Week five Flashcards
Pressure Gradient Force (PGF)
Areas of high and low pressure occur because of uneven heating of the Earth’s surface
Implication: Convection loops
- Created by unequal heating and cooling of the surface
- Pressure gradient controls horizontal air movement
- Air temperature controls vertical air movement
Coriolis effect
- Is an apparent force caused by the Earth’s rotation
- Object in motion on the surface always appear to be deflected to the right in the N Hemisphere
Coriolis “force”
Coriolis force is always perpendicular to the wind
As wind speed increases, Coriolis force increases to reach a maximum
magnitude when its equal to the pressure gradient force
C.F=PGF then wind blows parallel to the isobars with low pressure to the
left if you are in the Northern hemisphere
Geostrophic Wind
With no friction, upper atmospheric winds are geostrophic
- In other words, upper level winds run parallel to the isobars, with low pressure to the left when you are unfacing the wind in the Northern hemisphere
Friction Force
Friction from the surface slows wind speed and combines with the other forces to influence wind direction
The effect of friction extends to about 500m above the surface.
It explains the difference between surface winds and upper level winds.
Frictions slows down wind.
C.F<PGF so the wind does not blow parallel to the isobars
A low pressure centre
A low-pressure area, low or depression, is a region where the atmospheric pressure is lower than that of surrounding locations.
(see flash card)
A high pressure centre
Is a region where the atmospheric pressure is higher than that of surrounding locations.
(see flash card)
A high pressure centre
Is a region where the atmospheric pressure is higher than that of surrounding locations.
(see flash card)
Cyclones and anti-cyclones
Cyclones spin in opposite directions in the Northern and Southern Hemispheres due to the Coriolis effect
Example: Sea and land breeze
See breeze: afternoon wind that beings cool air off the water towards the land
Land breeze: nighttime wind that brings cooler air from land towards the water
Global surface patterns
1. Equatorial Low-Pressure Trough
- High insolation warms equatorial air; air rises and
produces a zone of low pressure - air from both hemispheres flows toward the surface
low and moves aloft as part of the Hadley cell (and
is then drawn poleward by the pressure gradient) - this area of convergence is known as the
intertropical convergence zone (ITCZ)
- Subtropical High-Pressure Belts
- air in descending part of Hadley cell is heated by
compression and becomes drier - the surface air diverging from the subtropical highs
generate Earth’s principle surface winds: - Mid-latitude westerlies
- Tropical trade winds
- Like the ITCZ, the high-pressure cells “follow” the
Sun, migrating by 5-10° in latitude
- Sub-polar Low-Pressure Cells
- dominant in the winter, weaker in the summer
- the contrast between cold, dry air masses and warm,
moist air masses forms the polar front - low-pressure cyclones are created by cooling and
condensation of warm air forced aloft
- Polar High-Pressure Cells
- Weak high-pressure cells with little energy
- Winds move away from the polar region in an
anti-cyclonic direction producing the polar easterlies
Upper Atmospheric Circulation
Two important facts explain upper level winds:
* Pressure decreases less rapidly with height in warmer air
* Differential heating causes air to flow poleward
Therefore, the pressure gradient is greater at altitude and winds tend to be stronger.
Jet streams
Jet streams are narrow, high speed upper level air flows.
The jet stream is a river of fast moving
air in the upper atmosphere. It steers storms and moves
weather patterns across the globe.
Jet streams and climate change
Jet stream is driven partly by the temperature differential between the
Arctic and the mid-latitudes.
If the temperature differential is large then the jet stream speeds up (zonal flow).
If the temperature differential reduces because of a warming Arctic then the jet stream weakens (trough/ridge) flow.
When the jet stream goes up (ridge) we see warm and dry weather. When the jet stream takes dip (trough) we see cooler and wetter weather.
Rossby waves
The undulations of the upper-level westerly winds
Seen from space, Rossby waves have about half the rotational speed of Earth.
Water Cycle and Precipitation
Hydrologic cycle
- Movement of water among the great global reservoirs
States of water
- A change of state from solid to liquid or liquid to gas absorbs latent heat from the surroundings
- That means atent heat is released when clouds from (condensation) and absorbed when water evaporates from the plant’s surface
Condensation releases energy
Humidity
- The measure of the water vapour in the air
- The maximum quantity of moisture that air can hold increases with the air temperature
Relative humidity = Actual water vapour X 100 / Maximum water vapour possible at that air temperature
Relative Humidity
- Imagine temperatures increasing over the day. The relative humidity would decline even if the amount of water vapour In the air stayed the same. This is because warmer air cab hold more water
When it is 100% it means that air cannot hold more water particles
Dew point temperature
The temperature at which a mass of air holds its maximum capacity of water vapour.
What do we use coasters?
- When cooling air reaches the dew point temperature condensation begins, forming dew or frost.
- End up with frost under 0 degrees
Example: Dew point and humidity
Relative humidity is often lower during the day
- T <Td and Td > 0 degrees C then dew formation
- T<Td, and Td<0 degrees C then frozen dew formations
Adiabatic Process
- An adiabatic process is a change in temperature within a gas that occurs only as a result of a change in pressure.
- This is important in studying weather and the motion of air because atmospheric pressure decreases with altitude.
- If you lift a parcel of air away from the surface, the pressure on the parcel decreases
- In an adiabatic process, there is no exchange of heat with the surrounding environment.
Vertical air motion
- Imagine there is an area of low pressure or convergence at the surface, causing air to rise
- Will it warm and contract or be cold and expand
Fog
A cloud layer close to the surface, formed when the air and dew point temperatures are nearly identical
Radiation fog
This forms when a surface cools the air directly above, often form at night.
A valley fog, in which cold air descends down a valley, is like a
radiation fog.
Advection fog
This forms when air migrates to a place where the conditions allow saturation to occur. This happens along some coastlines and is called a sea
fog. (where?)
Forms of precipitation
Once we have lifted air and generated a cloud, precipitation is possible.
Outside of the tropics, almost all precipitation begins as snow or ice.
The precipitation form on the surface depends on the temperature profile or the environmental lapse rate
Atmospheric Stability
Stability is the tendency of an air parcel, either to remain in place or change vertical position
Stable = resists vertical movement, returns to starting place if disturbed
Unstable = continues to rise until reaching an altitude where air has similar density
Absolute instability:
- Air parcel is warmer than the surrounding environment
- ELR>DALR>MALR
- Clouds with vertical development form (cumuliform)
- It is associated with thunderstorms and precipitation
Absolute stability:
Air parcel is cooler than the surrounding environment
- ELR<MALR<DALR
- Clouds with poor vertical development may form (stratiform)
- It is associated with calm weather
Making it rain
A parcel of air may be rise through convergence (a low), convection (local heating) a collision of air masses, or a physical barrier
