Lecture #3-4 [Weather and Climate] Flashcards
weather vs. climate
weather = specific condition of the atmosphere at a particular place and time. It is measured in terms of variables including
Climate= a statisical characterization of the weather, averaged over many years.
The Atmosphere
- supplies oxygen and carbon dioxide
- insulates against temperature extremes
- shields uv radiation
composition
gases, particulate matter
Thermal layers of atmosphere (5)
- troposphere (~18 km)
- Stratosphere (18-48 km)
- Mesophere (48-80 km)
- Thermosphere (80 km)
- Exosphere (into space)
Pressure
- basically, the weight of overlying air
- the taller the column of air above an object, the more pressure exerted
- Highest at sea level (1kg/cm2) decreases with increasing altitude
- Not a constant change with altitude: decreases at a decreasing rate
millibar (mb)
most common unit for atmospheric pressure
isobars
lines that connect points of equal pressure
how does wind move
air flows from H to L pressure
Direction of wind goverened by which three factors
- pressure gradient force
- coriolis effect
- friction
Pressure Gradient Force
- path of least resistance
- air flows at right angles to isobars
- high pressure= descending diverging
- low pressure= ascending+converging
coriolis effect
- deflects any object that flies or flows from straight path
- due to earth’s rotation eastward
- deflects to the right in N. Hem; left in S. Hem
- maximum at poles; zero at equator
Pressure Gradient + Coriolis
coriolis effect prevents surface winds from moving along pressure gradient
acts in the opposite direction to P.G force in upper troposphere
produces geostrophic winds: travel parralel to isobars
Friction does what
reduces wind speed
reduces coriolis effect
upsets geostrophic wind flow
winds move across isobars at an angle
Anticyclone vs Cyclone
winds spiral outwards from high pressure area clockwise (anticyclone)
winds spiral inwards into low pressure areas counterclockwise (cyclone)``
Wind speed
steep pressure gradient = fast moving winds
gradual pressure gradient= slow moving winds
Buoyancy
the tendency of any object to rise in a fluid
warm air parcel= less dense than surrounding air RISES
cool air parcel=denser than surrounding air SINKS
stability
stable air is non-buoyant (resists vertical movement)
in atmosphere: cold air beneath warm air
- temperature inversion
- cold winter night
Instability
mass of air heated
- becomes unstable
- a warm summer afternoon or at the equator
- air rises until it reaches equilibrium level
Equilibrium level
altitude where density, temp= surrounding air
while rising, air cools adiabatically
Adiabatic cooling
rising air, expansion because of less pressure, molecules spread, less collisions, drop in temp of air parcel
Adiabatic warming
descending air, compression because of more pressure, molecules get closer, more collisions, rise in temperature of air parcel
what is a visual determinatiopn of stability
clouds
unstable air, rising air ans vertical clouds
cumulous clouds=unstable air
stratiform clouds= stable air forced to rise
cloudless sky = stable air that is immobile
stable air
- non buoyant
- remains immobile unless forced to rise
- clouds: stratiform, cirriform
- precip: drizzle
unstable air
- buoyant
- rises without outside force
- clouds: cumuloform
- precip: showery
Air mass
-a distinct parcel of air >1600 km across -homogeneous with respect to -temp, humidity, stability -source regions= where air masses originate
warm fornts
advancing warm air
gentle slope
warm air ascends over retreating colder air
air cools adiabatically; clouds and precip
gradual uplift, clouds and precip, can last for days
[pressure decreases, cirrus clouds, thicker and lower clouds, pressure decreases,air is warmer and humid when parcel passes.]
Cold fronts
advancing cold air
hugs ground: dense air
steep face because of friction
causes rapid uplifiting of warm air as cold air hits it
rapid uplfit makes warm air very unstable
blustery & violent weather
higher intensity and shorter duration
[high cirrus clouds 1/2 days before, temp drops, fall in pressure, after air passes the air is colder.]
