Chapter 5 Lesson 4

Air PressurePgs. 170 – 175

Benchmark: SC.6.E.7.3

Vocabulary

• Density• Air pressure• Barometer• Mercury barometer• Aneroid barometer• altitude

Density

• Because air has mass, it has density and pressure.

• The more air molecules in a given volume, the greater the density.

• Fewer air molecules in a given volume, the density is less.

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Pressure

• The atmosphere is heavy, its weight exerts a force on us.

• This force of air is called air pressure.• A barometer is used to measure air pressure.• 2 types of barometers:1. Mercury2. Aneroid

Mercury Barometer

• Consists of a long glass tube that is closed at one end and open at the other. The open end of the tube rests in a dish of mercury. The closed end of the tube is almost a vacuum. The air pressing 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, so the level of mercury in the tube shows you the pressure of the air.

Mercury Barometer

Aneroid Barometer• Aneroid means “without liquid”.• It uses an airtight metal chamber that is

sensitive to changes in pressure.• When air pressure increases, the walls of the

chamber are pushed in. When the pressure drops, the walls bulge out. As the shape of the chamber changes, the needle on the dial moves.

Units of Air Pressure

• Weather reports use different units for air pressure.• Some use inches of Mercury, for example if the

column of Mercury is 30 inches high, the air pressure is “30 inches of Mercury” or “30 inches”.

• National Weather Service Maps use millibars. The pressure of the atmosphere is equal to one bar.

• One inch of mercury is about 33.86 millibars, so 30 inches of mercury is equal to about 1,016 millibars. (33.86 X 30)

How Does Altitude Affect Air Pressure?

• Air pressure decreases as altitude increases.• As air pressure decreases, so does density.• Sea level air has the weight of the whole

atmosphere pressing on it.• Air near the top of the atmosphere has less

weight pressing on it and thus has lower pressure. ←LOW AIR PRESSURE TOP OF MTN

HIGH AIR PRESSURE BOTTOM OF MTN

Altitude Affects Density

• As you go up through the atmosphere, the density of the air decreases.

• Gas molecules are farther apart at high altitudes than they are at sea level.

• Air contains 21% oxygen whether you are at sea level or on top of a mountain.

• Since the air is less dense at a high altitude, each cubic meter of air you breathe has fewer oxygen molecules than at sea level. So you would become short of breath more quickly at a high altitude.

Chapter 5 Lesson 5

WindsPgs. 176 – 179

Benchmark: SC.6.E.7.3

Vocabulary

• Wind• Anemometer• Windchill factor• Local winds• Sea breeze• Land breeze

Winds

• Differences in air pressure cause the air to move.

• Winds move from areas of high pressure to areas of low pressure.

• Most differences in air pressure are caused by the unequal heating of the atmosphere.

Measuring Wind

• Wind speed is measured using an anemometer.

• One type of anemometer has 3 or 4 cups mounted at the ends of spokes that spin on an axle. The force of the wind against the cups turns the axle. A meter connected to the axle shows the wind speed.

ANEMOMETER →

Windchill Factor

• The increased cooling that a wind can cause is called the windchill factor.

• A weather report may say that the temperature outside is 20˚ F, but if the wind speed is 30 mph, the windchill factor makes it feel like 1 degree above zero.

Local Winds

• Winds that blow over short distances are called local winds.

• The unequal heating of the Earth’s surface within a small area causes local winds.

• These winds only form when large-scale winds are weak.

• Two types of winds are: sea breezes and land breezes.

• A sea breeze or lake breeze is a local wind that blows from an ocean or lake.

• The flow of air from land to a body of water forms a land breeze.

Chapter 5 Lesson 6

Relative HumidityPgs. 180 – 183

Benchmark: SC.6.E.7.3

Relative Humidity

Humidity is a measure of the amount of water vapor in the air.

Relative humidity is the percentage of water vapor that is actually in the air compared to the maximum amount of water vapor the air can hold at a particular temperature.

Warm air can hold more water vapor than cool air.

Vocabulary

• Humidity

• Relative humidity

• Psychrometer

Measuring Relative HumidityRelative humidity can be measured with an instrument

called a psychrometer.A sling psychrometer has 2 thermometers, a wet-bulb

thermometer and a dry-bulb thermometer. The wet-bulb is covered by a moist cloth. When the psychrometer is “slung”, air blows over the thermometers. Because the wet-bulb thermometer is cooled by evaporation, its reading drops. If the relative humidity is high, the water on the wet-bulb evaporates slowly, and the wet-bulb temperature does not change much. If the relative humidity is low, the water on the wet-bulb evaporates rapidly, and the wet-bulb temperature drops by a large amount. The relative humidity can be found by comparing the 2 thermometers. (See page 182).

Relative Humidity

This chart shows how relative humidity changes with temperature.