Weather Forecasting SessionBoy Scout Merit Badge Day

Your Hosts:

Sean Potter and Eli Jacks

June 13, 2008

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What You’ll Learn Today

Requirement #3• Explain the difference between high and low pressure

systems in the atmosphere• Tell which is related to good and to poor weather• Draw cross sections of a cold front and a warm front• Show location and movement of cold and warm air• Understand frontal slope• Identify frontal cloud types tell where are they located, as well as

the location of precipitation associated with each front

Requirement #4• Tell what causes wind• Explain what it rains• Understand how lightning and hail are formed

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To Start – What is “Air Pressure” (And Why do we care about it?)

• Air pressure is the weight of air on top of you! - 14.7 pounds/square inch

• Cold air is “denser” than warm air.

- This means you can fit more air molecules

in a given space, so it’s heavier

• When cold air and warm air meet… Cold air wins!

• Waves of cold and warm air circle the globe due to uneven heating of the earth

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So What’s So Important About Pressure Differences?

• What happens when you open the fridge? - Cold air rushes out – why?

• The cold air is denser and of higher pressure! - This force is called the “pressure gradient force”

• This happens in the atmosphere too!

High

Pressure

Net

Pressure

Low

Pressure

1004 mb 1000 mbHere’s an example:

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/pgf.rxml

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And What About That Spinning Earth?

• Because the earth is rotating, air that starts out on a straight line actually curves!

• This is called the “Coriolis Effect”

Here’s another example:http://www.classzone.com/books/earth_science/terc/content/visualizations/es1904/es1904page01.cfm?chapter_no=visualization

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So This Means….

• Winds travel clockwise

around high pressure in the Northern Hemisphere…

• and counterclockwise

around low pressure in the Northern Hemisphere

(The opposite is true in the Southern Hemisphere.)

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High and Low Pressureon a Weather Map

Air moves fro

m high to low pressure…

Air moves fro

m high to low pressure…

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High and Low Pressureon a Weather Map

But also curvesas it moves!

But also curvesas it moves!

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Difference in Air PressureCauses Wind

StrongerWind

StrongerWindCalmer

WindCalmerWind

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But Something’s Happening in the Vertical Too…

Here’s an example:

http://www.mesoscale.iastate.edu/agron206/animations/12_CycAntCyc.html

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Rising Air Causes Clouds…And Precipitation

Convergent Convective Orographic

Three types of lifting…

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And When Air Masses Collide…

• We get a zone of rapid air mass change.

• We define these rapid areas of change as

“Fronts”

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What Happens Near Fronts?

Here’s an example in action:

http://www.mesoscale.iastate.edu/agron206/animations/05_cnWfronts.html

Cumulonimbus Clouds

Cold Front

Cold Air

Warm Air

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What Happens Near Fronts?

Here’s an example in action:

http://www.mesoscale.iastate.edu/agron206/animations/05_cnWfronts.html

Warm Front

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Fronts on a Weather Map

Steady Precipitation

ThunderstormsLightning & Hail

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Lighting and Thunder

• Lighting results from the connection of an

“Atmospheric Circuit”. It’s real electricity!

• Under normal conditions, the atmosphere is

a good “insulator”.

• This means charge does not normally flow from the

atmosphere to the ground.

• But – things change in the area of thunderstorms!

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How Cloud Charge Builds Up

Here’s an example:http://www.cartage.org.lb/en/themes/Sciences/Physics/Electromagnetism/Electrostatics/ElectroSpectrum/PhysicsLesson/PhysicsLesson.htm

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And Finally - Hail

• Hail is formed within extremely strong thunderstorm updrafts.

• Rain is carried high up in the cumulonimbus clouds above the freezing level…

• …and bounces up and down until heavy enough to fall!

Here’s an example:http://www.classzone.com/books/earth_science/terc/content/visualizations/es1805/es1805page01.cfm?chapter_no=visualization

Hailstone sizeMeasurement Updraft Speed

in. cm. mph m/s

bb < ¼ < 0.64 < 24 < 11

pea ¼ 0.64 24 11

marble ½ 1.3 35 16

dime 7/10 1.8 38 17

penny ¾ 1.9 40 18

nickel 7/8 2.2 46 21

quarter 1 2.5 49 22

half dollar 1¼ 3.2 54 24

walnut 1½ 3.8 60 27

golf ball 1 ¾ 4.4 64 29

hen egg 2 5.1 69 31

tennis ball 2½ 6.4 77 34

baseball 2¾ 7.0 81 36

tea cup 3 7.6 84 38

grapefruit 4 10.1 98 44

softball 4½ 11.4 103 46

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That’s the End of the Lesson!

Now, let’s look at today’s weather.

It’s your turn to make a forecast!

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Weather Mapfrom Iowa Tornado

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Credits

The graphics on slides 3, 6-15, and 17 are taken from Understanding Weather and Climate, 3rd edition, by Edward Aguado and James E. Burt.

© 1995-2008, Pearson Education, Inc., publishing as Pearson Prentice Hall