Weather is the sum of all the conditions of the air that may affect the earth's surface and its living things. Temperature, pressure, wind, and moisture are several of these conditions. Living things are affected by each of them. But the conditions of the air are always changing. The temperature goes up and down. The air pressure changes almost constantly. Winds pick up speed or slow down. The moisture in the air changes from hour to hour and from day to day. If living things were not able to adjust to these changes, they would not be able to survive.