Air pressure is the least noticed change that occurs in the weather.
However, changes in air pressure can cause, changes in the wind, which affects the amount of humidity in the air, the temperature, and in weather forecasting.
Air Pressure Defined
Air pressure is simply the pressure exerted by the weight of the air above.
Average air pressure at sea level is 1 kilogram per square centimeter.
Air pressure is not exerted straight down.
Air pressure is exerted in all directions. So the air pressure pushing up on the object balances air pressure pushing down on an object.
Measuring Air Pressure
A barometer is the instrument used to measure air pressure.
Normal air pressure is 1013.2 millibars or 29.92 inches of mercury.
Torricelli invented the barometer in 1643.
Torricelli found that when air pressure increases, the mercury in the tube rises.
When air pressure decreases, so does the height of the mercury column.
Today, a smaller more portable unit called an aneroid barometer is used and can easily be connected to a recording device.
Factors Affecting Wind
What causes wind?
Wind is the result of horizontal differences in air pressure.
Air flows from airs of high pressure to areas of low pressure.
Example: Opening and soda can or tennis ball container.
Wind is nature’s way of balancing inequalities in air pressure.
The unequal heating of Earth’s surface generates pressure differences.
Solar radiation is the ultimate energy source for most wind.
Angle of the Sun’s Rays
Energy from the sun strikes Earth most directly near the equator. Near the poles, the same amount of energy is spread out over a larger area.
What would happen if the Earth did not rotate?
There would be no friction between the moving air and the Earth’s surface.
Air would flow in a straight line between areas of high pressure to areas of low pressure.
There are three factors that combine to control wind:
1 – pressure differences
2 – Coriolis effect – affect direction only
3 – friction – affects wind speed and direction
Wind is created by differences in pressure……..
The greater the differences are, the greater the wind speed is.
Isobars are lines on a map that connect places of equal pressure.
Isobars that are closer together indicate a greater pressure gradient than those lines that are farther away
A steep pressure gradient causes greater acceleration of a parcel of air and higher winds
A less steep pressure gradient causes a slower acceleration and light winds.
The pressure gradient is the driving force of winds.
Pressure gradients have both magnitude and direction.
The spacing of the isobars represents magnitude .
The direction of force is always from areas of high pressure to areas of low pressure and at right angles to the isobars.
Wind does not always cross the isobars at right angles.
This movement is due to the Earth’s rotation and is named the Coriolis effect .
The Coriolis effect describes how Earth’s rotation affects moving objects.
All free moving objects or fluids, including the air, are deflected to the right (or clockwise) of their path of motion in the Northern Hemisphere.
In the Southern Hemisphere, winds are deflected to the left or counter-clockwise.
As Earth rotates, the Coriolis effect turns winds in the Northern Hemisphere toward the right.
The apparent shift in wind direction is attributed to the Coriolis effect four ways:
1 – The deflection is always directed at right angles to the direction of airflow.
2 – The deflection only affects wind direction not wind speed.
3 – The deflection is affected by wind speed - the stronger the wind, the greater the deflection .
4 – The deflection is strongest at the poles and weakens near the equator, becoming nonexistent at the equator.
Friction affects the wind only within a few kilometers of the Earth’s surface.
Friction acts to slow air movement , which changes wind direction.
Above the friction layer, the pressure gradient causes air to move across the isobars.
The pressure gradient and the Coriolis effect balance in high altitude air, and wind generally flows parallel to the isobars.
This produces the jet streams .
Jet streams are high altitude, fast moving rivers of air between 120 to 240 kilometers per hour in a west to east direction.
Close to the ground the shape of the terrain determines the angle of flow across isobars.
Over the ocean, friction is low and the angle of flow is low.
Over rough terrain, where friction is higher, winds move slower and move across isobars at greater angles.
Slower winds caused by friction decreases the Coriolis effect .
Pressure Centers and Winds
Pressure differences are basic to making observations about weather.
For example, low pressure is usually associated with cloudy conditions and precipitation.
