Page 245 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 8 Weather 209
Winter pattern
cA Continental arctic
Very cold, very dry, stable (avg. SH 0.1 g·kg–1)
cP Continental polar
(N. Hemi. only) cold, dry, stable, and high pressure (avg. SH 1.4 g·kg–1)
Great Lakes, it absorbs heat en- ergy and moisture from the lake surfaces and becomes humidi- fied. In what is called the lake ef- fect, this enhancement produces heavy snowfall downwind of the lakes, into Ontario, Québec, Michigan, northern Pennsylva- nia, and New York—some areas receiving in excess of 250 cm in average snowfall a year (Figure 8.2).
The severity of the lake ef- fect also depends on the pres- ence of a low-pressure system positioned north of the Great Lakes, with counterclockwise winds pushing air across the lakes. Climate change is ex- pected to enhance lake-effect snowfall over the next several decades, since warmer air can absorb more water vapour. In contrast, some climate models show that later in this century, lake-effect snowfall will de- crease as temperatures rise, but that rainfall totals will continue to increase over the regions leeward of the Great Lakes. Research is ongoing as to the effects of climate change on pre- cipitation in this region.
Atmospheric
Lifting
Mechanisms
When an air mass is lifted, it cools adiabatically (by expan- sion). When the cooling reaches the dew-point temperature, moisture in the saturated air can condense, forming clouds and perhaps precipitation. Four principal lifting mechanisms, illustrated in Figure 8.3, operate in the atmosphere:
(a)
Summer pattern
cA Continental arctic (avg. SH 0.3 g·kg–1)
mP Maritime polar
Cool, humid, unstable all year (avg. SH 4.4 g·kg–1)
(b)
Hot, low relative humidity, stable aloft, unstable at surface, turbulent in summer
(avg. SH 10 g·kg–1)
mP
Maritime polar
Cool, humid, unstable all year
(avg. SH 4.4 g·kg–1)
mT cT mT
Maritime tropical
Warm, humid, stable to conditionally unstable (avg. SH 13 g·kg–1)
Maritime tropical
Warm, very humid, very unstable (avg. SH 17 g·kg–1)
▲Figure 8.1 Principal air masses affecting North America. Air masses and their source regions influencing North America during (a) winter and (b) summer. (SH = specific humidity.)
an invading winter cP air mass from the north. How- ever, that air mass warms to above the –50°C of its win- ter source region in northern Canada, especially after it leaves areas covered by snow.
Modification of cP air as it moves south and east pro- duces snowbelts that lie to the east of each of the Great Lakes. As below-freezing cP air passes over the warmer
• Convergent lifting results when air flows toward an area of low pressure.
• Convectional lifting happens when air is stimulated by local surface heating.
• Orographic lifting occurs when air is forced over a barrier such as a mountain range.
• Frontal lifting occurs as air is displaced upward along the leading edges of contrasting air masses.
Tropical
mP Maritime polar
Cool, humid, unstable all year (avg. SH 4.4 g·kg–1)
mT
Maritime tropical
Warm, humid, unstable (avg. SH 14 g·kg–1)
mT Maritime tropical Warm, humid, stable to conditionally unstable (avg. SH 10 g·kg–1)
cP Continental polar Cool, dry, moderately stable (avg. SH 1.4 g·kg–1)
mP Maritime polar
Cool, humid, unstable all year (avg. SH 4.4 g·kg–1)