Weather for Kids

Subject: Inversions

Normally, temperatures are warmest near the ground and much colder at higher elevations. However, on occasion this scenario becomes reversed when cold air settles into the lower valleys and warmer air remains trapped above it. Meteorologists often refer to this phenomenon as an inversion.

Inversions are most common when a high pressure system moves over Montana, bringing mainly clear skies and light winds. In the evenings, the air cools rapidly and sinks toward the valley floor while warmer air remains trapped above against the mountain slopes. You can think of the valleys and mountains acting like a bowl with a lid on top.

Inversions are known to affect our weather in different ways. Moisture and tiny particles in the air become trapped in the valleys under the inversion. This can create low clouds and fog in the short term, however pollution and poor air quality can be a problem if the inversion continues for several days. Cold nighttime and daytime temperatures in the valleys are common occurrences with inversions, too. Sometimes the temperature can be 10 to 20 degrees warmer on a mountain top compared to a valley location. You could be skiing or sledding in the mountains enjoying the sunshine and 40 degree temperatures, while back home in the valleys, clouds and fog with temperatures only in the 20s would be common. Inversions are usually responsible for a variety of precipitation types that can occur when a storm system begins to move into the area. Since temperatures are warmer aloft during an inversion, precipitation that falls from the clouds as snow may melt into sleet or rain before reaching the ground. If the temperatures at the ground are below freezing, rain drops can instantly freeze into a sheet of ice.

What does it take to get rid of an inversion? Sometimes it's not easy, because cold air that is settled in a valley needs a force to push it out. A change in the weather pattern where low pressure takes the place of a high pressure system usually does the trick. Low pressure will bring cooler air into the mountains and warmer air to the valley floors, having a similar effect as if you mixed the air with a blender.