Robert W. Christopherson Charlie Thomsen Chapter 8 Weather.

Robert W. ChristophersonCharlie Thomsen

Chapter 8Weather

The day-to-day atmospheric condition (temperature, wind speed, precipitation) is referred to weather. It can be violent bringing disasters as a result of heavy precipitation (rain fall, snow storm, ice storm) or high wind speed.

Weather Air Masses  

Atmospheric Lifting Mechanisms  

Midlatitude Cyclonic Systems  

Violent Weather  

Air Masses Definition: A large body of atmosphere (>1000 miles across and several km thick) with homogenous physical properties (in particular temperature and moisture content).Source Region: Areas where air masses originate.

It must be an extensive and physically uniform area Because atmosphere is heated chiefly from below and gains

moisture from evaporation or transpiration from Earth surface.The area is characterized by a general stagnation of air circulation

so the air will stay long enough to reach certain level of equilibrium with the surface.

Air mass weather: Due to the size of the air mass, it may take several days for it to pass an area, during this period, the surface experience constant weather conditions.

We all experienced summer heat waves (several days of hot weather) and winter cold spells (several days of bitter cold weather)

Air Mass Classification Classification Criteria:

Latitude of source region: Arctic (A), Polar (P), and Tropical (T)Nature of surface: Marine (m) and Continential (c)

Common Air Masses. cA: continental Arctic cP: continental PolarcT: continental TropicalmT: marine TropicalmP: marine Polar

Note: No mA because the arctic ocean is covered by ice throughout the year. Thus we only have cA air mass.

North America Air Masses

Figure 8.2

Role of NA Air Masses mT: warm, humid, unstable, thus brings a lot of rain

Source region: Gulf-Caribbean-AtlanticWinter: cP dominates the central and eastern US, mT only occasionally enters part of the country.Summer: affects a much wider area of NA, primary sources of precipitation for eastern 2/3 of US

Source Region: North PacificCompared to the North Atlantic mT, this mT have much less impact on NA weather.Winter: only influences NW Mexico and the extreme southwestern US.Summer: reaches Gulf of California and in July and August can reach interior western US, monsoon character.

cT: warm, dry, unstable, but no rainOnly exist in summer over northern interior Mexico and adjacent parts of US.Confined to source region.Occasionally move to southern great plains. If it stays long, drought may occur.

Role of NA Air Masses mP: cool to cold, humid, unstable

Source region: North Pacific.Due to the general west-to-east wind, North Pacific mP influences larger area than North Atlantic mPWinter: the air begins as cP from Siberia and moves through the vast ocean region gradually modified as mP. When move inland, orographic rain results.Summer: retreats

Source Region: North AtlanticOnly occasional affects weather of North America. Its influence is confined east of Appalachians and north of Cape Hatteras, NC. Known as nor’easter locally

cT: warm, dry, unstable, but no rainOnly exist in summer over northern interior Mexico and adjacent parts of US.Confined to source region.Occasionally move to southern great plains. If it stays long, drought may occur.

Role of NA Air Masses 

cP: cold, dry, and stableSource region: snow-covered interior regions of Canada and Alaska.

Winter: Can reach much of central and eastern US

Summer: Properties of source region changes, thus the air mass. Occasional invasion brings cooling relief.

cA: cold (colder than cP), very dry, stable Source region: arctic basin and Greenland ice cap

brings cold waves in winter.

Air Mass Modification Once an air mass leaves its source region, it not only modifies the weather of the region it passes, but also is warmed or cooled from below and gains/loses moisture in the meantime. cA/cP moves over ocean in winter:

Evaporation adds moisture to the otherwise dry airWarmer water heats the air from below

Causing instability and vertical ascending currents that rapidly transport heat and moisture to higher levels. In matter of days, cA/cP can be transformed to become mP air mass.

Lake-Effect Snow:Lake-absorb more heat during the summer Lake surface temperature is much higher than surround land (8~17 degrees higher in the Great Lakes than land)cP picks up the moisture in the lake.The low temperature on land in the leeside of the lake force the extra-moisture to drop as snow, thus called Lake-effect snow.The strips of land most affected are called snowbelts.

Lake-Effect Snowbelts

Figure 8.5

Atmospheric Lifting Mechanisms Convergent Lifting  

Convectional Lifting  

Orographic Lifting  

Frontal Lifting (Cold and Warm Fronts)  

Atmospheric Lifting Mechanisms 

Figure 8.6