Air Pressure and Wind Pages 523 -537

WHAT IS PRESSURE? Pressure is the amount of Force per unit area.

Example: What pressure does a 50 kg person exert on the ground when standing on one foot that measures 20 cm x 8cm? Pressure = weight/area

Answer: What pressure does a 50 kg person exert on the ground when standing on one foot that measures 20 cm x 8cm? 50kg/ (20cmx8cm) = 0.31 kg/cm2 What if person is standing on both feet?

Answer: What pressure does a 50 kg person exert on the ground when standing on one foot that measures 20 cm x 8cm? 50kg/ (20cmx8cm) = 0.31 kg/cm2 What if person is standing on both feet? Half the pressure: 0.156 kg/cm2

WHAT IS AIR PRESSURE? Air Pressure is the weight of the atmosphere per unit area. At the surface, the atmospheric pressure is about 1kg/cm2

Air Pressure is exerted in all directions

Atmospheric Pressure = Barometric Pressure  

Measuring Air Pressure p523 Air pressure is measured with an instrument called a Barometer. There are two main types of barometers

Measuring Air Pressure p523 Aneroid barometer measures the atmospheric pressure using the expansion of a metal.

Measuring Air Pressure p523 Mercury barometer measures the atmospheric pressure by adjusting the height of mercury inside a tube.

Air Pressure is measured in millibars p524 Millibar is a measure of the actual force of the atmosphere pushing down on a surface. Standard pressure at sea level is 1013.2 millibars

Air Pressure drops with altitude At higher elevations, there are fewer air molecules above a given surface than a similar surface at lower levels. An altimeter is an instrument used by air craft to measure height above sea level. Barometer reading drops about 1 cm for every 123 meter within the first few km of atmosphere. Then pressure changes more slowly as air thins rapidly. Half the weight of the atmosphere is within 5.5 km of surface of Earth.

Factors Affecting Air Pressure p525 Changing temperature: warm air is lighter than cold air because the molecules of warm air are farther apart. So when warm air replaces an equal volume of cold air, the air pressure at the ground falls. Conversely, the arrival of colder air higher up causes the pressure t ground to rise.

Factors affecting Pressure p525 Changing humidity: the more water vapor in the air the lighter it is. When water vapor enters the atmosphere, it pushes out an equal volume of dry air. Water vapor is lighter than the nitrogen and oxygen it pushes out. Therefore, humid air weighs less than dry air and exerts less pressure.

Pressure indicates weather High pressure associated with “fair weather” Low pressure associated with “rain” Falling pressure associated with increasing cloudiness Rising pressure associated with clearing conditions

Indicating Pressure pressure reading in millibars (mb).

Isobars Isobars are lines that connect areas of equal air pressure. Pressure Gradient is the spacing between the lines, indicate pressure change over a given distance.

What is the isobar interval on this map?

What is the isobar interval on this map? 4 millibars

Cyclones and Anticyclones Isobars that decrease in value inward indicate a low-pressure center (L), or cyclone . If the value increases toward center, this indicates a high-pressure center (H), or anticyclone.

Cyclones – add this to your notes Cyclones - low pressure in center, spin inward Effects: heavy rain, strong wind, storms, tornadoes.  Anticyclone -  high pressure in center, spin outward. air slowly circulates in a clockwise (northern hemisphere) or counterclockwise (southern hemisphere).  Associated with calm, fine weather.

Anticyclones a weather system with high atmospheric pressure at its center, around which air slowly circulates in a clockwise (northern hemisphere) or counterclockwise (southern hemisphere) direction.  Anticyclones are associated with calm, fine weather.

What's the difference between cyclone and hurricane or typhoon? the location where the storm occurs.  Hurricanes, cyclones, and typhoons are all the same weather phenomenon;  we just use different names for these storms in different  places.  In the Atlantic and Northeast Pacific, the term “hurricane” is used.

Where are the areas of high and low pressures? Is A an area of high pressure or Low? B? A B

Practice Read pages 523-526 Answer 1-5 p526

Wind Direction and Isobars Winds flow roughly parallel to the isobars.