trouble in Weather World

Listen to what weather-wise Pennsylvanians had to say:

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The Facts Behind Stability?

The stability of the atmosphere depends upon how temperature changes as you move up from the ground. First, however, we need to learn some basics. Rising air always cools, and, conversely, sinking air always warms. This type of temperature change, which is caused simply by ascent or descent in the atmosphere, is called adiabatic cooling or warming. When air moves vertically, its pressure changes. Since the atmospheric pressure always decreases with height, sinking air will compress as its pressure increases. Like the compressed air at the bottom of a bicycle pump, sinking air in the atmosphere warms. The converse is true for the cooling of rising air in the atmosphere. The exact rate of temperature change for vertically moving air depends on whether or not a cloud is forming. In the absence of clouds, rising air cools at the rate of 5.5 degrees Fahrenheit for every thousand feet of ascent. Under similar conditions, sinking air will warm at 5.5 degrees for every thousand feet of descent. In the presence of clouds, the rate of adiabatic temperature change is closer to 3 or 4 degrees for every thousand feet of vertical displacement.

Since meteorologists usually talk about an unstable atmosphere with respect to thunderstorms and other inclement weather, we should assume that there are clouds forming in the atmosphere. If the temperature falls slowly or rises with height (in other words, if the temperature falls more slowly than 3 or 4 degrees every thousand feet), the atmosphere is said to be stable. Air moving upward will find itself colder than its environment and will sink back toward the ground. Vertical currents are suppressed in a stable atmosphere.

If the temperature falls rapidly with height (more rapidly than 3 or 4 degrees every thousand feet), the atmosphere is unstable. Air moving upward will find itself warmer than the air around it. The upward-moving air will be buoyant and will therefore continue to rise. Vertical currents are encouraged and will accelerate in an unstable atmosphere.

Atmospheric stability greatly affects the weather. Precipitation falling from a stable atmosphere typically affects a large region and falls with nearly steady intensity ( an all-day rain or snow). However, localized showers and thunderstorms with rapidly varying rainfall rates indicate that the atmosphere is unstable.

Examples of clouds forming in stable atmosphere.

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Stable clouds

Unstable clouds

How do meteorologists determine the stability of the atmosphere?

Hundreds of weather balloons all over the world are launched every twelve hours. They measure temperatures and other meteorological quantities at various heights above the ground. The temperature at a given location is then organized in graphical form on a Skew-T diagram. (T stands for temperature, skew just means that temperature lines on the graph are skewed from being oriented vertically). Meteorologists can then look at temperature profiles (temperatures throughout the lowest 10 miles of the atmosphere) provided by the Skew-T to diagnose atmospheric stability.

Stable Skew-T

Unstable Skew-T