Download Wind, Water Vapor, and Humidity

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 highpressure zones above and below the
equator.
Global Wind Zones
W
E
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.
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.