1. Weather

2. Overview
Air Masses
mP, cP, mT, cT
Lifting Mechanisms
convectional, convergent, orographic,
frontal
Mid-Latitude Cyclonic Systems
Severe Weather
Thunderstorms
Tornadoes
Tropical Cyclones

3. Weather
Weather vs. Climate
Weather refers to atmospheric conditions at any
given place and time
Climate is a long term average of rainfall and
temperature patterns
Weather is the product of different air masses
coming into contact through pressure induced
movement, such that atmospheric instability
is produced through lifting.
Air masses
Lifting mechanisms
Pressure systems

4. Air Masses
The characteristics of air are influenced
by the region of origin
latitude (temperature)
A, AA – Arctic, Antarctic
P – Polar
Mid-latitudes receive air masses from higher
and lower latitudes
T- Tropical
E- Equatorial
surface, either oceans or continents
(moisture)
m – maritime
c- continental

5. For North American weather, important air
masses include:
cP (continental polar): cold, dry, stable air
mP (maritime polar): cold, moist, unstable
mT (maritime tropical): warm, moist, variable
stability
mT Gulf/Atlantic – warmer, very unstable (Gulf Stream)
mT Pacific – less moist, stable to conditionally unstable
(cold ocean currents)

8. Influence of mT Gulf/Atlantic air mass on US rainfall

9. Lifting Mechanisms
For instability, adiabatic cooling,
saturation, condensation, cloud
formation and precipitation to occur, air
must rise.
Convectional Lifting
Convergent Lifting
Orographic Lifting
Frontal Lifting

10. Convectional Lifting
Land masses heat up more quickly than oceans
when maritime air masses cross onto land, they experience
surface heating
creates low pressure, and air rises
Convectional lifting common over
Urban heat islands
surfaces with low albedo

11. Convergent Lifting
Winds converge on an area of low pressure
Collision of winds force air upwards
Common in the ITCZ
Produces a high amount of precipitation

12. Orographic Lifting
If a pressure gradient crosses a
mountain range, wind flow is forced
over the mountains
Wind travels upslope (windward side), then
drops back downslope (leeward side)
Cooling, saturation, condensation, cloud
formation and precipitation on windward
side
Removes moisture from the air
Dry wind drops down leeward side,
warming as it drops
Rain shadow

14. For islands, orographic rainfall is the primary source of rainfall.
During times of high prevailing winds, a definite rain shadow
effect is present. One side of the island can experience a tropical
rainforest climate while the other experiences tropical monsoon
patterns. Atolls suffer especially, since the highest point of
elevation rarely exceeds 5 m above sea level.

15. Frontal Lifting
The leading edge of an advancing air mass is called
a front.
Cold Fronts
Cold, stable air advances into warm air mass, causing
abrupt and rapid uplifting and lowering of pressure
Vertically developed clouds, thunderstorms
Warm Fronts
Warm air advances over cold air, less abrupt lifting
Form a variety of stratiform clouds at various altitudes
Warm air creates an inversion layer for cold air beneath
With cyclonic systems:
Stationary fronts: warm and cold fronts meet head-on
Occluded fronts: cold front overtakes warm front in
cyclonic system

19. Midlatitude Cyclonic Systems
The movement and collision of air masses
create moving centers of low pressure
Tend to move from West to East, creating paths
called storm tracks
Cyclonic rotation of air around the low brings cold
air masses from the north and west and warm air
masses from the south
Creates a rotation of warm and cold fronts around the
low pressure center
Cold front tends to move faster than warm front, so the
fronts eventually become occluded
This pattern is called a Midlatitude Cyclone or
a Wave Cyclone

20. Cycle
Storm tracks
Higher latitudes during the summer
Greatest frontal activity during the Spring
Cyclogenesis
Form on polar front, eastern slopes of mountains
Intensification of low pressure, commencement of
rotation
Open Stage
Cold air masses drawn from North and West, Warm air
from the South
Occluded Stage
Cold front overtakes warm front, forming an occluded
front
Dissolving Stage
Lifting energy is spent, weather patterns dissipate

29. Severe Weather
Thunderstorms
Occur wherever there is pronounced lifting by any of the
mechanisms
Common along cold fronts
Rapid condensation liberates large quantities of heat energy
Thunderstorms produce
Lightning
charge separation due to ice crystals at top layer of cloud
Positive charges migrate to top of cloud, electrons to bottom and ground
Hail
Strong updrafts hold rain drops aloft, recirculating them and freezing
them
Atmospheric Turbulence
Incoming gusts from convergent winds
Outgoing downdrafts pulled along by falling precipitation

34. Tornadoes
As faster upper level winds overtake slower
surface winds, horizontal-axis turbulence
develops
This horizontal rotating air mass can be lifted
by the updrafts of thunderstorms into a
vertical orientation
Accelerated by the cyclonic uplift of the storm
(positive feedback)
Forms a mesocyclone
Smaller turbulent rotations within the mesocyclone
form Funnel Clouds
A tornado occurs when a funnel cloud
reaches the ground
Fujita Scale (F scale)

37. Tropical Cyclones
Form in the Tropics
Coriolis force is insufficient near the equator for cyclone
formation
Energy comes from hot ocean water
Low pressure centers migrate across oceans
Winds converge, uplift, reach tropopause, diverge
Divergence aloft accelerates updraft, decreases pressure
If a jet stream is overhead, further acceleration occurs
Heat energy released by condensation further fuels
process
Extreme drop in pressure causes a steep pressure
gradient, converging winds accelerate to high speeds,
and a tight, spiral structure is formed

41. Structure
Eye
An area of calm at the
center
Eye wall
Band of intense
thunderstorms
surrounding the eye
Most intense activity
Rain bands
alternating zones of
rainfall in spiral
structure
Storm surge
Low pressure raises
sea level below center