Unit 4 :Meterology

A. Air Pressure1. Air pressure – the weight of air pressing down on Earth2. temperature and altitude determine air pressure3. the more air particles are present, the more air density or pressure exists4. the less air particles are present, the less air density or pressure exists

5. High pressure a. air is more denseb. associated with cold air(more dense than warm air)

c. associated with sinking air

6. Low pressure a. air is less denseb. associated with warm air (less dense than cold air)c. associated with rising air

7. warm air rises, cold air sinks8. air cools as it rises9. air pressure is measured using a tool called a barometer10. winds are formed when air from a high pressure (more dense) moves to areas of low pressure (less dense)

B. Air Mass

1.air mass – large mass of air that takes on the temperature and moisture characteristics of the surface below it2. air moves from an area of high pressure (more dense) to areas of lower pressure (less dense)

3. a high pressure systema. rotating mass of cool, dry air rotating in a downward clockwise motionb. creates sunny and pleasant weather

4. a low pressure systema. rotating mass of warm, moist air rotating an upward counterclockwise

motionb. creates stormy, windy weather

5. air masses are classified by their temperature and humidity (moist or dry)6. air masses in the US move west to east

C. Fronts1. there are 4 major air masses that effect the weather in North America

a. continental polar (cP)b. continental tropical (cT)c. maritime polar (mP)d. maritime tropical (mT)

2. most weather occurs along the border of these air masses at boundaries called fronts

3. Cold front a. occurs when a cold air mass overtakes a warm air massb. this pushes the warm air mass upward c. creates severe weather lasting a short period of timed. represented by blue triangles on a weather map

4. Warm fronta. occurs when a warm air mass slides up and over a cold air massb. this creates rainy weather that lasts for several daysc. represented by red semicircles on a weather map

5. Stationary fronta. occurs when a cold and warm air mass meet but do not move in either directionb. this creates sluggish winds and precipitationc. represented by alternating blue triangles and red semicircles

6. Occluded front a. occurs when two cold air masses merge and force warmer air between

them to riseb. this creates high winds and heavy precipitationsc. represented by purple alternating triangles and semicircles

D. Clouds1. cloud – a visible mass of condensed water in the atmosphere made up of

mixtures of tiny droplets of water or tiny crystals of ice2. form when water evaporates from Earth’s surface

3. classified according to shape and altitude4. prefixes:

a. cirro – describes high clouds (above 6000m)b. alto – describes middle clouds (between 2000m and 6000m)c. strato – describes low clouds (below 2000m)

E. Types of Clouds1. cirrus

a. high, white, feathery cloudb. usually associated with fair weather

2. cumulusa. thick, puffy massesb. usually associated with fair weather

3. stratusa. occurs in layers and often cover the whole skyb. usually associated with fair weather

4. nimbusa. dark, grey clouds with ragged edgesb. associated with precipitation

F. Fronts and Clouds1. cold front – cumulonimbus clouds are large puffy rain clouds associated with

severe thunderstorms along a cold front2. warm front – cirrus, stratus, and nimbostratus clouds are associated with the

passing of a warm front

G. Water Vapor1. humidity – the amount of water vapor in the air2. relative humidity – ratio of water vapor in a volume of air relative to how much

water vapor that volume of air is capable of holding

a. warm air is capable of holding more moisture than cool airb. if you increased the temperature of a room without adding more

moisture, the relative humidity of the room would decrease

c. relative humidity is expressed as a percentaged. measured by a hygrometer

3. saturation – point at which the air holds as much water vapor as it possibly cana. warm saturated air contains more water vapor than cold saturated air

4. dew point – temperature to which air must be cooled at the same pressure to reach saturationa. if the same air continued to cool past the dew point, the air’s excess water

vapor would condense to lead to precipitation

5. condensation (gas to a solid) occurs only when air is saturated a. this leads to precipitation (rain, snow, freezing rain (glaze), sleet, hail)

6. saturation, relative humidity, and dew point are used to measure humidity