Air pressure and wind Chapter 19 Mrs. Schwartz

What is Air Pressure? Air Pressure=the pressure exerted by the weight of air above. its very important factor in predicting the weather! Air pressure at sea level is 14.7 lbs per square inch. Air pressure is exerted in all directions— down, up, and sideways The air pressure pushing down on an object exactly balances the air pressure pushing up on the object

Measuring Air Pressure Air pressure is measured by using a barometer. 2 TYPES: 1. Aneroid—canister like device, pressure changes on the spring move the arrow

Measuring Air Pressure 2. Mercury—liquid device, pressure changes make the liquid go up(high) & go down(low) As pressure increases the liquid in the barometer rises. As pressure decreases the liquid lowers in the barometer.

What Causes Wind Wind is the result of horizontal differences in air pressure Air flows from areas of higher pressure to areas of lower pressure Wind is nature’s way of balancing out pressure inequalities The unequal heating of Earth’s surface generates pressure differences Solar radiation is the ultimate energy source for most wind If Earth did not rotate, and if there were no friction between moving air and Earth’s surface, air would flow in a straight line from high to low pressure areas Three factors combine to control wind: pressure differences, the Coriolis effect, and friction

https://www.youtube.com/watch?v=D52rTzibFRc https://www.youtube.com/watch?v=RzSqhrn2dDM

3 factors affecting wind Pressure differences Coriolis effect Friction

Pressure differences Pressure Differences—greater the difference the greater the wind. Air moves from high to low pressures Isobars=lines of equal pressure Closely packed = windy Widely spaced = not as windy Pressure Gradient=steep means windy Pressure Gradient=weak/not steep means Not windy

2. Coriolis Effect—Earth’s rotation affects moving objects --Free moving objects move to the right in the northern hemisphere & to the left in the southern.

Friction—only important near Earth. Friction slows wind & can change wind direction. JET STREAMS=high up & fast moving!

Jet Streams – fast-moving rivers of air that travel between 120 to 240 kilometers per hour in a west-to-east direction The roughness of the terrain determines the angle of airflow across the isobars; the smoother the terrain, the smaller the angle of airflow Slower wind speeds caused by friction decrease the Coriolis effect

19.2

19.2 Pressure Centers & Wind Pressure Centers—help to predict weather Low (Cyclones) = air is unstable & rises, cools, condenses, clouds = stormy, cloudy, air moves counterclockwise in N. hemisphere, winds move inward Pressure decreases toward center

High (Anti-Cyclones) = air is stable & descends, warms, evaporates, clear/fair, air moves clockwise in N. hemisphere, winds move outward Pressure increases toward center

Global Winds on a Non-Rotating Earth The underlying cause of wind is the unequal heating of Earth’s surface The atmosphere balances these differences by acting as a giant heat-transfer system The system (atmosphere) moves warm air toward high latitudes and cool air toward the equator

Global Winds on a Rotating Earth Trade Winds – two belts of winds that blow almost constantly from easterly directions and are located on the north and south sides of subtropical highs

Global Winds on a Rotating Earth Westerlies – dominant west- to-east motion of the atmosphere that characterizes the regions on the poleward side of the subtropical highs

Global Winds on a Rotating Earth Polar Easterlies – winds that blow from the polar high toward the subpolar low

Global Winds on a Rotating Earth Polar Front – stormy frontal zone separating cold air masses of polar origin from warm air masses of tropical origin

https://earth. nullschool https://earth.nullschool.net/#current/wind/surface/level/orthographic=- 49.59,10.00,300

19.3

Local Winds Local Winds—small-scale winds are produced by a locally generated pressure gradient. Local Winds—are caused by 2 things: 1. by topographic effects (mtns. Etc.) 2. by variations in surface composition (land or water, etc.) ** Named for where they COME FROM!

4 types of Local winds 1. Sea Breeze—happens during the day, land heats up quicker & causes air to rise allowing cooler air to come from the sea. 2. Land Breeze—happens during the night, land cools of quicker & causes the warmer air over the sea to rise allowing cooler air to come from the land.

3. Valley Breeze—occurs during the day, air rises from valley floors. 4. Mountain Breeze—occurs at night, air sinks from the mountain tops toward the valley floor.

Measuring Wind Wind Direction—wind vane  wind direction is given by the direction the wind is coming FROM, (N,S,E,or W) or sometimes given in degrees! Wind Speed—anemometer. Wind Socks—see them at airports measure wind speed & direction!

El Nino At irregular intervals of three to seven years, the warm countercurrents, along the coasts of Peru and Ecuador, become unusually strong and replace normally cold offshore waters with warm equatorial waters – El Niño The warm waters block the nutrients from reaching the surface waters, causing many fish to die off, and greatly affects the fishing industries of Peru and Ecuador Some inland areas that are normally arid get an abnormal amount of rain, increasing their crop production These episodes mostly effect the eastern tropical Pacific, but is a part of the global circulation and affects the weather all over the world

La Nina The opposite of El Niño is an atmospheric phenomenon known as La Niña Researchers have come to recognize that when surface temperatures in the eastern Pacific are colder than average, a La Niña event is triggered that has a distinctive set of weather patterns A typical La Niña winter blows colder than normal air over the Pacific Northwest (with more precipitation) and the Northern Great Plains It warms much of the rest of the United States

Global Distribution of Precipitation Areas dominated by the convergent Trade winds (equatorial low) have mainly rain forests and abundant precipitation Areas dominated by the subtropical high-pressure cells are regions of extensive deserts The interiors of large land masses commonly experience decreased precipitation You will be able to explain much about global precipitation through your knowledge of global winds and pressure systems

Global Distribution of Preciptiation