Every drop of rain and every flake of snow starts as water that rose into the air from oceans, lakes and rivers. Water vapor is water in the air as a gas, and you cannot see it. Warm air can hold more water vapor than cold air. Relative humidity compares how much vapor is in the air with the most the air could hold at that temperature. At 100 percent the air is full.

When air cools enough, it cannot hold all its vapor. The dew point is the temperature at which air is full of water vapor and the vapor starts to turn into drops. That is how dew forms. Dew is little drops of water on grass and leaves in the morning, and it forms from water in the air, not from rain.

Air has weight, and wind blows from places with high air pressure toward places with low air pressure. Low-pressure areas can be stormy, and high-pressure areas usually bring calm, clear weather.

A hurricane is a huge spinning storm that forms over warm ocean water. A tropical storm has winds of 39 mph or more. When its winds reach 74 mph it is a hurricane. The same kind of storm is called a hurricane in the Atlantic, and a typhoon or tropical cyclone in other parts of the world.

Along the coast, the sea itself can rise. Storm surge is the rise in sea level pushed toward the coast by a storm's winds. It causes much of the flooding along a coast.

A forecast begins with knowing what the weather is doing right now, and forecasters use that to work out where storms will be tomorrow. Satellites in space gather information about the air and clouds below, day and night. When a storm could be dangerous, a watch means be ready; a warning means it is happening or about to, so take action.

Words to know

• humidity: wetness of air
• vapor: gas of water
• surge: sudden rise
• hurricane: huge spinning storm
• typhoon: a hurricane elsewhere