Air Pressure and Moisture

Air PressureAir Pressure&&

MoistureMoisture

Air PressureAir Pressure Air PressureAir Pressure (also known as (also known as atmospheric atmospheric

pressurepressure) is a measure of the force of the air ) is a measure of the force of the air pressing down on the Earthpressing down on the Earth’’s surface.s surface.

Air Pressure depends on the density Air Pressure depends on the density (d = m/v)(d = m/v) of the air. Less dense air exerts less air of the air. Less dense air exerts less air pressure.pressure.

Factors that Affect Air Pressure:Factors that Affect Air Pressure:1.1. TemperatureTemperature – places of high – places of high

temperatures usually have lower air temperatures usually have lower air pressure than places of low temperatures.pressure than places of low temperatures.

2. 2. WaterWater vaporvapor – less dense air can hold more – less dense air can hold more water than colder denser air. Air with a large water than colder denser air. Air with a large amount of water vapor in it exerts less amount of water vapor in it exerts less pressure than dryer air. (Low pressure pressure than dryer air. (Low pressure systems)systems)

3.3. ElevationElevation or or altitudealtitude – as elevation increases, – as elevation increases, the air becomes thinner, or less dense. Air the air becomes thinner, or less dense. Air pressure decreases with increasing elevation.pressure decreases with increasing elevation.

Higher elevation – less dense and lower pressure

Lower elevation – more dense and higher pressure

The Rockies

Thin air and low pressure makes it difficult for climbers to breathe (Oxygen level drops).

Helicopters will not work due to the lack of air pressure.

Measuring Air PressureMeasuring Air Pressure

Air pressure changes with Air pressure changes with changes in changes in temperaturetemperature and and elevationelevation; the standard air ; the standard air pressure is measured at a pressure is measured at a temperature of temperature of 00 degrees degrees CelsiusCelsius at at seasea levellevel. .

Not on notes…Not on notes…Thinking in terms of air Thinking in terms of air molecules…molecules…

As the number of air molecules As the number of air molecules onon the the surfacesurface increasesincreases, there is an increase in force on that , there is an increase in force on that surface, and as a result the pressure increases. surface, and as a result the pressure increases.

In contrast, a In contrast, a reductionreduction in the number of air in the number of air molecules molecules aboveabove a surface will result in a a surface will result in a decreasedecrease in pressure. in pressure.

Atmospheric pressure is Atmospheric pressure is measured with an measured with an instrument instrument called a called a ""barometerbarometer," which is why ," which is why atmospheric pressure is atmospheric pressure is also referred to as also referred to as barometric pressure. barometric pressure.

2 types:2 types:a) Mercurya) Mercury – Evangelista – Evangelista

Torricelli. At sea level, the Torricelli. At sea level, the weight of the atmosphere weight of the atmosphere forces mercury 760 mm.forces mercury 760 mm.

When air pressure When air pressure increasesincreases, the column of , the column of mercury mercury risesrises in the tube. in the tube.

AneroidAneroid - - In an In an aneroid barometer, a aneroid barometer, a partially evacuated partially evacuated metal drum expands metal drum expands or contracts in or contracts in response to changes response to changes in air pressure.in air pressure.

Average Air Pressure on EarthAverage Air Pressure on Earth(not on notes…)(not on notes…)

Average sea-level pressure is 1013.25 (mb) or 29.921 inches of mercury (in Hg).

Average pressure on Earth ranges between 900 mb and 1040 mb.

Air Pressure and WeatherAir Pressure and Weather BarometersBarometers help us forecast the weather. help us forecast the weather.

If If pressure is highpressure is high, it prevents warm, moist air , it prevents warm, moist air form rising into the upper atmosphere. (form rising into the upper atmosphere. (good good fair weather occursfair weather occurs))

If the If the pressure is lowpressure is low, the air masses move , the air masses move apart in the upper atmosphere. This reduces apart in the upper atmosphere. This reduces the pressure on the layers of warm air below. the pressure on the layers of warm air below. Warm air rises, clouds form, and Warm air rises, clouds form, and rainy rainy weatherweather may occur. may occur.

