Fast Facts for the Temperature Team
Surface and Upper Air

Temperature is important when forecasting precipitation type. Most precipitation begins as snow in clouds where temperatures are below freezing.
The difference between rainfall and snowfall depends on the temperatures of the layers of air the snow falls through on its way to the surface. More often than not, a raindrop is a melted snowflake.
East of the Rocky Mountains when the temperature is below 0 degrees Celsius at 850 mb, there is a good chance that any precipitation will be be snow.
Generally, weather systems move across the United States from the west to the east.

Fronts

Fronts are narrow zones between two air masses. The air masses have different temperatures, humidity, or both. Clouds and precipitation often form along a front.
Cold air is more dense than warm air. A cold front moves like a bulldozer. It pushes up a warm air mass in its path. The cold air advances, and the warm air retreats.
Warm air is less dense than cold air. That means a warm front slides over top of a cold air mass. The warm air advances, and the cold air retreats.
Warm air is less dense than cold air. That means cold air pushes warm air up. Next the warm air rises until it eventually becomes cold. That makes it dense enough to move downward.