GEOG101: Physical Geography
Lecture 10: Wednesday, February 25, 2009
Topics
1. Weather vs. Climate
2. Air Masses
3. Atmospheric Lifting Mechanisms
4. Midlatitude Cyclonic Systems
5. Violent Weather
A question from last lecture
• Why does the ice sublimation happen?
1. Very dry (< 10% relative humidity, for example) air passing through the surface of ice;
2. Intense sunlight provides enough energy to activate water molecules, e.g., > (80+540) = 620 calories/gram
3. Single H2O molecule can not be liquid but gas--Because the air is DRY! It has no friends around -- therefore fly away!
Weather vs. Climate
• Climate: long-term average (over decades) of weather conditions and extremes in a region
• Weather: short-term, day-to-day condition of the atmosphere
• What are included in a weather system?
Temperature, air pressure, relative humidity, wind speed and direction, seasonal factors such as changes in insolation receipt
Air Masses
• Air mass: A regional mass of air that has specific conditions of temperature, humidity and salinity
It is named after its initial source region, like winds
Air masses are classified according to moisture and temperature of their source region:
Moisture – m for maritime (wet) and c for continental (dry)
Temperature (latitude factor) – A (arctic), P (polar), T (tropical), E (equatorial), and AA (Antarctic)
• Air Masses Affecting North America:
Continental Polar Air Mass (cP): cold, stable air, clear skies, high pressure
Maritime Polar Air Mass (mP): cool, moist, unstable, low pressure
Two Maritime Tropical Air Masses (mT):
mT Gulf/Atlantic: Extremely unstable, high energy, moist, lot of rain from late spring to early Fall
mT Pacific: stable to conditionally stable, lower in moisture and energy, low average precipitation
• Air Masses Modification: As air masses migrate from their source region, their moisture and temperature characteristics change
Examples: cP air mass passes the Great Lake areas
Atmospheric Lifting Mechanisms
• For air masses to cool adiabatically (by expansion) and to reach the dew-point temperature and saturate, condense, form cloud, and precipitate, they must lift and rise in altitude
• Convergent Lifting: Air flows toward an area of low pressure, displacing air upward
Effects: tropical convergent lifting of warm, moist air can produce a tropic storm
Intertropical convergence zone (ITCZ): NE and SE trade winds converge
• Convectional Lifting: Air lifting stimulated by local surface heating
Effects: Ascending air masses cool to cause showers and thunderstorms
Example: Florida– land is surrounded by ocean, heats up fast during day time; Urban heat island; ITCZ
• Orographic Lifting: Air lifting is forced over a barrier such as a mountain
Effect: Causing rain shadow in the mountains areas
Examples: Rain shadow in Sierra Nevada, Rocky Mountains; Rain pattern in Washington state
• Frontal Lifting (Cold and Warm Fronts): Air lifting occurs along the leading edges of contrasting air masses
Cold Fronts: Cold air forces warm air aloft
Effects: Cold Front can cause severe precipitation, thunderstorms, and tornadoes;
Warm Fronts: Warm air moves up and over cold air
Effects: Warm front may be much more mild, causing drizzly rain showers
Midlatitude Cyclonic Systems
• The conflict between contrasting air masses develops midlatitude cyclones or wave cyclones
• Life cycle of a midlatitude cyclone: it take 3-10 days and can be divided into several stages:
Cyclogenesis: cold-warm air meets cause convergent lifting
Open stage: cold air advances causing anticlockwise flow – mature cyclone
Occluded stage: cold air moves faster than warm air, overtakes the warm fronts
Dissolving stage: Lifting mechanism is cut off from warm airs, remains of the cyclones dissipate in the atmosphere
Storm tracks: Cyclonic storms changes their path with the Sun and the seasons.
Violent Weather
• Thunderstorms: The condensation of large quantity of water vapor in clouds releases tremendous energy. WHY?
• Tornadoes: Winds in high altitude flow faster than the surface winds due to the friction with ground -- cause updrafts with the rotation axis perpendicular to the ground
• Tropical Cyclones: Surface airflow converges into low-pressure area, ascends, and flow outward as it diverges aloft.