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Bill Nye on AtmosphereBill Nye on Atmosphere

Air PressureAir Pressure Air pressureAir pressure

What is air What is air pressure?pressure?

Air Pressure is Air Pressure is

Differences in air pressure Differences in air pressure cause Earth’s winds and cause Earth’s winds and weather to change.weather to change.

Air has pressure b/c air has Air has pressure b/c air has mass.mass.

Is defined as force per unit Is defined as force per unit area. area. The less mass a The less mass a volume of air has the less volume of air has the less dense the parcel.dense the parcel.

The weight of the atmosphere The weight of the atmosphere per unit area. At the surface , per unit area. At the surface , 1 kg per square centimeter is 1 kg per square centimeter is the average air pressure.the average air pressure.

Measuring Measuring Air pressureAir pressure

Mercury Mercury BarometerBarometer

At sea level At sea level the height the height of the of the column column normally is normally is 76 cm. 76 cm.

The instrument used to The instrument used to measure air pressure is the measure air pressure is the BAROMETERBAROMETER

There are two types of barometers—mercury and aneroid.

The Aneroid The Aneroid BarometerBarometer

BarographBarograph

Uses a thin metal can and by Uses a thin metal can and by pumping most of the air out of pumping most of the air out of the can, the can is made the can, the can is made sensitive to air pressure,sensitive to air pressure,

A spring or similar device A spring or similar device keeps the can from collapsing keeps the can from collapsing inward. inward.

As the shape of the can As the shape of the can changes with the changes in air changes with the changes in air pressure, a pointer attached to pressure, a pointer attached to the can moves over a scale.the can moves over a scale.

Is a recording aneroid Is a recording aneroid barometer. Its pointer is a pen barometer. Its pointer is a pen that writes on a chart.that writes on a chart.

http://www.stuffintheair.com/Atmosphere-Lesson-Plans.html

AltimeterAltimeter

Since air pressure is the Since air pressure is the weight of the air overhead, weight of the air overhead, what happens to air pressure what happens to air pressure as you go up in altitude??as you go up in altitude??

The air pressure drops with The air pressure drops with an increase in altitude.an increase in altitude.

Is a aneroid barometer that Is a aneroid barometer that reads height above sea level.reads height above sea level.

Air Air Pressure Pressure UnitsUnits

Air pressure is reported in Air pressure is reported in two different ways. The first two different ways. The first way gives the height of the way gives the height of the mercury column in the mercury column in the barometer. It may be given barometer. It may be given in centimeters or in inches.in centimeters or in inches.

The second way uses a The second way uses a metric unit called a metric unit called a millibarmillibar .

1000 millibar of air pressure is roughly equivalent to the average air pressure at sea level.

F. Understanding Pressure F. Understanding Pressure MapsMaps

1.1. Isobars- areas with equal air pressureIsobars- areas with equal air pressure

Sea- level = 1013.3 mbSea- level = 1013.3 mb

Denver, CO = 835 mbDenver, CO = 835 mb

Meridian, ID = 975 mbMeridian, ID = 975 mb

2. Isobar map rules:2. Isobar map rules:

a. If # is 499 or less, place 10 in fronta. If # is 499 or less, place 10 in front

b. if # is 500 or more, place 9 in frontb. if # is 500 or more, place 9 in front

c. decimal is moved before last digit.c. decimal is moved before last digit.

+ Isobars always make a circle!+ Isobars always make a circle!

Ex. 950 = 995.0 Ex. 950 = 995.0

ISOLINE ACTIVITY:

1. In PEN, uncode each pressure: 103= 1010.3

2. In PENCIL, label the highest pressure H and the lowest pressure L.

3. For the H draw the following isobars: 1016, 1012, 1008 mb (each in a different color)

4. For the L draw the following isobars: 996, 1000, 1004 mb (each in a different color)

5. For each wind indicator- show wind movement.

HL

1016 mb

HL

1012 mb

1016 mb

HL

1008 mb

1012 mb

1016 mb

HL 996 mb

1008 mb

1012 mb

1016 mb

HL 996 mb

1008 mb

1012 mb

1016 mb

1000 mb

HL

1004 mb

1000 mb

996 mb

1008 mb

1012 mb

1016 mb

HL

1004 mb

1000 mb

996 mb

1008 mb

1012 mb

1016 mb

Why does air Why does air pressure pressure change?change?

Changing TempsChanging Temps.. Warm air is lighter than cold air because Warm air is lighter than cold air because

the molecules are farther apart. the molecules are farther apart. When cold air replaces a warm air mass When cold air replaces a warm air mass

then the air pressure at the ground goes then the air pressure at the ground goes up.up.

Changing humidity.Changing humidity. The more water vapor the air contains The more water vapor the air contains

the the lighterlighter it is. it is. How?How? When water vapor enters the When water vapor enters the

atmosphere it pushes out and equal atmosphere it pushes out and equal volume of dry air. volume of dry air.

Dry air has more nitrogen and oxygen Dry air has more nitrogen and oxygen and both of these elements in their and both of these elements in their gaseous state weigh more than does gaseous state weigh more than does water vapor.water vapor.

In General scientist have found that In General scientist have found that a falling barometer means warmer a falling barometer means warmer weather, rain or snow. weather, rain or snow.

2020

Changing PressureChanging Pressure

A A rising barometerrising barometer = increasing air pressure. = increasing air pressure.

This usually means: Clear Cool Dry.This usually means: Clear Cool Dry.

2121

Changing PressureChanging Pressure

A A falling barometerfalling barometer = decreasing air pressure. = decreasing air pressure.

This usually means: warm, wet, cloudyThis usually means: warm, wet, cloudy

Tut tut it looks like

rain

Air is forever trying to find…Air is forever trying to find… EquilibriuEquilibriu

mm Air travels from high to low Air travels from high to low

pressure. pressure. This affects wind direction This affects wind direction

and speed.and speed.

Low & High Comparison Low & High Comparison TableTable

Atmospheric condition

Pressure

Temp. Humidity Gasses Result

Lowwarmer Greater Water vapor

(low mass)Cloudy & precipitation, unsettled

HighCooler Lesser Oxygen &

nitrogen (high mass)

Clear & stable

Follow up questions:

1. Describe the wind motion in the H.

2. Describe the wind motion in the L.

3. In what area is the wind speeds the greatest?

4. Winds go from ___ to ____ for pressure.

5. What type of weather is associated with high pressure systems?

6. What type of weather is associated with low pressure systems?

* Answer questions 17-23 pg 433 blu text.