At sea level air pressure is normally around At sea level air pressure is normally around

760 mm.760 mm. Extremely strong hurricanes have air pressures Extremely strong hurricanes have air pressures

between 30 and 70mm.between 30 and 70mm.

A Look at Air PressureA Look at Air Pressure

Air Pressure and Weather (2 minutes)

Moisture in the AirMoisture in the Air EvaporationEvaporation – the process by which water – the process by which water

molecules escape into the air. (The molecules escape into the air. (The atmosphere holds 14 million tons of moisture!)atmosphere holds 14 million tons of moisture!)

HumidityHumidity – – MoistureMoisture in the air. in the air.

RelativeRelative humidity humidity – describes how – describes how farfar the air the air is from is from saturationsaturation (at a given temperature).(at a given temperature).

It is a useful term for expressing the amount of It is a useful term for expressing the amount of water vapor when discussing the water vapor when discussing the amountamount and and raterate of of evaporationevaporation.  . 

Why and how?Why and how? RelativeRelative humidity is commonly stated during humidity is commonly stated during

weather reports because it is an important weather reports because it is an important indicator of the rate of indicator of the rate of moisturemoisture and and heatheat loss loss by plants and animals.by plants and animals.

Example: 1 kg of air can hold 12 g of water Example: 1 kg of air can hold 12 g of water vapor, but is actually holding 9 g. The RH vapor, but is actually holding 9 g. The RH would be 9/12 x 100, or 75 %. If it were 12/12 x would be 9/12 x 100, or 75 %. If it were 12/12 x 100, it would be 100%.100, it would be 100%.

Air Pressure and Air MoistureAir Pressure and Air Moisture

Measuring Relative humidityMeasuring Relative humidity Meteorologists measure RH Meteorologists measure RH

with a with a psychrometerpsychrometer.. It uses the It uses the differencedifference in in

readings between two readings between two thermometers, one having a thermometers, one having a wetwet bulb and the other bulb and the other having a having a drydry bulb, to bulb, to measure the moisture measure the moisture content or relative humidity content or relative humidity of air.of air.

If humidity is low, If humidity is low, evaporation is quick. If high, evaporation is quick. If high, evaporation is will take place evaporation is will take place more slowly.more slowly.

Tdb           dry bulb temperature (°C)Twb           wet bulb temperature (°C)

Interpretation of pyschrometer readings…

Let’s do some examples:

Example DryBulb

Wet Bulb

Difference Relative Humidity

1. 15 @C 5 @C 10 @C 14 %

2. 30 @C 28 @C 2 @C 86 %

Water Vapor 2 min

Measuring Relative Humidity

Dry Bulb Wet Bulb Relative Humidity

40 38 Difference is 2 RH = 83%

48 34 Difference is 14RH =12%

56 50 Difference is 6, RH = 65%

60 46 Difference is 14, RH = 30%

62 56 Difference is 6, RH = 69%

66 64 Difference is 2, RH = 90%

68 52 Difference is 16, RH =31%

Mini Lab: Determining Relative Humidity1. Define humidity:

2. Define relative humidity:

1. Relative humidity changes due to:

1. Warm air is capable of holding more or less moisture thancool air?

5. Relative humidity is expressed as a:

The amount of water vapor in air.

Tells you how close air is from total saturation. The ratio of water vapor in the air in volume or air to how muchWater vapor or air is capable of holding.

Temperature.

More. There are less molecules in warm air.

Percentage (%)

Mini Lab: Determining Relative Humidity

1. You will determine the average relative humidity of the classroom.

2. Dampen “wet” bulb. 3. Spin for one minute.

4. Record dry and wet bulb readings.

5. Determine relative humidity (use chart).

6. Repeat 2 more times.

7. Obtain average for all readings.