THE ATMOSPHERE

IN MOTION

Air Pressure and Wind

• Wind which is the horizontal movement of air, helps to moderate surface temperatures, distribute moisture, and generally cleanse the atmosphere.

• Though there are several forces that affect the direction in which air moves, differences in air pressures are what set the air in motion.

Air Pressure• Air pressure is the weight of the atmosphere as it pushes

down upon Earth exerting a force per unit of area.• Air pressure is exerted in all directions because air

molecules move in all directions.• Air pressure at any point on Earth’s surface is dependent on

the weight of the air above, note air pressure decreases as elevation increase

• Air pressure is most noticeable to people when they experience a change in the air pressure. – A rapid ascent, the air pressure outside of your body rapidly

decreases causing your eardrums to hurt or pop because of the greater internal pressure.

– A rapid descent causes an increase in external pressure causing greater pressure on the outside of your eardrum.

Air Pressure• Air pressure is measured with an instrument

called a barometer. There are two types– Mercury – Aneroid

• When recording air pressure there are several different units of measure.– Inches and millimeters are use to record the height

of the mercury column in a mercury barometers.– Millibars are used on U.S. weather maps. The air

pressure recorded on weather maps is adjusted to eliminate the effect of elevation.

Air Pressure

• Air pressure changes are not only caused by elevation but also temperature and humidity. – At sea level air pressure decrease as temperature

increases. • In warmer air the water molecules are further apart ,

thus exerting a lower force on the Earth’s surface. • In colder air the water molecules are closer together,

thus exerting a greater force on the Earth’s surface.

Air Pressure– As humidity increases (more water vapor in the

air) the air pressure decreases because water vapor has less mass than oxygen or nitrogen molecules. • Water vapor molecules replace the more massive

oxygen and nitrogen molecules thus causing the air to be lighter and exerting less force on the Earth’s surface.

Air Pressure

• Changes in air pressure allow for simple but not necessarily an accurate way of forecasting the weather.– A decrease in air pressure often signals the

approach of warmer, more humid air, along with rain or snow.

– An increase in air pressure often signals the approach of cooler, drier air and fair weather.

Air Pressure• Meteorologist analyze air pressures by plotting

isobars on weather maps.– Isobar is a line that joins points of equal air pressure.– When the air pressure steadily increases towards the

center of a set of closed isobars, the area defined by the isobars is called a high-pressure area or high.

– When the air pressure steadily decreases towards the center of a set of closed isobars, the area defined by the isobars is called a low-pressure area or low..

– Closely spaced isobar lines indicate an area where the air pressure changes quickly.

– Dividing the pressure change by the distance over which the air pressure changes is called the pressure gradient.

Air Pressure

Air Pressure and Wind

• The wind blows due to the air moving from a high pressure are to a low pressure area assuming there are not other forces acting on it.– The greater the difference in air pressure between

two points the stronger the wind the blows between them

– Closely spaced isobars indicate a strong pressure gradient and strong winds.

– Widely space isobars indicate a weaker pressure gradient and weaker winds.

Air Pressure and Wind• Pressure differences and winds are ultimately

caused by the unequal heating of Earth’s surface.– The Earth’s land absorbs more solar radiation than

the surrounding water. – The air over the land heats up due to radiation and

becomes less dense. – The hot air rises and the cooler air over the ocean

blows in to replace the rising warm air. – Thus air flows from high

pressure to low pressure

Air Pressure and Wind

• Meteorologist use two instruments to measure wind speed and direction.– The wind vane is use to determine the direction

the wind is blowing from.– The anemometer is used to measure the speed of

the wind 10 meters above the ground.• Wind speed can also be estimated based on the wind’s

effect on water, smoke, trees, and other objects.

Factors Affecting Winds

• If the Earth was perfectly smooth and did not rotate, then air would flow straight from high pressure to low pressure.

• Earth is not perfectly smooth and it does rotate so the winds never flow in a straight path.

• The Coriolis Effect: is the tendency of an object moving freely over the Earth’s surface to curve away from its path of travel.

• The rotation of the Earth is what causes the Coriolis effect.

