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