Mountain Windstorms

• Foehn: a warm, dry downsloping wind

• Bora: a cold, dry downsloping wind

• Chinook: a warm, dry downsloping wind along the east slopes of the Rockies; can exceed 100 knots sustained; common in eastern CO, from Fort Collins, down to Denver; the highest winds affect Boulder, CO, located at the base of the Rockies, only 20 miles from the Continental Divide; Figures 16.1-2

• Lee Waves form as air flows over a mountain barrier; altocumulus lenticularis clouds form at the crest of the waves; Figures 16.3-4

• Hydraulic Jump occurs when the cross-mountain flow is strong and a very stable layer (e.g., inversion) is present upstream at a level just above the mountaintop; shooting flow occurs down lee side of the mountain (i.e., the foothills) and areas of rotation, or rotors, can form, creating severe low-level turbulence; changes in upstream wind flow can cause these features to move back and forth over the foothills; Figure 16.5

• Breaking Waves can occur over the mountaintop above the inversion layer, creating severe turbulence (like an ocean wave crashing onto the shore) and can have an effect on the shooting flow. modifying its shape & channel (i.e., its intensity, creating gustiness)

• Cold vs. Warm Chinook: 1) the temp of the air upstream of the mountains (i.e., its source region), in comparison with the air temp on the leeward side; 2) latent heat release as the air moves up the windward side of the mountains and creates precip (if no precip, then the air would evaporate on the leeward side, thus cooling the airstream); 3) adiabatic warming during descent (10 C / km); air can warm 27 to 36 F from crest of Rockies to the plains, while the RH can drop below 10%; an existing snowcover can "disappear" quite rapidly; Figure 16.6-7

• Forecasting Chinooks: ones needs: 1) a stable layer upstream of the mountain barrier, 1-2 km above the mountaintop (Figure 16.5); 2) a strong surface pressure gradient across the mountain barrier (usually High pressure over the Pacific or Great Basin and a Low pressure or trough on the Great Plains); 3) the wind flow near the mountaintop (approx. 700 mb) must be strong and westerly (note that a strong jet stream is not required, which would disrupt the wave structures necessary to support downsloping winds); Figures 16.8-10

Mountain Wind (continued)

• Santa Ana Winds: downsloping windstorms over southern CA; have an easterly component; not as strong as the Chinooks of CO, but can gust to near 100 knots; can spread wild fires during extremely dry conditions

• Santa Ana winds occur when High pressure is located over the Great Basin and a strong north-south pressure gradient is present over the southern CA mountains; a stable layer (i.e., inversion) is probably necessary for strong Santa Ana windstorms

• Christmas Windstorm of 2000: Figures 16.11-13

• Katabatic Winds: cold, dry downsloping wind off ice sheets on elevated plateaus or glaciers (e.g., Antarctica & Greenland); can gust over 100 knots; air cools over ice sheet during clear, calm period (i.e., maximum radiational cooling); ice sheet emits infrared radiation and cools; the air over the ice also cools by emitting infrared radiation to space; at some point, the cold, dense air "spills over" the edge and down the slope, driven by the force of gravity; if loose snow is present, can create a "ground blizzard"; Figure 16-14

Local & Regional Circulations

Thermal Circulations: thermal highs & lows

• Atmosphere warm or cools locally.

• Shallow system (few km above ground).

• Weaken with height.

Sea (Lake) & Land Breezes: a type of thermal circulation, due to uneven heating rates of land and water.

• Sea breeze is usually stronger than land breeze, due to greater contrasts in temp during daytime.

• Begins at approximately mid-morning.

• By late afternoon, cool ocean air may reach a depth of 1000 feet and extend inland more than 12 mi.

• Sea Breeze Front: leading edge of the sea breeze as it moves inland ... temp drop of approx. 10 F in one hour is common.

• Sea Breeze Convergence Zone: common on Pacific coast, where air flow around hills rather than over them; also, common over Florida's interior, where thunderstorms often form in summer; can happen over the Great Lakes too.

Mountain & Valley Breezes: develop along mountain slopes.

• Mountain heats up before valley and wind blows up mountain slope.

• Reverse is true at night, as mountain top cools faster.

• Winds are usually in the 5-10 mph range.

Katabatic Winds: technically any downsloping wind, but is stronger than typical mountain breeze.

• Elevated plateau provides cold source, PGF causes air to spill down the mountain slope; Columbia Gorge and Cascades in US.

• Can reach hurricane force wind speeds, but is usually 10-15 mph or less.

• Can exceed 100 mph in Greenland and Antarctica.

• Also known as Bora (Russia), Mistral (Rhone Valley of France).

Chinook Winds: a warm, dry wind descending a mountain range.

• Through adiabatic compression, air warms and dries out; can cause a temp rise of over 35 F in one hour on the eastern slopes of the Rockies; relative humidity can drop to less than 5%.

• Usually found with strong westerly winds blowing over a north-south mountain range (i.e., Rockies)

• Can reach speeds of 50-100 mph.

• Also known as Foehn (Germany), Zonda (Argentina).

Santa Ana Winds: a warm, dry wind that blows from the east or northeast over southern California.

• Warms and dries out through adiabatic compression;

• High pressure usually centered over Great Basin; wind funnels through mountain canyons; can reach speeds of 50-100 mph.

• Creates dust storms; can aggravate brush fires due to dried out vegetation; can "fan" existing fires.

Dust Devil: a swirling column of air common to the hot, dry desert

• Created by a strong updraft during the heat of the day, usually under a clear sky.

• The spinning vortex sucks dust and sand upwards.

• The diameter can range from 10 feet to as much as 100 feet and the vortex can reach from less than 1000 feet to several thousand feet high.

• Unlike a tornado, the dust devils winds increase with height.

• Are generally weak and do little damage.

• Can also occur between buildings or other objects, that create a swirling motion due to forced wind flow.

• Also known as Willy-Willy (Australia)

Haboob: a desert wind that creates blinding sandstorms.

• Cold downdrafts along leading edge of thunderstorms lifts dust & sand into a huge, tumbling dark cloud that may extend horizontally over 150 km and rise vertically to the base of the thunderstorm.

• Dust devils and tornadoes are possible along the turbulent cold-air boundary.

• Most common in African Sudan and the Southwest U.S. (southern Arizona).

Other Desert Winds: can be exceedingly hot and can raise the temp to 120+ F, while lowering the relative humidity to under 10%.

• Leste (Morocco)

• Leveche (Spain): a Leste that crosses the Mediterranean Sea

• Sirocco (Sahara Desert - North Africa)

• Khamsin (Egypt, Red Sea & Saudi Arabia)

• Sharav (Israel)

• The above tend to be most common in Spring and Fall, due to the dry Mediterranean Summers.