Mid-latitude cyclones
westerly winds almost always move to east day to day weather changes migratory L-pressure systems air mass convergence 35-70 degrees lat
Northern hemisphere mid latitude cyclones
converging counter clock wise
cool air from N warm air from S
convergence of 2 air masses; 2 fronts
cold front [center to SW]
warm front [center to NE]
2 zones of clouds and precip
Mid latitude anticyclones
migratory H-pressure cell [w to e]
air subsiding, diverging, clockwise rotation
no fronts
weather clear and dry
summer= warm temps; winter= very cold
Hurricanes
tropical cyclones
very low pressure centres: steep pressure gradients outward from centre
strong winds spiraling inward
must be >119 km/h
Hurricanes vs. Mid-latitude cyclones
all warm air; no fronts
much smaller
eye where weather is calm
strongest winds at eye wall
Global atmospheric circulation
without rotation or even land and water distribution
- excess radiation at equator- creates low pressure belt around the world
- poles: high pressure
- surface winds would travel along pressure gradient
- air would rise at equator, flow toward poles, sink into polar highs
Hadley cells
warm air rises at equator
creates low pressure
ascends and cools
moves poleward
begins descending at 30* N and S
creates high pressure
flows back toward equator
Surface components of atmospheric circulation
- polar high
- polar easterlies
- subpolar low
- westerlies
- sub trop high
- trade winds
- inter trop convergence zone
subtropical highs
h pressure belt at 30*
huge anticyclones
develop from descending hadley cell air
weather warm, clear, calm little rain
desert locations
“horse latitude”
source of tradewinds and westerlies
Trade winds
equator-ward side od STHs
diverge toward W and toward equator
25N- 25S
easterly
warm, drying winds
capable of holding lots of moisture
don’t release moisture unless
intertropical convergence zone
air from 3 hemispheres meets
where NE and SW trades come together
doldrums
weak airflow erratic winds
L pressure belt circling globe
over oceans, defined cloud band
over continents, associated with T-storms
Westerlies
30-60 n and s
surface winds: all directions
upper atmosphere winds: geostrophic, prominently WE
polar front jet stream
sub trop jet stream
Polar front jet stream
9-12 km high in troposphere
basically WE but shifts regularyly
curves= rossby waves [separate cold polar air from warm tropical air]
influence on surface weather
Polar Highs
H pressure over both poles
antarctic high
-strong and persistent
arctic high
-less pronounces
anticyclonic air flow
air sinks into high pressure and diverges near surface
Polar easterlies
winds move EW
occupy area between polar high and 60*
cold and dry
Subpolar lows
l pressure at 50-60* n and s
rising air
clouds
precipitation
generally stormy conditions
Local winds that affect BC
sea and land breezes
tropical coastlines, midlatitudes in summer
day= sea breeze (coming from sea)
night= land breeze (coming from land)
During day, land warms up fast heats air above air expands, rises creates L pressure breezes blow in from water at night land cools more quickly relatively higher pressure over land
Valley breezes
day: radiation from land
mountain slope air heats up faster than valleys
heated air rises, creates L pressure, air flows upslope from H to L pressure
Mountain Breezes
night: mountain slopes lose heat rapidly
adjacent air chills and creates H pressure
air sinks downslope into valley
Chinook winds
steep pressure gradient
air flows from windward to leeward side
warm and dry has lost moisture
adiabatic warming
Orographic lifting
air forced upslope
if ascending air is cooled to dewpoint, precipitation results
as air moves down leeward side, it warms, rain stops
rain shadow= dry area on leeward side
El nino- southern oscillation (ENSO) and La Nina
[El Nino] warming of the surface equatorial ocean surface off the coast of southern america
[La Nina] conditions involve upwelling of cold water in the same region
In BC, winters following the oset of an El Nino event are generally warmer and drier normal and La Nina winters are generally cooler and wetter
It looks like we are experiencing an el Nino year in 2019
Pacific Decadal Oscillation (PDO)
-warm phase of the PDO is characterized by below-normal sea surface temperatures in the central and western north pacific and unusually high ones along the west coast of North America
positives PDO phases are associated with warmer temperatures throughout western canada and with less precipitation in the mountains and interior, which also reduce the snowpack
currently in a positive phase
