1. Fronts Front The Boundary between two different air masses.
The front is the area between the two air masses where temperatures and humidity change.
An approaching front means a change in the weather.

4. Fronts and Precipitation
Fronts almost always bring precipitation.
At the frontal surface, warm air rising high into the troposphere means cooling.
What happens to water vapor when air cools?
Condensation then clouds and precipitation.

5. Cold front
Cold air is advancing and displacing warmer air. Weather is wet with heavy precipitation.

6. Warm front
Warm air is pushing ahead displacing colder air. Warm temps follow the passage of the front.

7. Stationary Front
If neither air mass is being displaced.
Temperatures remain stagnant and winds are gentle to nil.

8. Rain Shadow – A area of dry land on the leeward (or downwind) side of a mountain
Winds carry air masses up and over the mountain range
Air is driven upward over the mountain, falling temperatures cause the air to lose much of its moisture as precipitation

9. Air Pressure & Wind
Hurricane Irma 9/5/2017

10. Air Pressure Air Pressure
Weight of the air as it pushed down upon the Earth’s surface.
14.7 pounds/inch2 (1013 mb)
Measured w/ barometer
Mercury
Aneroid
High pressure vs. low pressure
Changes are due to
Elevation
Temperature
Humidity

11. Dry air is heavier than wet
Dry vs. wet air
Dry air is heavier than wet
water molecule has less mass than O2 or N2 molecules.
The more water vapor in the air the less the:
Density
Pressure
Mass
Less than 1% water vapor produces all precipitation
Drives storm
Nitrogen
Oxygen
Water

12. Calculate the Weight of the Air
Molecular weight of Gases: N2= 14 amu, O2=16 amu, H2O=10 amu

13. Using Pressure to Predict Weather Changes
Low Pressure
High Pressure
Air is rising
Warmer and more humid air
Warmest air found at the top of the classroom
Rain or snow
Rotation: Counter clockwise
Air is falling
Cooler and dryer air
Fair weather
Rotation: Clockwise

14. Reading Weather Maps Isobars Pressure gradient
A line that joins points having the same air pressure.
Weather maps
Closely spaced isobars indicate an area where pressure changes quickly.
Strong winds (closely spaced)
Pressure gradient is calculated by dividing the distance by the pressure changes.
Always moves from High to Low

15. Reading Isobars
At all the points on this isobar, the air pressure is 1020 mb.
The pressure in a high is greater than that in surrounding regions.
The pressure in a low is less than that in surrounding regions.
Isobars that are close together indicate a strong pressure gradient.
Isobars that are far apart indicate a weak pressure gradient.

16. What Makes the Wind Blow?
“air in motion relative to the Earth’s surface”.
Horizontal motion
Caused by pressure differences between two pressure systems.
Uneven heating of the Earth’s surface
Isobars show wind
Three forces that cause wind to blow in the direction it does:
Pressure Gradient Force
Coriolis Force
Friction
Named for the direction they come from.
Measured with an anemometer or wind vane.

17. Pressure Gradient Force (PGF)
Force is caused by differences in pressure within the atmosphere.
Force is trying to move the air to eliminate differences (reach equilibrium).
On a surface map isobars packed close together equals a strong PGF.

18. Coriolis Force Coriolis Force
“The tendency of an object moving freely over the Earth’s surface to curve away from its path of travel.”
Caused by the rotation of the Earth
Curve is to the right in the N. Hemisphere.
Strongest near the poles and zero at the equator.
Faster the speed the stronger the effect.
Generally noticeable only for objects traveling over great distances.
Nearly equal and opposite the PGF

19. Friction
Friction
The opposing force, due to contact, that causes objects to slow down.
Only applies to air near the surface.
Cause air to slow down and spiral into a low and out of a high.

20. Continental Winds Monsoons Seasonal winds that affect a continent.
Caused by differential heating of the land and water.
Summer
Land heats up, wind blows from ocean (warm, moist)
Winter
Land cools down, wind blows from land (cold, dry)

21. Local Winds Local Winds Extend for a distance of 100 km
Caused by temperature differences
Examples:
Ocean breeze
Mid-morning to sunset
Land breeze
Midnight to after sunrise
Mountain breeze
Night (Santa Ana winds)
Valley breeze
day