Chapter 2 Section 3 Winds

What are winds? A wind is the horizontal movement of air from an area of high pressure to an area of lower pressure Winds are caused by differences in air pressure

Convection Currents Form when an area of Earth’s surface is heated by the Sun’s rays Air over the heated surface expands and becomes less dense. As the air becomes less dense, its air pressure decreases

Convection Currents Cool dense air with a higher pressure flows underneath the warm, less dense air= warm air rising

How do we measure wind speed? Winds are described by their direction and speed Anemometers are used to measure wind speed

Wind Direction The name of a wind tells you where the wind is coming from For example: A west wind comes from the west and blows toward the east Wind direction is determined with a wind vane The wind swings the wind vane so that one end points into the wind

Wind Direction Represented on a Map Wind direction is shown by a line leading into a circle The example below shows that the wind blew in from the east

Wind Speed Represented on a Map The wind speed is shown by the “feathers” on the wind direction line One “feather” =10 knots Half of a “feather”=5 knots A circle within a circle= calm, no wind A flag=50 knots (1 knot=1.15mph) (feathers are also sometimes called “barbs”)

Weather Symbol Examples Example #1: What is the wind speed and direction? Wind coming from the east at a speed of 85 knots

Pressure and Wind Isobars are lines joining places on a weather map that have the same air pressure Meteorologists use isobars on weather maps to observe atmospheric pressure changes over an area and to make predictions about wind speed Remember-Wind is a result of air pressure differences and wind blows from high to low pressure The closer the isobars are= the faster the wind speed The further apart the isobars are= the slower the wind speed

Wind Chill Factor Wind blowing over your skin removes body heat The stronger the wind, the more friction is created from the air molecules hitting your body This creates heat energy which is absorbed by the water near the surface of your skin, causing it to evaporate This evaporation removes heat from your body and makes you feel colder

Local Winds Local winds are winds that blow over short distances Local winds are caused by the unequal heating of Earth’s surface within a small area Unequal heating often occurs along the shore of a large body of water It takes more energy to warm up a body of water than it does to warm up an equal area of land (water has a high specific heat) During the day the land warms up faster than the water (unequal heating) At night, the land cools faster than the water (unequal heating)

Local Winds Land Breeze –Local wind that blows from the land to a body of water during the night Sea Breeze -Local wind that blows from an ocean or lake to the land during the day

Global Winds Global winds are winds that blow steadily from specific directions over long distances Global winds are created by the unequal heating of Earth’s surface Warm air rises at the Equator and cold air sinks at the poles

Jet Streams About 10 km above Earth’s surface are bands of high-speed winds called jet streams Meteorologists use the location of some of the jet streams as an aid in weather forecasting Jet streams generally blow west to east Jet streams form at the boundaries of adjacent air masses with significant differences in temperature (polar region vs. Equator) Jet streams have a meandering shape

Jet Stream Example

Chapter 3 Section 1 Weather

Air Masses A huge body of air that has similar temperature (warm or cold), humidity (dry or wet), and air pressure (high or low) at any given height is called an air mass Four major types of air masses influence the weather in North America: Maritime tropical Continental tropical Maritime polar Continental polar

Types of Air Masses Continental: land (dry) Mar: sea (wet) Tropical: warm/hot Polar: cold The area that the air mass forms over will determine its characteristics For example: If the source region is an ocean, the air mass will have a lot of moisture If the source region is land, the air mass will be drier

Putting it together Maritime Polar… Continental Polar… Moist and cold Continental Polar… Dry and Cold Maritime Tropical… Moist and warm Continental Tropical… Dry and warm

Air Masses and Their Origin Maritime Tropical Gulf of Mexico and Atlantic and Pacific Ocean Continental Tropical Southwest and Northern Mexico Maritime Polar North Pacific and North Atlantic Continental Polar Central and Northern Canada and Alaska

North American Air Masses 1 6 2 4 5 3

How Air Masses Move Prevailing Westerlies- push air masses west to east Jet stream

Fronts The boundary where 2 unlike air masses meet is called a front 4 types of fronts Cold Warm Stationary Occluded Storms and changeable weather often develop along fronts

Symbols for Fronts

Front Symbols and Direction Front symbols point in the direction of movement Example: In the picture below, the warm, cold, and occluded fronts are moving north and the stationary front is not moving in any direction

Front Symbols and Direction Draw a cold front moving east: Draw a warm front moving south: Draw an occluded front moving northeast:

Cold Fronts A cold front is when a fast-moving cold air mass takes over a warm air mass Possible weather: Clouds with possible thunderstorms with heavy rains or snow; cooler weather on the way

Warm Fronts A warm front is when a warm air mass overtakes a slow-moving cold air mass Possible weather: Humid, light rain or snow for several days; warmer weather on the way

Stationary Fronts A stationary front is when cold and warm air masses meet, but neither can move the other Possible weather: The air masses remain stalled over an area and may bring many days of clouds and precipitation

Occluded Front An occluded front is when a warm air mass is caught between two cooler air masses Possible weather: Cloudy and rain or snow

Front Symbols on a Weather Map

Low Pressure and Weather If you look at a weather map, you will see areas marked with a letter L and the L stands for “low pressure” Low pressure systems lead to cloudy conditions that often bring precipitation Here’s how it works: Areas of low pressure are caused by massive amounts of air rising from the ground into the atmosphere As the air molecules rise, they take their mass with them Air pressure is a force Force = mass x acceleration Less mass of air molecules left at ground level means low air pressure As this air rises it takes whatever water vapor it is holding along with it As altitude increases in the troposphere, the temperature decreases Eventually the air mass reaches a temperature at or below the dew point and the water vapor condenses Relative humidity rises because colder air cannot hold as much water vapor This condensation forms clouds If there is enough water vapor in the air, it might also bring precipitation

High Pressure and Weather If you look at a weather map, you will see areas marked with a letter H and the H stands for “high pressure” High pressure systems lead to clear and calm weather Here’s how it works: Areas of high pressure are caused by massive amounts of air sinking from the upper troposphere down towards the ground As the air molecules sink, they take their mass with them Air pressure is a force Force = mass x acceleration More mass of air molecules coming to ground level means high air pressure As this air sinks it takes whatever water vapor it is holding along with it As altitude decreases in the troposphere, the temperature increases The higher the temperature is, the farther from the dew point you get Warmer air is less dense so it has more space to hold water vapor so relative humidity drops This means that condensation does not occur so no clouds form Because there are no clouds, there will be no precipitation

Weather Map Example

Weather Map Example