|BACKYARD METEOROLOGY: INSTABILITY
METEOROLOGIST JEFF HABY
Instability when present is increased by several factors including warming surface temperature, increasing surface dewpoint and
cooling aloft. The characteristics of clouds can be used to assess instability and stable layers (called a cap) that prevents the
instability from being released. Many clues for instability and capping can be obtained by looking at the sky.
Instability release that develops into thunderstorms often starts from air rising from near the surface. Warm surface temperature and
humid surface air help increase instability and increase the chance that instability release (thunderstorms) will occur. In backyard
meteorology, it is a good idea to know what wind directions bring certain air masses. For example, a surface southeast wind may tend
to bring in more humid air into a specific location while a surface northwest wind may tend to bring in cooler and drier air into a
specific location. The temperature often warms during the day thus instability tends to be highest in the afternoon when the greatest
amount of cumulative warming has occurred. The feeling of very warm and humid air along with breezy winds can be a good indication
that the lower atmosphere is helping build instability.
The vertical development of clouds can be used to indicate where in the atmosphere instability release is occurring. In thunderstorm
situations it is common to have low level cumulus clouds that have some vertical development but are limited in their vertical
development. The limit is provided by a capping inversion that slows and then halts the upward convection. These are the fair weather
cumulus that are common in the warm season. They do indicate that if given more instability, the fair weather clouds could then develop
into thunderstorms. This can be provided for example by a lifting mechanism, increasing surface temperature or the breaking of the
It is difficult to assess the temperature profile above the surface of the atmosphere with only looking at the sky. This is important
to know since it is where a cap may be in place. Knowing the temperature profile will also help indicate the amount of instability
since cold air aloft will help enhance instability. Examining forecast soundings before observing the sky will help
with this information.
Once thunderstorms form then instability is released. In this situation the air rises throughout the troposphere and is only capped once
the storm rises to the upper reaches of the troposphere. The next writing will expand upon the process of instability release.