2. Air Pressure
Air Pressure (also known as atmospheric
pressure) is a measure of the force of the air
pressing down on the Earth’s surface.
Air Pressure depends on the density (d =
m/v) of the air. Less dense air exerts less air
pressure.
3. Factors that Affect Air Pressure:
1. Temperature – places of high
temperatures usually have lower air
pressure than places of low temperatures.
4. 2. Water vapor – less dense air can hold more
water than colder denser air. Air with a large
amount of water vapor in it exerts less
pressure than dryer air. (Low pressure
systems)
5. 3. Elevation or altitude – as elevation increases,
the air becomes thinner, or less dense. Air
pressure decreases with increasing elevation.
Higher elevation
– less dense and
lower pressure
Lower elevation –
more dense and
higher pressure
The Rockies
6. 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.
7. Measuring Air Pressure
Air pressure changes with
changes in temperature and
elevation; the standard air
pressure is measured at a
temperature of 0 degrees Celsius
at sea level.
8. Not on notes…Thinking in terms of air
molecules…
As the number of air molecules on the surface
increases, there is an increase in force on that
surface, and as a result the pressure increases.
In contrast, a reduction in the number of air
molecules above a surface will result in a
decrease in pressure.
9. Atmospheric pressure is
measured with an
instrument called a
"barometer," which is why
atmospheric pressure is
also referred to as
barometric pressure.
2 types:
a) Mercury – Evangelista
Torricelli. At sea level, the
weight of the atmosphere
forces mercury 760 mm.
When air pressure
increases, the column of
mercury rises in the tube.
10. Aneroid - In an
aneroid barometer, a
partially evacuated
metal drum expands
or contracts in
response to changes
in air pressure.
11. Average Air Pressure on Earth
(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.
12. Air Pressure and Weather
Barometers help us forecast the weather.
If pressure is high, it prevents warm, moist air
form rising into the upper atmosphere. (good
fair weather occurs)
If the pressure is low, the air masses move
apart in the upper atmosphere. This reduces
the pressure on the layers of warm air below.
Warm air rises, clouds form, and rainy
weather may occur.
At sea level air pressure is normally around
760 mm.
Extremely strong hurricanes have air pressures
between 30 and 70mm.
13. A Look at Air Pressure
Air Pressure and Weather (2 minutes)
14. Moisture in the Air
Evaporation – the process by which water
molecules escape into the air. (The
atmosphere holds 14 million tons of moisture!)
Humidity – Moisture in the air.
Relative humidity – describes how far the air
is from saturation (at a given temperature).
It is a useful term for expressing the amount of
water vapor when discussing the amount and
rate of evaporation.
15. Why and how?
Relative humidity is commonly stated during
weather reports because it is an important
indicator of the rate of moisture and heat loss
by plants and animals.
Example: 1 kg of air can hold 12 g of water
vapor, but is actually holding 9 g. The RH
would be 9/12 x 100, or 75 %. If it were 12/12 x
100, it would be 100%.
17. Measuring Relative humidity
Meteorologists measure RH
with a psychrometer.
It uses the difference in
readings between two
thermometers, one having a
wet bulb and the other
having a dry bulb, to
measure the moisture
content or relative humidity
of air.
If humidity is low,
evaporation is quick. If high,
evaporation is will take place
more slowly.
18. Interpretation of
pyschrometer
readings…
Tdb dry bulb temperature (°C)
Twb wet bulb temperature (°C)
Let’s do some examples:
Example Dry Wet Difference Relative
Bulb Bulb Humidity
1. ̊ ̊
15 C 5 C ̊
10 C 14 %
2. ̊ ̊
30 C 28 C ̊
2C 86 %
20. Dry Bulb Wet Bulb Relative Humidity
40 38 Difference is 2
RH = 83%
48 34 Difference is 14
RH =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%
21. Mini Lab: Determining Relative Humidity
1. Define humidity:
The amount of water vapor in air.
2. Define relative humidity:
Tells you how close air is from total saturation. The ratio
of water vapor in the air in volume or air to how much
Water vapor or air is capable of holding.
1. Relative humidity changes due to:
Temperature.
1. Warm air is capable of holding more or less moisture than
cool air?
More. There are less molecules in warm air.
5. Relative humidity is expressed as a: Percentage (%)
22. 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.