Factors Affecting Winds

Factors Affecting Winds• The Coriolis Effect can described as follows:– In the Northern Hemisphere, the Coriolis effect deflects

objects to their right; in the Southern Hemisphere, to their left– The Coriolis effect is greatest near the poles and least near the

equator.– The effect increases as the speed of the object increases.– The effect does not depend on an object’s direction of

movement.– The effect is generally noticeable only for objects traveling

over great distances, such as planes and winds.• Due to the Coriolis Effect winds in the Northern

Hemisphere rotate clockwise out of high pressure and counterclockwise into areas of low pressure. (The pattern is reversed in the Southern Hemisphere).

• Wind is also affect by friction between the air mass and the Earth’s surface. The friction causes surfaces winds to slow down.

• The Coriolis effect is weaker on slower moving objects, thus frictions reduces the Coriolis effect on surface winds.

• The more friction the less the deflection of surface winds due to the Coriolis effect.

• The smoother the surface the faster the surface winds thus the stronger the Coriolis effect.

• The higher the altitude the less friction so the stronger the Coriolis effect on the winds, they are deflected more.

Factors Affecting Winds

• Coriolis effect on wind in the Northern Hemisphere:

• In the top of the troposphere the friction has literally no affect on the wind speeds so the Coriolis effect is very strong on these winds.

• The winds at this altitude form what is call the Jet Stream. – The jet stream is bands of swiftly moving winds that are typically

thousands of kilometers long and hundreds of kilometers wide and about one kilometer from top to bottom.

– The Polar-Front jet stream is cool air from the poles that mixes with warmer air to the south.

– This polar-front jet stream has a great effect on the United States weather, it supplies the energy to storms and directing their paths.

– The polar-front jet stream is fastest in the winter when there is the greatest temperature contrasts on the Earth’s surface.

– There is a tropical-easterly jet stream that is weaker than the polar-front jet stream.

Factors Affecting Winds

Global Wind Patterns• Many factors affect wind speeds, including the

temperature differences between equatorial and polar regions, the rotation of Earth, the location of the continents, the time of year, and local topography.

• Lets first look at Earth’s wind patterns if Earth did not rotate. – Warm air at equators would

rise allowing for the cooler air that is sinking at the poles to move towards the equators to replace the rising warm air.

– This would result in one large convection cell in each hemisphere.

• Because of the Coriolis effect and the fact that air cools and sinks long before it reaches the poles a three-celled circulation model in each hemisphere better represents what is happening.

Global Wind Patterns

Global Wind Patterns• Circulation cells are caused by alternating bands of

high and low pressures at Earth’s surface.• The Coriolis effect causes the winds in the Northern

Hemisphere to deflect to the right– The air flowing southward from the pole is deflected to the

right producing easterlies (prevailing winds blowing from east to west).

– Between 30o and 60o the northward air is shifted to the right to produce westerlies (prevailing winds blowing from west to east).

– Between 30o and the equator the southward flowing air that is shifted to the right produces easterlies (prevailing winds blowing east to west).

– In the Southern Hemisphere the pattern is reversed.

Global Wind Patterns• Like with any model the three-cell model is

accurate in some ways and inaccurate in other ways.– The main weakness is with the mid-latitudes (30o

and 60o ) circulation is a simplified view which does not take into account the continents and seasons.

– The main strength of the three-cell model is that it gives fairly accurate view of general patterns of surfaces winds and pressures outside of the mid-latitudes.

Global Wind Patterns• Special areas of wind circulation occur at and/or

near the equator.– Intertropical convergence zone (ITCZ) is an area that the

surface winds of the two hemispheres come together. Here it is hot, humid and there is little to no wind present. This area can be referred to as the doldrums.

– Trade Winds are areas where there is are constant and steady winds. This is where merchants wanted to sail their ships.

Continental and Local Winds

• Because of Earth’s tilt and relative position of the sun changes over the year, Earth’s surface temperatures change with seasons so does the global winds.– The seasonal changes in temperatures cause

seasonal changes in air pressures.– Because air pressures change seasonally thus the

winds change seasonally.

Continental and Local Winds• Local wind is a wind that extends for a distance of 100

kilometers or less. – Local winds are caused mainly by differences in temperature.– Example of local winds are sea and land breezes.– Temperature difference also cause mountain and valley breezes.

• At night the mountain cools so the air cools and sinks causing the breezes to flow down the mountain, thus called a mountain breeze.

• During the day the mountain warms so the air rises causing the breeze to flow up the mountain, thus called valley breeze.

• Valley breezes are much lower in speed than that of a mountain breeze.