High pressure generally means clear skies and good weather.
Highs and Lows
Lows - are cyclones (centers of low pressure)
Highs - are anticyclones (centers of high pressure)
Lows – pressure decreases from the outer isobars to the inner isobars.
Anticyclones – pressure values of the isobars increase from the outside to the center
Cyclonic and Anticyclonic Winds
When the pressure gradient and the Coriolis effect are applied to pressure centers in the Northern Hemisphere, winds blow counter-clockwise around a low .
Around a high , they blow clockwise .
In either hemisphere, friction causes the net flow of air inward around a cyclone and a net flow of air outward around an anticyclone .
Weather and Air Pressure
Rising air is associated with cloud formation and precipitation, whereas sinking air produces clear skies.
A surface low-pressure system where air is spiraling inward causes the area occupied by the air mass to shrink.
This process is called horizontal convergence .
When air converges horizontally it must increase in height to allow for the decreased area it occupies.
This produces a taller heavier column of air.
A surface low can exist only as long as the column of air above it exerts less pressure than does the air surrounding it.
In order for a surface low to exist for very long, converging air at the surface must be balanced by outflows aloft .
Surface convergence around a cyclone causes a net upward movement.
Rising air is often associated with cloud formation and precipitation.
Lows are associated with unstable air and stormy weather .
So what happens around an anticyclone or high-pressure area?
Low pressure areas can produce bad weather during any season of the year.
Because surface conditions are linked to the conditions of the air above, it is important to understand total atmospheric circulation.
The underlying cause of winds is the unequal heating of Earth’s surface.
The tropics receive more solar radiation then it radiates back into space.
The poles radiate more energy back into space than it receives.
The atmosphere balances these differences by acting as a giant heat-transfer system.
This system moves warm air toward high latitudes and cool air toward the poles.
Non-Rotating Earth Model
If the Earth did not rotate…………….
Air at the equator would rise until it reached the tropopause, which would deflect this air toward the poles where it would spread in all directions until it reached the equator where it would begin to rise again.
Rotating Earth Model
Since the Earth rotates, the two-cell circulation system is broken into smaller cells.
Three pairs of cells would carry on the task of redistributing heat on Earth.
Near the equator , rising air produces a pressure zone known as the equatorial low – which is an area characterized by an abundant amount of precipitation.
As this air reaches 2—30 degrees north or south of the equator, it sinks back toward the surface.
This sinking air and its associated heating due to compression produce hot, arid conditions.
The center of this zone is the subtropical high . This high encircles the globe at about 30 latitude.
The deserts of Arabia, Australia, and the Sahara in North Africa exist because of the stable conditions associated with subtropical highs.
At the surface, airflow moves outward from the center of the subtropical high.
Some of the air travels toward the equator and is deflected by the Coriolis effect producing the Tradewinds .
Tradewinds are two belts of winds that blow almost constantly from easterly directions.
The Tradewinds are located between the subtropical highs and the equator.
The remainder of the air travels toward the poles and is deflected, generating the prevailing westerlies in the middle latitudes.
The westerlies make up the dominant west-to-east motion of the atmosphere the atmosphere on the pole side of the subtropical high.
As the westerlies move toward the poles, they encounter the cool polar easterlies in the region of the subpolar low.
The polar easterlies are winds that blow from the polar high toward the subpolar low.
These winds are not constant. In the polar region, cold polar air sinks and spreads toward the equator.
The interaction of these warm and cold air masses produces the stormy belt known as the polar front .
Global Wind Belts
A series of wind belts circles Earth. Between the wind belts are calm areas where air is rising or falling.
Four pressure zones dominate this global circulation.
Subtropical and polar highs – dry sinking air that flows outward at the surface, producing prevailing winds.
The low-pressure zones at the equatorial and subpolar regions are associated with inward and upward airflow accompanied by clouds and precipitation.
Influence of Continents
The only truly continuous pressure belt in the subpolar low in the Southern Hemisphere.
Here the air is uninterrupted by landmasses.
