Monday

Rocket bursts into the lab with his jacket half off. "Yesterday I needed a coat. Today I am sweating. The weather broke!"

Raven is already at the log. "Two days, two very different readings," she says. "What do you notice about the wind?"

Rocket checks. "Yesterday it blew from the north. Today it is from the south."

Nova brightens a map on the wall screen. A huge blob of air sits over the land to the north, another over the warm sea to the south.

"The weather did not break," she says. "A different body of air arrived. Where do you think it came from?"

Rocket stares at the southern blob. "From somewhere warm," he says. "And it brought its warmth along."

An air mass is a large body of air with generally uniform temperature and humidity.

The area where an air mass forms gives it its character. The longer it sits over that source region, the more it takes on the properties of the surface below.

Air that forms over an ocean picks up moisture. Air that forms over land stays dry.

Air that forms near the poles is cold. Air that forms in the tropics is warm or hot.

Air masses do not stay home. They drift across the land and carry their temperature and moisture with them.

When a warm moist air mass replaces a cold dry one, your area feels the change in a day or less.

Rocket's sweaty morning was not a mistake in the log. A new air mass had arrived.

Good start. Tomorrow you learn what happens at the line where two air masses meet.