Air Pressure and Wind D 34-45

How can air pressure change? 1. Volume  large volume of air means there will be less pressure 2. Height above Earth’s surface  depends on the weight of the molecules pressing down on a given area; molecules are closer together at sea leavel Denser air weighs more than an equal volume of less dense air and pushes down harderDenser air weighs more than an equal volume of less dense air and pushes down harder

3. Temperature  When air is heated, the molecules speed up 4. Amount of water vapor  Air is a mix of nitrogen, oxygen, and other gases; water molecules weigh less than oxygen and nitrogen molecules Lighter molecules exert less pressure because they weigh lessLighter molecules exert less pressure because they weigh less In moist air lighter water molecules take the place of heavier oxygen and nitrogen molecules so there is less pressure than dry airIn moist air lighter water molecules take the place of heavier oxygen and nitrogen molecules so there is less pressure than dry air

Why do winds blow? Wind  air that moves horizontally  Convection cells  the unequal heating and cooling of the air often makes a pattern of rising air, sinking air, and wind –A part of the atmosphere where air moves in a circular pattern because of the unequal heating and cooling

What are sea and land breezes?  Sea breeze  a wind that blows from the sea toward the land

 Land breeze  a wind that blows from the land toward the water

What is the Coriolis effect?  Earth’s rotation affects winds blowing across its surface  The Earth rotates clockwise from the North Pole; the same thing happens with the winds blowing from the North Pole; because the Earth rotates the winds seem to curve to the right as they head southward

 No matter which way the wind blows it will curve to the right in the Northern hemisphere (This is the Coriolis effect)  In the Southern hemisphere it causes the winds to curve to the left

How are global wind patterns produced?  Year round the equator is being heated strongly by the sunlight; this causes the air to be very warm; it also caused evaporation, which leads to moist air  warm moist air over the equator creates a zone of low pressure around the globe  As this air warms it becomes less dense and starts to rise; as it rises it begins to spread out north and south

 At about 30 degrees north and sound latitudes it begins to cool and sink towards the surface; this causes a high pressure zone on both sides of the equator –This causes a belt of winds to be set in motion around the Earth by air moving from the high pressure zones towards low pressure at the equator. These are know as trade winds

 The poles get very low angle sunlight and the air is very cold –This air is cold and dry so it has high pressure  Air at the poles moves towards 60 degrees latitude forming winds called easterly winds (winds that blow from the east)  Other winds occur between 60 degrees and 30 degrees latitudes called westerly winds (winds that blow from the west) (The US is in this zone)

What are isobars?  Knowing where the air pressure is high or low allows you to predict which way the air will move –Meteorologists start by plotting the air pressure at many different locations then connect all the places with the same air pressure with a line –A line on a map connecting places with equal air pressure is called isobar

 High-pressure systems (H) have a center of higher pressure than the surrounding area and the wind in the center blows clockwise  Low-pressure systems (L) the center is surrounded by higher pressure and the winds in the center blow counterclockwise

How do we study the wind? 1. Weather vane  has a pointer that blows around in the wind; the pointer is mounted to it can point to the different compass markings

2. Anemometer  a series of cups mounted on a shaft that can spin freely; when the wind blows against the cups they spin like a pinwheel (used to measure wind speed)

3. Beaufort Scale  created in 1805 by Sir Francis Beaufort of the British Navy; a way to measure wind speed by observing it’s affect on the sea