Large landmasses create seasonal temperature differences that disrupt the pressure patterns.
Asia becomes cold in the winter when a seasonal high develops.
This surface high deflects winds off shore.
In summer, landmasses are heated and develop low-pressure cells, which permit air to flow onto the land.
These seasonal changes in wind direction are known as monsoons .
During the summer , the air over the continent becomes much warmer than the water surface, so the surface air moves from the water to the land. The humid air from the water converges with dry air from over the continent and produces precipitation over the region, over 400 inches at some locations ! During the winter the flow reverses and the dominant surface flow moves from the land to the water.
Regional Wind Systems
Between 30-60 degrees latitude, migrating cyclones and anticyclones interrupt the general west-to-east flow, known as the westerlies.
In the Northern Hemisphere, these pressure cells move from west to east around the globe.
Small-scale winds produced by a locally generated pressure gradient are known as local winds .
Local winds are usually caused by either topographic effects or by variations in surface composition – land and water – in the immediate area.
Land and Sea Breezes
Summer – land surfaces are heated more intensely than the adjacent body of water during the daylight hours.
As a result, air above land heats, expands, and rises, creating an area of low pressure.
A sea breeze develops because cooler sea air has higher pressure and moves toward the low-pressure air on land.
The breeze starts before noon and increases in intensity till mid to late afternoon. These breezes tend to moderate the temperatures in coastal areas.
At night, the reverse occurs.
Example: Chicago experiences lake effect temperature moderations especially near the lake.
Valley and Mountain Breezes
During daylight hours – the air along the slopes of a mountains are heated more intensely than air at the same elevation over the valley floor.
Because the warmer air along the slopes is less dense, it glides up along the slope generating a valley breeze .
Cumulus clouds forming over the adjacent mountain peaks can identify upslope breezes.
During nighttime the reverse effect occurs generating mountain breezes .
Example: Grand Canyon at night.
Cool air drainage can occur even on modest slopes.
The coolest air is usually found in the deepest spots.
As mountain and valley breezes are usually more modest in the winter.
How Wind is Measured
Two basic wind measurements
1 – direction
2 – speed
Winds are labeled (named) by the direction from which they blow.
The instrument used to determine this is a wind vane .
Toward which direction does a SE wind blow?
To the NW
Prevailing wind – when the wind blows consistently more often from one direction more than any other.
Example: In the U.S., the westerlies consistently move weather from west to east across our continent.
Along with this westward flow, cells of high and low pressure along with their wind characteristics are moved along.
As a result wind direction can change often.
An anemometer is used to measure wind speed.
El Nino and La Nina
The cold Peruvian current flows toward the equator along the coast of Ecuador and Peru.
This flow encourages upwelling of cold nutrient-filled waters that are primary food sources for million of fish and anchovies.
During the end of the year, a warm current that flows southward along the Ecuador and Peru coast replaces the cold Peruvian current.
At irregular intervals of three to seven years, these warm counter-currents become unusually strong and replace normally cold offshore waters with warm equatorial waters.
These unusually strong warm undercurrents block the upwelling of colder, nutrient filled water.
As a result, anchovies starve wrecking the local fishing industry. At the same time, usually arid inland areas receive more rainfall than usual which substantially increases the yields of cotton and pastures.
El Nino is actually part of the global circulation affecting the weather at great distances from Peru and Ecuador.
Example: In 1997 – jet streams steering weather patterns in North America brought three times the normal precipitation to the Gulf Coast in Florida.
The mid-latitude jet stream pumped warm air far north, bring higher than normal temperatures west of the Rocky Mountains.
is the opposite of El Nino.
When surface temperatures in the eastern Pacific are colder than average, a La Nina event is triggered that has a distinctive set of weather patterns.
A typical La Nina winter blows colder than normal air over the Pacific Northwest and the northern Great Plains including increased amounts of precipitation.
At the same time, it warms much of the rest of the U.S.
La Nina activity can increase hurricane activity.
The cost of hurricane damage is 20 times greater in La Nina years as compared to El Nino years.