Low pressure system over Iceland Air Pressure & Wind Low pressure system over Iceland

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

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 Nitrogen Oxygen Water

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

Using Pressure to Predict Weather Changes Low Pressure High Pressure Air is rising Warmer and more humid air Rain or snow Rotation: Counter clockwise Air is falling Cooler and dryer air Fair weather Rotation: Clockwise

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

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.

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.

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.

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

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.

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)

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

Global Winds Winds named by the direction from which they blow. Earth encircled by six major wind belts Three in each hemisphere From pole to equator, they are the polar easterlies, the westerlies, and the trade winds. Move warm air away from the equator to the poles

Draw the diagram into your composition book