Weather Forecasting Session Boy Scout Merit Badge Day Your Hosts: Sean Potter and Eli Jacks June 13, 2008

2 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

3 To Start – What is “Air Pressure” (And Why do we care about it?) Air pressure is the weight of air on top of you! 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

4 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 mb1000 mb Here’s an example:

5 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:

6 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.)

7 High and Low Pressure on a Weather Map Air moves from high to low pressure…

8 High and Low Pressure on a Weather Map But also curves as it moves!

9 Difference in Air Pressure Causes Wind Stronger Wind Calmer Wind

10 But Something’s Happening in the Vertical Too… Here’s an example:

11 Rising Air Causes Clouds… And Precipitation ConvergentConvectiveOrographic Three types of lifting…

12 And When Air Masses Collide… We get a zone of rapid air mass change. We define these rapid areas of change as “Fronts”

13 What Happens Near Fronts? Here’s an example in action: Cumulonimbus Clouds Cold Front Cold Air Warm Air

14 What Happens Near Fronts? Here’s an example in action: Warm Front

15 Fronts on a Weather Map Steady Precipitation Thunderstorms Lightning & Hail

16 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!

17 How Cloud Charge Builds Up Here’s an example:

18 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: Hailstone size MeasurementUpdraft Speed in.cm.mphm/s bb< ¼< 0.64< 24< 11 pea¼ marble½ dime7/ penny¾ nickel7/ quarter half dollar1¼ walnut1½ golf ball1 ¾ hen egg tennis ball2½ baseball2¾ tea cup grapefruit softball4½

19 That’s the End of the Lesson! Now, let’s look at today’s weather. It’s your turn to make a forecast!

20 Weather Map from Iowa Tornado

21 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. © , Pearson Education, Inc., publishing as Pearson Prentice Hall