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Course Syllabus can be found at: http://www.wx4sno.com/portfolio/BSU/Fall_2011/

This lecture will be posted AFTER class at:http://www.wx4sno.com/portfolio/BSU/Fall_2011/Lectures/

Lesson 13Air Pressure

Hess, McKnight’s Physical Geography, 10 ed. 97-103 pp.

Air PressureAir pressure is the force exerted

by the atmosphere along a surface◦Can be at ground level◦Or can be at any height above

ground level (AGL)Air pressure is caused by

atmospheric gases being pulled toward Earth by gravity◦Force can be measured, usually in

either inches of mercury or millibars

Air Pressure, cont.

Factors Influencing Air PressurePressureDensityTemperatureAll three are related

◦If one changes, the other two change as well

The following are generalizations not laws pertaining to air pressure

Air Pressure GeneralizationsRising air produces low pressure near a

surface◦ Strong rising air can lead to the development of

dynamic lowsDescending or “subsiding” air produces high

pressure near a surface◦ Strong subsidence results in dynamic highs

Warm temperatures produce low pressure near a surface◦ Known as thermal lows

Cold temperatures produce high pressure near a surface◦ Known as thermal highs

Dynamic High & Low Pressure

Descending air leads to surface high pressure

Rising air leads to surface low pressure

Thermal High & Low Pressure

Warm air rises, which produces low pressure at a surface

Cold air sinks, produces high pressure at a surface

High

Low

Measuring Air PressureTwo systems of

measurement:◦ Inches of mercury (inHg)

Height of mercury within a vacuum column

Sea level: 29.92 inHg◦ Millibars (mb)

Measure of force pressing down on a surface

1mb = 1000 dynes cm-2

1 dyne = force required to accelerate 1 gm-2 1 cm-2

Sea level: 1013.25 mb◦ The larger the number, the

higher the pressure

IsobarsDifferences in pressure can be

mapped with lines of equal pressure, known as isobars

Elongated areas of high pressure are known as ridges

Elongated areas of low pressure are known as troughs

Isobar Map

Station modelsWeather observing stations

(human-operated and automatic) are located around the world

These stations report temperature, pressure, and a lot more information about the current weather

Station models, cont.

For this lesson, we’re only concerned with temperature and pressure

Temperature is located in upper-left◦ Always measured in degrees Fahrenheit

Pressure is located on upper-right◦ Given as an abbreviated measurement…needs to

be converted…

Station models, cont.

To read the correct pressure:◦ Add either a “9” or a “10” in front depending on

which would bring the value closer to 1000.0.◦ Then add a decimal before the very last digit

In the example above, 998 is given on the station plot◦ Adding a “9” in front and a decimal before the last

digit give us 999.8 mb Pressures generally fall between 950.0 mb and

1050.0 mb

Station Model Pressure Examples

986.5 mb

1013.8 mb

Drawing Isobars

Connect station plots with equal pressure values

Some isolines will fall between stations

Lesson 14Humidity

Hess, McKnight’s Physical Geography, 10 ed. 134-138 pp. & A8, A-9

HumidityHumidity is the amount of water

vapor in a sample of airTwo important ways to measure

it for this lab◦Mixing ratio (g/kg)◦Relative humidity (%)

Mixing RatioMixing ratio is the actual amount of water

vapor in a sample of air◦ Grams water vapor per kilogram dry air (g/kg)

Mixing ratio does not change as the volume of air changes

The greatest amount of water vapor a parcel of air can hold is known as the saturation mixing ratio◦ At this point the air is completely saturated and

condensation occursRecall, saturation is when there is 100%

water vapor in air and when that happens, water vapor goes from a gas to a liquid through condensation

Relative HumidityA comparison between actual amount

of water vapor in the air to the maximum amount of water vapor the air can hold at a given temperature◦ This is also known as capacity

Expresses the amount of saturation in a percentage (%)

Calculating Relative Humidity

For example, if we have a mixing ratio of 13.5 g/kg and a saturation mixing ratio of 22.5 g/kg, relative humidity would be:

Things to ConsiderAs temperature increases, water

vapor capacity also increases◦This means that as temperature

increases, relative humidity decreases

As temperature decreases, water vapor capacity decreases◦As temperature decreases, relative

humidity increases

Dew PointTemperature at which relative

humidity is 100%Water vapor content = water vapor

capacityDew point temperature can never be

higher than actual air temperature◦ When air temperature = dew point

temperature, condensation occurs

Sling PsychrometerTwo thermometers

mounted side-by-sideThe bulb of one

thermometer is exposed to the air, like a normal thermometer◦ Dry-bulb thermometer

The bulb of the second thermometer is wrapped in cloth soaked in distilled water◦ Wet-bulb thermometer

See page 74 for more information

Finally…Omit problem 3d on page 77Only do Part 1 & 2 problems (S.I.

Units)…omit pages 79 and 80.◦You can do pages 79 and 80 for up to

1 point extra credit