Atmospheric pressure

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15 Terms

1
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Define atmospheric pressure

Atmospheric pressure is the force per unit area exerted by the atmosphere on the surface in contact with it

2
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Atmospheric pressure and weather

Changes in pressure are linked to changes in weather. Low pressure brings clouds, rain and wind, while high pressure tends to bring dry, clear and calm conditions

3
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Unit for pressure

N/m² = pascal (Pa)

4
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Different units for pressure

Average atmospheric pressure at sea level is 101,325 pascals (Pa)

1hPa=100Pa

Millibars (mb) same as hPa. 1013.25mb=1013.25hPa

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Inches of mercury (inHg)

1013.25hPa/1013.25mb = 29.92inHg

6
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How does the mercury barometer work?

As atmospheric pressure increases, mercury is forced up inside the vacuum tube

As atmospheric pressure decreases, the mercury in the tube drops

<p>As atmospheric pressure increases, mercury is forced up inside the vacuum tube</p><p>As atmospheric pressure decreases, the mercury in the tube drops</p>
7
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How does the aneroid barometer work?

It is a series of partially evacuated capsules. The higher number of capsules the higher the accuracy. As pressure increases, the capsules squeeze which moves the needle upwards

<p>It is a series of partially evacuated capsules. The higher number of capsules the higher the accuracy. As pressure increases, the capsules squeeze which moves the needle upwards</p>
8
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How does the altimeter work?

An altimeter is a barometer that shows an increase with height with a decrease in pressure and vice versa.

<p>An altimeter is a barometer that shows an increase with height with a decrease in pressure and vice versa.</p>
9
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Relationship of pressure and height

As height increases, pressure decreases as there’s less mass above you

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What are isobars?

These are lines showing the same atmospheric pressure on a surface pressure chart

  • when isobars are closer together, there’s a high pressure gradient, so strong winds (as wind is just air equalising the high and low pressure)

  • when further apart, there a low pressure gradient, so winds are weaker

<p>These are lines showing the same atmospheric pressure on a surface pressure chart</p><ul><li><p>when isobars are closer together, there’s a high pressure gradient, so strong winds (as wind is just air equalising the high and low pressure) </p></li><li><p>when further apart, there a low pressure gradient, so winds are weaker </p></li></ul>
11
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Pressure and altitude relationship

  • High altitude low pressure

  • Low altitude high pressure

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What are the ISA pressure values (hPa)?

  • MSL - 1013

  • 5,000ft - 850

  • 10,000ft - 700

  • 18,000ft - 500

  • 20,000ft - 480

  • 24,000ft - 400

  • 30,000ft - 300

  • 38,000ft - 200

  • 58,000ft - 100

<ul><li><p>MSL - 1013</p></li><li><p>5,000ft - 850</p></li><li><p>10,000ft - 700</p></li><li><p>18,000ft - 500</p></li><li><p>20,000ft - 480</p></li><li><p>24,000ft - 400</p></li><li><p>30,000ft - 300</p></li><li><p>38,000ft - 200</p></li><li><p>58,000ft - 100</p></li></ul>
13
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What are the pressure lapse rate?

  • MSL - 27ft/hPa

  • 20,000ft - 50ft/hPa

  • 40,000ft - 100ft/hPa

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How does temperature affect pressure?

Warm air

  • expands pressure levels

  • pressure falls slowly with height (because the atmosphere is less dense and expanded vertical so it’s gradual)

Cold air

  • compacts pressure levels

  • pressure falls rapidly with height (because the atmosphere is more dense and compact so it’s more quick)

15
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Calculating temperature lapse rate?

H = 96T/P

<p>H = 96T/P</p>