Atmospheric pressure

Define atmospheric pressure

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

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

Unit for pressure

N/m² = pascal (Pa)

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

Inches of mercury (inHg)

1013.25hPa/1013.25mb = 29.92inHg

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

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

How does the altimeter work?

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

Relationship of pressure and height

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

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

Pressure and altitude relationship

• High altitude low pressure

• Low altitude high pressure

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

What are the pressure lapse rate?

• MSL - 27ft/hPa

• 20,000ft - 50ft/hPa

• 40,000ft - 100ft/hPa

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)

Calculating temperature lapse rate?

H = 96T/P