Lec 6 - Winds, Moisture, Precipitation Flashcards
What is wind?
the expression of atmospheric motion
In what dimensions does wind move?
- x & y: latitude and longitude
- z: vertical component
What causes parcels of air to move?
parcels of air move as a result of forces acting on it!
How can you tell from looking at isobars on a map whether a pressure gradient is strong or weak?
- Weak: bars spaced out
- strong: many bars close together
What is the Pressure Gradient Force?
- the initial force that causes air to move
- equal to inverse of density *(change in Pressure / change in distance)
What factors influence air movement?
- Pressure gradients
- Coriolis force
- Geostrophic flow
What is the Coriolis Force?
- objects in atmosphere = influenced by Earth’s rotation
- deflective force that causes objects to ‘drift’ from their original path
What direction does the Coriolis force act in?
- N hemisphere: right
- S hemisphere: left
How does geostrophic flow work?
- air moving along a pressure gradient is subject to Coriolis force
- air eventually flows parallel to straight isobars
What is geostrophic flow?
- the wind vector achieved when forces of pressure gradient and Coriolis force come in to balance
What are supergeostrophic/subgeostrophic winds?
- supergeostrophic: high pressure centres where Coriolis Force dominates the pressure gradient force
- subgeostrophic: low pressure centres: low pressure centres where pressure dominates over Coriolis force
- BOTH kinds of conditions result in airflow parallel to the curved height contours
What happens to wind when isobars are curved?
the balance of forces must include centrifugal force (directed toward the centre of rotation)
- results in gradient wind around curved isobars
What wind forces are balanced in low pressure areas?
Fc (Coriolis) and Fcen (Centrifugal) balance Fp (pressure gradient force)
- generates ‘gradient winds’ (curved around isobars)
What wind forces are balanced in high pressure areas?
Fp (gradient force) and Fcen (centrifugal) balance Fc (Coriolis)
What effect does friction have on wind?
creates drag in planet boundary layer (1.5km of air closest to surface)
What is the planet boundary layer, and when is it relevant?
the air within 1.5km of the planet’s surface
this is the area affected by drag
What are cyclones?
- Low pressure areas at the surface with roughly enclosed isobars
- air converges toward low P centers (cyclones)
- ascending air, precipitation
What are anticyclones?
- areas of high pressure with roughly enclosed isobars
- air moves out from high pressure center, creates clear skies
What direction does are move in cyclones?
counterclockwise in N hemisphere
clockwise in S hemisphere
What direction does air move in anticyclones?
clockwise in N hemisphere
counterclockwise in S hemisphere
What is the maximum amount of water the air can contain?
Differs depending on temperature!
(different saturation points)
What 2 factors determine the amount of water vapour in the air?
- water supply/evaporation rate
- Temperature
What is the difference between absolute humidity and specific humidity?
- absolute humidity (vapour density) is the density of water vapour in g/m^3
(changes with air volume) - specific humidity is the portion of air mass made of water vapour, expressed in g/kg
What is the difference between specific humidity and mixing ratio?
specific humidity is the portion of air made up of water vapour (g/kg)
mixing ratio is the ratio of water vapour to dry air (g/kg)
What is relative humidity?
the amount of water vapour in the air relative to the total amount that can be held at that temperature
When is relative humidity highest?
in early morning, at the coolest time of day (ie at saturation point)
What is the dew point temperature? (Td)
the temperature air must be cooled to in order to reach saturation of water vapour
How do water droplets form and shrink?
condensation and evaporation
What kind of process happens when a parcel of air is cooled to its dewpoint temperature?
a diabatic process where energy is gained/lost
What is a diabatic process?
a process involving the addition/removal of energy from a system
- ex when water condenses/evaporates it adds/removes energy from its surroudnings
Where is nighttime loss of longwave radiation the greatest?
close to the surface
What is fog?
surface cloud when air cools to dew point, gains moisture, or when cool/warm air are mixed
When does cloud formation occur?
when a parcel of air is cooled to its dewpoint temperature
What kind of process is cloud formation?
adiabatic
(as a result of air rising through the atmosphere, since as air rises it cools to dewpoint T)
How does adiabatic warming/cooling occur in clouds?
- dry adiabatic warming when air sinks/compresses/warms
- drya diabatic cooling when air rises/expands/cools
What is the dry adiabatic lapse rate? (DALR)
- the rate of cooling/warming that occurs with a change in height (no energy exchange with other parcels of air)
- equal to 9.8C / km)
What is the saturated adiabatic lapse rate?
- the rate of cooling with change in height for saturated parcels of air
- condensation occurs since parcel is saturated, so a change in height also means energy is released into the environment)
- 0.5 C/km
between DALR and SALR, which is larger?
DALR
- when saturation occurs (SALR), condensation releases heat into the environment, so SALR has a slower temp change
What is the lifting condensation level?
LCL is the height at which rising parcels of air reach saturation (T = Tdew)
- the point where SALR applies and DALR stops applying
What is the environmental Lapse Rate (ELR)?
the temperature profile of the atmosphere in terms of temperature changes with height (you can measure with a balloon)
How does the ELR (environmental lapse rate) change throughout the say?
temperatures close to the surface change, but changes are smaller the higher you go
What 4 mechanisms cause air to lift?
- Orographic
- Frontal
- Convection
- Convergence
What is orographic uplift?
- when air is deflected upwards by high terrain, it results in adiabatic cooling, promoting clouds and precipitation
- creates rain shadow (no rain) on leeward side, while windward side is v wet
What is frontal lifting?
- when two masses of different densities converge (ex cold and warm)
- warm air is lifted upward
- most prominent in mid-latitudes, happens everywhere
How is uplift caused by local convection?
when differential heating at surface causes warmer air to rise, expanding and forming clouds
- most prominent in prairies/low topography
What is convergence uplift?
- when air is forced to move horizontally and meets more air, some of it has to go up
- adiabatic cooling, etc
- not common in Canada, can happen on edges of mountain ranges
What part of precipitation is explained by geography?
the temporal characteristics of precipitation (how/when it forms, due to orographic, frontal, convection uplift)
