1.04 Atmospheric Pressure

8/12/2019 1.04 Atmospheric Pressure

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Atmospheric pressureis defined as the force per unit area exerted against a

surface by the weight of air above that surface at any given point in the Earth's

atmosphere.

Low pressure areas have less atmospheric mass above their location, whereas

high pressure areas have more atmospheric mass above their location.

Similarly, as elevation increases there is less overlying atmospheric mass, so that

pressure decreases with increasing elevation.

Atmospheric Pressure

The average atmospheric pressure at sea level is about 1 atmosphere (atm) = 101.3

kPa (kiloPascals) = 76cm of mercury (symbol Hg).


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The pressure exerted by the whole atmosphere on the Earths surface is approximately

100,000 Pa. Usually, atmospheric pressure is quoted in millibars (mb). 1 mb is equal to

100 Pa, so standard atmospheric pressure is about 1000mb. In fact, actual values ofatmospheric pressure vary from place to place and from hour to hour. At sea level,

commonly observed values range between 970 mb and 1040 mb. Because pressure

decreases with altitude, pressure observed at various stations must be adjusted to the

same level, usually sea level.

Atmospheric pressure is measured by a barometer. A mercury barometer measures

the pressure by noting the length of mercury which is supported by the weight of the

atmosphere. One centimetre of mercury is equal to 13.33 mb, so normal atmospheric

pressure can support a column of mercury about 76cm (or 30 inches) high.


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Air blows from regions of high atmosphere pressure ("highs" or anticyclones) to regions

of low atmospheric pressure. In a high-pressure system, air pressure is greater than the

surrounding areas. This difference in air pressure results in wind, or moving air. In a high-

pressure area, air is denser than in areas of lower pressure. The result is that air will movefrom the high-pressure area to an area of lower density, or lower pressure. Conversely,

winds tend to blow intolow-pressure areas because air moves from areas of higher

pressure into areas of lower pressure. As winds blow into a low, the air can be uplifted.

This uplift of air can lead to the development of a depression with clouds and rain.

Air moving from high to low pressure does not however, follow a straight-line path. In fact,the air moving from high to low pressure follows a spiralling route due to the rotation of

the Earth beneath the moving air, which causes an apparent deflection of the wind to the

right in the Northern Hemisphere, and to the left in the Southern Hemisphere.

Temperature also affects atmospheric pressure. Warmer temperatures will increase

atmospheric pressure.


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Atmospheric Circulation

Atmospheric circulationis the large-scale movement of air, and the means (together

with the smaller ocean circulation) by which thermal energy is distributed on thesurface of the Earth.

The wind belts girdling of the planet are organised into three cells: (1) the Hadley cell,

(2) the Ferrel cell, and (3) the Polar cell.

(1) Hadley cell

The Hadley cellmechanism is well understood. The atmospheric circulation pattern

that George Hadley described to provide an explanation for the trade winds matches

observations very well. It is a closed circulation loop, which begins at the equator

with warm, moist air lifted aloft in equatorial low pressure areas (the Intertropical

Convergence Zone, ITCZ) to the tropopause and carried poleward. At about 30N/Slatitude, it descends in a high pressure area. Some of the descending air travels

equatorially along the surface, closing the loop of the Hadley cell and creating

the Trade Winds.


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The tropopauseis the atmospheric boundary between the troposphere and

the stratosphere. Going upward from the surface, it is the point where air ceases

to cool with height, and becomes almost completely dry. More formally, it is the

region of the atmosphere where the environmental lapse rate changes (0

C/km)from positive (in the troposphere) to negative (in the stratosphere).

The trade winds(also called trades) are the prevailing pattern of easterly

surface winds found in the tropics, within the lower portion of the Earth's atmosphere,

in the lower section of the troposphere near the Earth's equator.


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H: High pressures on earth surface

L: Low pressures on earht surface


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When the air reaches the polar areas, it has cooled considerably, and descends

as a cold, dry high pressure area, moving away from the pole along the surface

but twisting westward as a result of the Coriolis effect to produce the Polareasterlies.

3. Polar cell

2. Ferrel cell

The Ferrel cell,theorized by William Ferrel (1817-1891), is a secondary circulation

feature, dependent for its existence upon the Hadley cell and the Polar cell. It

behaves much as an atmospheric ball bearing between the Hadley cell and the

Polar cell, and comes about as a result of the eddy circulations of the mid-latitudes

(In fluid dynamics, an eddyis the swirling of a fluid and the reverse current created

when the fluid flows past an obstacle). For this reason it is sometimes known as

the "zone of mixing."

While the Hadley and Polar cells are truly closed loops, the Ferrel cell is not.


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Horse Latitudesor Subtropical Highare subtropical latitudes between 30 and

35 degrees both north and south. This region, under a ridge of high pressure called

the subtropical high, is an area which receives little precipitation and has variable

winds mixed with calm.

The consistently warm, dry conditions of the horse latitudes also contribute to the

existence of temperate deserts, such as the Sahara Desert in Africa, the

southwestern United States and northern Mexico, and parts of the Middle East in

the Northern Hemisphere; and the Atacama Desert, the Kalahari Desert, and

the Australian Desert in the Southern Hemisphere.


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Normal atmospheric pressure at sea level is called 1 atmosphere or 1 atm

which = 76cm Hg = 10 m H20 = 101 325 Pascals in the SI system. (H20= 1

g/cm3= 1000 kg/m3, Hg= 13.6 g/cm3= 13,600 kg/m3)

Why cant we feel air pressure on our body....its

76cm of mercury which is quite a lot,why cant

we feel it?

1. We are accustomed to it, drastically increase or decrease it and you will feel the

difference.

2. Intenal pressure in our body neuteralized it . in addition the pressure of air act

over all direction and neuteralized each other.

3. The air around us is constantly pushing against everything; atmospheric pressure.

When you blow air into a balloon, you're pushing gas molecules at high pressure

into the balloon. These air molecules crash into each other and the walls of the

balloon, causing it to inflate. The air pressure in the balloon is higher than

atmospheric pressure; or it would be crushed.


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Can You Survive in Space Without a Spacesuit?

Yes, for a very short time. The principal functions of a spacesuit are to create a

pressurized, oxygenated atmosphere for astronauts, and to protect them from

ultraviolet rays and extreme temperatures. Without it, a spacewalker would

asphyxiate from the lack of breathable air and suffer from ebullism, in which a

reduction in pressure causes the boiling point of bodily fluids to decrease below

the body's normal temperature. Since it takes a bit of time for these things to kill

you, it's possible to make it through a very quick stint in outer space.

At most, an astronaut without a suit would last about 15 seconds before losing

conciousness from lack of oxygen. (That's how long it would take the body to use up

the oxygen left in the blood.) Of course, on Earth, you could hold your breath for

several minutes without passing out. But that's not going to help in a vacuum

An astronaut who fell unconscious from lack of oxygen would last for a few minutesmore before dying from asphyxiation or the effects of the pressure reduction.

Ebullism would result in the formation of bubbles in the moisture found in the eyes,

mouth, and skin tissue. One NASA test subject who survived a 1965 accident in

which he was exposed to near-vacuum conditions felt the saliva on his tongue begin

to boil before he lost consciousness after 14 seconds.


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1.04 Atmospheric Pressure