Wind, Water Vapor, and Humidity (Pages D 57 – D 60 & D 38 – D 41) Weather

Coriolis Effect Earth’s rotation affects winds blowing across its surface. Imagine you are in an airplane. While you are in the air, Earth is rotating, or spinning, underneath you. Earth rotates counterclockwise. No matter which way the wind blows, it will curve to the right in the Northern Hemisphere. This curving is known as the Coriolis Effect. In the Southern Hemisphere, the Coriolis Effect causes winds to curve left.

Global Wind Patterns At the equator it is very hot. The heat causes evaporation, so the air becomes moist. The warm moist air produces a low pressure near the equator. As the warm air moves away from the equator it cools off, moving to the north and south. The cool air cools and becomes denser. The air sinks toward the surface causing high- pressure zones above and below the equator.

The top and bottom of the Earth have polar easterly winds. 30º & 60º latitudes have westerly winds. The continental United States is in the zone of westerly winds. Global Wind Zones WE

Isobars Knowing where the air pressure is high or low allows you to predict which way air will move. A line on a map connecting places with equal air pressure is called an isobar. Isobars make pressure patterns easier to see.

Big differences in air pressure over short distances cause strong winds. This is shown on a map by drawing closely spaced isobars. Widely spaced isobars represent gentle winds. High-pressure system is an air mass of higher than normal pressure where the winds blow outward from the center in a clockwise pattern. Low-pressure system is an air mass of lower than normal pressure; often brings precipitation. High and low-pressure systems are represented with circular patterns on an isobar map. Air pressure is measured in millibars (mb).

Weather Station Model A weather station model shows the current weather conditions at a particular place at a particular time. This includes information on temperature, precipitation, cloud cover, air pressure, wind speed, and wind direction.

Interpreting Symbols The circle inside the weather station model symbol is used to determine cloud cover.

Water Vapor Water that forms around a cool glass of liquid comes from the air around the glass. When warm air touches the cold glass, the air cools. Droplets of water form, run down the side of the glass, and make a puddle on the table. The water in the air is called water vapor. Water vapor is water in the form of a gas. It is invisible, colorless, odorless, and tasteless. The amount of water vapor in the air is called humidity. More than 2/3 of the planet is covered with liquid.

The changing of a liquid to a gas is called evaporation. Condensation is the changing of a gas into a liquid. You see condensation on shower doors, on cold drink glasses, and dew on grass in the early morning. Plants transport the liquid in the ground their roots and stems to their leaves. The leaves then give off water in the process called transpiration.

Humidity Two factors determine the amount of humidity in the air: 1. water available to evaporate 2. warm temperature Warm air will take on more water vapor than cold air. Relative humidity is a comparison between how much water vapor is in the air and how much the air could hold—at a given temperature. Relative humidity can be used to predict how the air will feel to a person.

Forming Clouds As air rises and cools, the water vapor in it condenses into tiny water droplets, forming clouds. If the temperature is below the freezing point of water, its water vapor will form a cloud of tiny ice crystals.