Air Pressure

The Force of the Air Above

What is Air Pressure?The force that is exerted on a surface by atoms and molecules.

Surface

Pres

sure

How Does Density relate to Air Pressure?

More dense air = More PressureMore particles of air in a specific space (volume), therefore more mass pushing on a surface.

Less dense air = Less PressureLess particles of air in a specific space (volume), therefore less mass pushing on a surface.

Remember in class - When more people piled up, the person on the bottom felt the most pressure, because as the mass increased over the same area, the force increased.

F = M/A or F=W/AForce = Mass/Area or Force = Weight/Area

If the area is = (Constant) and the Mass Increases,

the Force Increases

If the area is = C(Constant) and the Mass Decreases,

the Force DecreasesMore Mass = More Force

Surface Surface

More Molecules in Same Volume

Mor

e Pr

essu

re

Less

Pre

ssur

e

Less Molecules in Same Volume

Density

How Does Altitude relate to Air Pressure?Gravity pulls air molecules closer to the earth, therefore there are more molecules in the lower part of the atmosphere resulting in more mass spread over a small volume of space (More Dense Air)In general air pressure...•Increases as altitude decreases (since gravity pulls more air molecules closer to the earth)•Decreases as altitude increases (since there are less air molecules higher in the atmosphere)

How Does Altitude relate to Air Pressure?

air pressure decreases as altitude increases

credit: National Weather Service

How Does Temperature relate to Air Pressure?

We learned that in a closed container,

•If the volume is kept the same (constant) - as

temperature increases, pressure increases

(Gay-Lussac)

•If the temperature is kept the same (constant) -

as pressure increases, volume decreases

(Boyle)

•If pressure is kept the same (constant) - as

temperature increases, volume increases.

(Charles)

However, the atmosphere is not a closed container!!!•As temperature of an air mass increases, air molecules spread out and the volume of air becomes less dense.•Less dense air has less molecules and less mass.•Less dense air applies less pressure on the surface

•Areas of high pressure can be caused when cool air is

sinking and pressing on the ground. At this time, the weather

is usually dry and clear. •Areas of low pressure can be caused when warm air rises, pressing less on the ground. With low pressure, the weather is often wet and cloudy.

Air Pressure on a Can

Remember•When the temperature inside the can increases, the air molecules rose - volume of gas increased - leaving less air molecules in the can since the can was not closed.•This less dense air had less pressure than the air outside the can.•The greater outside pressure caused the can to crush.

How is Air Pressure Measured?Pascals (Pa), Millibars (MB), Inches of Hg (mercury)Air pressure at weather stations is converted to a value that would be observed if that instrument were located at sea level.

Air pressure at sea level is about 14.7 pounds per square inch (psi), (1.03 kg/cm2), or...

Inches of Mercury29.92 (760 mm) in of Hg (mercury)Height of a column of mercury measured in hundredths of inches

Millibars1,013.25 millibarsValues used in meteorology range from about 100 to 1050

credit: National Weather Service

Common units used by meteorologists

credit: National Weather Service

Mercury Barometer•Consists of a glass tube open at the bottom end and partially filled with mercury.• The air pressure pushing down on the surface of the mercury in the dish is equal to the pressure exerted by the weight of the column of mercury in the tube.

• When the air pressure increases, it presses down more on the surface of the mercury.

• Greater air pressure forces the column of mercury higher.

Mercury Barometer

credit: National Weather Service

Aneroid Barometer•The word aneroid means “without liquid”• An aneroid barometer has an airtight metal chamber that is sensitive to changes in air pressure• As the shape of the chamber changes, the needle on the dial moves.

Aneroid Barometer

credit: National Weather Service

LOW PRESSURE•Areas of low pressure can be caused when warm air rises, pressing less on the ground.•Not enough force, or pressure, to push the clouds and storms away. •Results in cloudy, rainy, or windy weather.•Winds push in a counterclockwise direction. (northern hemisphere)

How Does Air Pressure Affect Weather?

credit: National Weather Service

HIGH PRESSURE•Areas of high pressure can be caused when cool air is sinking and pressing on the ground.•Higher pressure in the atmosphere pushes all the clouds away.•Usually creates cool, dry air and clear skies. •Winds push in a clockwise direction (northern hemisphere)

How Does Air Pressure Affect Weather?

credit: National Weather Service

1. In the atmosphere, as the temperature of air increases, the air molecules spread apart.This causes the pressure at the surface to:

•Increase•Decrease•Stay the same•None of the above

2. In general, areas of high pressure usually indicate:

•Fair to good weather•Rainy, stormy weather•Tornado may be approaching•Hurricane may be approaching

Reading Surface Pressure MapsSurface pressure maps are marked with H's and L's which indicate high and low pressure centers. Surrounding these "highs" and "lows" are lines called isobars. "Iso" means "equal" and a "bar" is a unit of pressure so an isobar means equal pressure. These areas of equal pressure are marked with a line. Everywhere along each line is constant pressure. The closer the isobars are packed together the stronger the pressure gradient is. Pressure gradient is the difference in pressure between high and low pressure areas. Wind speed is directly proportional to the pressure gradient. This means the strongest winds are in the areas where the pressure gradient is the greatest or where the isobars are closest together.

Isobars & Isotherms• Curved lines on a weather map that group areas or locations together.

• Isobars – areas of equal air pressure• Isotherms –areas of equal temperature

Iso means equalTherm means heatBar is the unit used to measure air pressure

Isobars

www.weather.comL = Lowest Pressure

H = Highest Pressure

credit: National Weather Service

Links to Current Surface Pressure Maps

HAM WEATHER