Weather

Chapters 12 &13

Air Masses

• Large body of air that takes on the charateristics of the area where it was formed (over land masses are drier)

• Classifying air masses– Temperature

• Polar (P), Tropical (T)

– Location • Continental (c), Maritime (m), Arctic (a)

Wind Systems• Coriolis Effect- Earth’s rotation causes wind

patterns– N. Hemisphere wind to right– S. Hemisphere wind to the left

• Polar Easterlies (Westerlies)- 60°- poles- NE in N Hemis.; SW in S Hemis.

• Prevailing Westerlies- 30-60°- westward winds• Trade winds- 30-0° - Easterlies• Intertropical Convergence Zone- “doldrums”-

rising air causes frequent rain rain forests (near equator)

• Jet streams- high altitude winds, move weather patterns

Fronts

• Narrow region separating 2 air masses of different densities (temperatures)

• Cold front– Cold dense air, displaces

(pushes) warn air up along STEEP front; as warm air rises it condenses showers, clouds

– Larger temperature differences more “powerful” cold front

Fronts- Cont’d

• Warm Front- – Warm air pushes cold

air (slowly) upwards gradual slope

– Very cloudy, precipitation

Fronts-Cont’d

• Stationary- two air masses meet and neither forces the other out (temperature (density) difference between them is very small) cloudy

• Weather doesn't change until another front comes through

Pressure Systems

• High Pressure- air sinks (few clouds) and spreads out; circulates in RIGHT in N Hemis.

• Low pressure- air rises replaced in and upward motion (LEFT in N hemis.)

Cyclone (Nor'easter)

• Develops along a stationary front imbalance (temp, pressure, density) causing part of the front to move south as Warm front moves North upper and lower level winds must be favorable fro a low pressure cyclone to form

Measuring Weather

• Thermometer- measures temperature

• Barometer- measures air pressure

• Anemometer- measures wind speed

• Hygrometer measures humidity

• Radar

• Infrared- temperature sensitive assists with measuring cumulonimbus clouds

Thunderstorms• Formation

– 1. Need abundant source of moisture in lower atmosphere

– 2. Air lifting allows for condensation– 3. unstable atmosphere

• Limitations– 1. Rising air meets stable air– 2. Rate of condensation not able to keep

cloud warmer than air (limits cumulonimbus to 18,000 m)

Types of Thunderstorms

• Air Mass- caused by unequal heating (late afternoon thunderstorms)– Mountain T-storms- orographic lifting– Sea-breeze T-storms- extreme temp

differences at shore

• Frontal- mostly advancing cold fronts- rapid upward flow line of T-storms at leading edge

Development

• 1. Cumulus- air rises vertically creating updrafts- moisture at the top of cloud

• 2. Mature- precipitation forces air under cloud to cool creates downdraft

• 3. Dissipation– downdrafts cool surrounding air no steady, warm moist air

Severe T-storms

• Super cells- cold front powers updrafts

• 10 % of all T-storms

• Hail- frozen layered precipitation

Lightning

• Caused by friction between updrafts and downdrafts– Positive and negative ions form– Stepped leader[-] (invisible) extends towards ground– Return stroke [+] reaches up to meet stepped leader– When they meet channel forms lightning bolt

• Thunder- sound caused by rapid expansion/contracting of air

Tornadoes

• Violent, whirling wind column of air with contact to ground– Often associated with Super cells– Caused by a sudden change in wind

speed/direction wind shear– Causes horizontal swirling winds near ground– Interaction with updrafts force tornado upright– More likely in Spring and late

afternoon/evening• Spring b/c of clash of polar and maritime air over

Great Plains

Hurricanes

• Tropical cyclone- large, rotating low pressure system– Form b/c large amount of energy and

moisture join with a tropical disturbance– Pressure decreases at center causing

circulation (low pressure)

• Needs:– 1. Abundance of very warm water– 2. Disturbance to lift warm air

Stages

• 1. traveling tropical disturbance

• 2. Circulation

• 3. Winds of 65 km/hr tropical storm

• 4. Winds of 120 km/hr hurricane– Eye development– Eye wall- highest winds

• 5. decline- after landfall no water supply

Hurricane Hazards

• Storm surge- hurricane force winds drive ocean waters inwards sometimes 6 m higher than usual

• Flooding

• Wind

• Rain