lecture 8- atmospheric pressure and precipitation Flashcards
A droplet diameter of about 500 mm is characteristic of
drizzle
the average droplet size is about 1,000 to 2,000 mm
for rain
droplets can reach a maximum diameter of about 7,000 mm
thunderstomrs
Ice storms occur when
the ground is frozen and the temperature in the lowest air layer is also below freezing.
virga
On some occasions, wisps or streaks of rain or snow fall from a cloud, but evaporate before reaching the ground (this phenomenon is called virga).
blowing, drifting snow, blizzard
Blowing snow is a condition in which horizontal visibility is restricted to 10 km or less because the wind has raised snow particles to a height greater than 2 M (wind speeds of 35 to 39 kph).
Wind speeds below 35 kph are associated with drifting snow, which moves at a height of less than 2 m.
Blizzard conditions develop at wind speeds above 40 kph, when visibility is reduced to less than 1 km and temperatures are below -10 °C.
Condensation and precipitation will occur when a parcel of moist air is forced to rise and cool by the adiabatic process past the point where it becomes saturated (4 methods).
1) air can be forced to rise over a range of hills or mountains resulting in orographic precipitation.
2) air is forced to rise when there is a net inflow due to horizontal convergence (i.e. northeast and southeast trade winds along the ITCZ).
during convection, a parcel of air rises when it is heated by the underlying surface, making it less dense than the air around it.
in mid-latitudes, air masses of contrasting types meet along frontal zones where colder air tends to be forced under warmer air, forcing it to rise.
Orographic precipitation develops when
winds move moist air up and over high terrain.
Moist air rises on the windward side and is cooled at the dry adiabatic rate until it reaches the lifting condensation level where clouds form.
Cooling now proceeds at the wet adiabatic rate and precipitation begins and continues to fall as the air rises further up the slope.
After passing over the mountain summit, the air begins to descend the leeward slopes of the range and is compressed and warmed, causing cloud droplets and ice crystals to evaporate or sublimate.
This takes up latent heat so the descending air warms at the wet adiabatic rate, but eventually the sky clears, and warming continues at the dry adiabatic rate (the air is now warmer and drier creating a rain shadow - Chinook winds).
A parcel of air is stable when
it is denser than the surrounding air aloft.
A parcel of air is unstable when
unstable when it is less dense than the surrounding air.
absolute stability
A condition of absolute stability develops when the temperature of the surrounding air is warmer than the air parcel.
Absolute instability develops when the rising air remains warmer and less dense than the surrounding air.
thunderstorms
are intense convective storms associated with tall, dense cumulonimbus clouds in which there are very strong updrafts.
The key to the convectional precipitation process is latent heat.
When water vapour condenses into cloud droplets or forms ice crystals, it releases latent heat to the rising air parcel.
By keeping the parcel warmer than the surrounding air, this latent heat fuels the convection process, driving the parcel even higher.
The most dangerous type of thunderstorm is the supercell, which, as well as causing heavy rain, severe hail, and powerful winds, also has a high probability of tornadoes.
Supercells are single cell thunderstorms usually 20 to 50 km in diameter that last for 3 to 4 hours.
The rotational updraft is a unique feature of a supercell and forms the core of the system.
Once it is large enough to be detected on Doppler radar, it is termed a mesocyclone; at this stage the sky appears as an ominous dark, swirling cloud mass.
microbursts
The downdraft that accompanies a thunderstorm can sometimes be very intense, and on occasion has been linked to aircraft accidents.
Such intense downdrafts are called microbursts.
