5 Minutes – Silent Study Time Quiz #3 Atmosphere Composition and Structure - use your notes and quiz template to prepare.

II. Air Pressure

Can you feel air pressure? When you take off or land in an aircraft, your ears may hurt or feel uncomfortable. This is because your eardrums can feel changes in air pressure as the aircraft moves quickly up and down. But what is air pressure?

A. Air Pressure Basics 1. Definition: weight of all the air in the atmosphere pressing down on Earth. 2. The reason air has pressure because it has mass (you can weigh it!)

B. Altitude & Air Pressure Since air pressure is the weight of the air overhead, what happens to air pressure as you go up in altitude?? The air pressure drops as altitude increases (molecules spread out.) Altimeter

C. Air Gasses 1. Low pressure air masses are made up of water vapor. a. water vapor has a low mass 2. High pressure airs masses are made up of nitrogen & oxygen a. N & O are high mass molecules H20 vapor Nitrogen & Oxygen

Why does air pressure change? Changing Temps. Warm air is lighter than cold air because the molecules are farther apart. When cold air replaces a warm air mass then the air pressure at the ground goes up. Changing humidity. The more water vapor the air contains the lighter it is. How?? When water vapor enters the atmosphere it pushes out an equal volume of dry air. Dry air has more nitrogen and oxygen and both of these elements in their gaseous state weigh more than does water vapor. In General scientist have found that a falling barometer means warmer weather, rain or snow.

Changing Pressure A rising barometer = increasing air pressure. This usually means: Clear Cool Dry. Rising barometer readings indicate that a high pressure system is approaching. Higher atmospheric pressure is usually associated with fair weather and clearing skies.

Tut tut it looks like rain Changing Pressure A falling barometer = decreasing air pressure. This usually means: warm, wet, cloudy Tut tut it looks like rain Falling barometer readings usually indicate the approach of an area of low pressure. Low pressure readings are usually associated with storm systems. Tornadoes and hurricanes can produce very low barometric readings.

Air is forever trying to find… Equilibrium Air travels from high to low pressure. This affects wind direction and speed.

Low & High Comparison Table Atmospheric condition Pressure Temp. Humidity Gasses Result Low warmer Greater Water vapor (low mass) Cloudy & precipitation, unsettled High Cooler Lesser Oxygen & nitrogen (high mass) Clear & stable

E. Wrap-Up 1. Air moves from high pressure to low pressure forming winds. 2. As a result, the greater the difference between the high pressure and low pressure areas is, the higher the wind speed is.

How Winds Form Earth’s atmosphere is nearly always in motion. Air is free to move in any direction. The horizontal movement of air along Earth’s surface is called wind. Although a greater volume of air moves horizontally, it can also move vertically. Winds form as cool, heavy air moves toward warm, light air. Cool air moves along Earth’s surface toward warm air. Winds are caused by differences in air pressure. Regions of cold, heavy air have high air pressure, and regions of warm, light air have low air pressure. Therefore, they are called “highs” and “lows”.

Air Currents Up and down movements of air are called air currents. Because of the size of the Sun and the tilt of the Earth, no parts of the Earth are heated the same. As air over the warmer regions is heated, it expands and becomes less dense. Air over cooler regions is heavier and denser. Therefore, it moves under the warm air and pushes it upward. As the warm air mixes with the cool air, it becomes heavier and moves downward. Do you see the Convection Currents?

3. Barometer- instrument used to measure air pressure a 3. Barometer- instrument used to measure air pressure a. units = millibars (mb) There are two types of barometers—mercury and aneroid.

CBA: Awesome Atmosphere Assignment What does the atmosphere block? Describe Bill’s “atmosphere cylinder” experiment, & what were his findings? How do pilot’s determine their current altitude? Describe the difference of air pressure between Death valley & on top of a mountain. Why the difference? Why do your ear’s pop in a plane? Order the five layers of the atmosphere from highest to lowest. Why is it warmer in low cities than in the high mountains? How are Earth’s atmosphere and a greenhouse similar? Bill must run around the track _______ times to simulate the height of the troposphere. Name three topics of the atmosphere that were described in the “weather rap.”

F. Understanding Pressure Maps Isobars- areas with equal air pressure Sea- level = 1013.3 mb Denver, CO = 835 mb Meridian, ID = 975 mb

2. Isobar map rules: a. If # is 499 or less, place 10 in front b 2. Isobar map rules: a. If # is 499 or less, place 10 in front b. if # is 500 or more, place 9 in front c. decimal is moved before last digit. Ex. 950 = 995.0 mb d. low pressure- #’s get lower toward the middle e. high pressure- #’s get higher toward the middle

ISOLINE ACTIVITY: In PEN, uncode each pressure: 103= 1010.3 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.

1016 mb H L

1016 mb H L 1012 mb

1016 mb H L 1012 mb 1008 mb

1016 mb H L 996 mb 1012 mb 1008 mb

1016 mb H L 996 mb 1012 mb 1000 mb 1008 mb

1016 mb H L 996 mb 1012 mb 1000 mb 1008 mb 1004 mb

1016 mb H L 996 mb 1012 mb 1000 mb 1008 mb 1004 mb

Isobar Mapping Questions Follow up questions on separate piece of paper: Describe the wind motion in the H. Describe the wind motion in the L. In what area is the wind speeds the greatest? Winds go from ___ to ____ for pressure. What type of weather is associated with high pressure systems? What type of weather is associated with low pressure systems?