Precipitation Processes & Forms

Droplets in clouds Fall Speed

0.1 microns (smallest) 0.0001 cm./S

Cloud droplet 10-50 microns 1 cm./S

Rain droplet 1,000 microns 650 cm./S

Responsible for Cloud Growth

1.) Collision-Coalescence Process:

- warm clouds above 0oC

- collector droplet will collide with other droplets

- most often results in coalescence (merging)

2.) Bergeron Process:

- mid latitudes, cold clouds or cool-cold

- both droplets & ice crystals co-exist

- water is drawn to the ice crystal

- ice crystal gets more mobile

- “RIMING”-ice crystals collide with

super cooled droplets of

water which freeze onto them.

- “AGGREGATION”- two crystals, each with

a thin coating of liquid water,

may join to form a single larger crystal.

Precipitation Forms

1.) SNOW – frozen crystalline precipitation that remains in the ice stage throughout

descent. Takes different shapes depending on temperature & moisture. 50 microns to 5

mm.

Warm = (dense, large flakes)

Cold = (smallest, powder flakes)

Snow Belts

1.) Western Mountains

2.) Eastern Canada & NE States

3.) Great Lakes Snow Belts

LAKE EFFECT SNOW (ESSAY QUESTION)

TEMPERATURE

- Temp. difference between lake surface & overlaying air

- Best when air Temperatures are 15oC

– 25oC

cooler than lake surface.

- Dew point between the 850 mb level & surface 13oC

- (Greater the temperature difference the heavier the snow)

WIND

- Wind has to be over the lake to transport warm moist air to shore

- Minimum wind speed 5 m/Sec. for significant snow over land

- If wind is to strong, air isn’t over the lake long enough to take effect

DIRECTION

- Wind direction, downwind portion of the lake receives the most snowfall

- “FETCH” – distance crossing open lake

- “STABILITY” – effects mixing & convection

- RH determines amount of moisture required to saturate

- (Dry cold air requires long residence times over water)

EXTENT OF ICE COVERAGE

- Surface must be unfrozen & atleast 0oC

FETCH

- Wind must pass across atleast 80 km. (55 miles) of lake surface before lake

effect snow will form

SHORELINE CHARACTERISTICS

- Surface roughness of land (friction), enhances vertical uplift

- Orographic Enhancement, hills cause uplift resulting in more snow

2.) RAIN - precipitation arriving at the surface in the form of liquid drops.

- Ranges from 0.5 – 5 mm.

3.) GRAUPEL – ice crystals that have grown by rimming to produce a spongy, somewhat

translucent particle, which may contain an air bubble that gives a spongy texture.

- Ranges as great as 5 mm.

4.) HAIL – ice pellets formed in roughly concentric layers. Almost always associated with

thunderstorms (cumulonimbus clouds).

5.) SLEET – ice pellets resulting when raindrops freeze before reaching the surface. Forms

when raindrops fall in a layer of air that is below 0oC

and freezes before the ground.

(WARM FRONT)

6.) FREEZING RAIN – rain droplets freeze at surface after they fall below an inversion

and pass into air having a temperature below 0oC

. Freezes after it hits the ground.

Atmospheric Circulation & Wind Systems

Three –Cell Model

Hadley – developed original one cell model (1700’s).

Ferrel – a model with 3 distinct cells (1865).

3 TYPES OF CELLS

Hadley Cells

Ferrel Cells

Polar Cells

EXAM

Inter-tropical Convergence Zone (ITCZ) - At the equator, strong solar heating cause’s air to

expand and rise, creating a zone of low pressure.

Sub-tropical Highs (STHs) - The sinking air forms large areas of surface high pressure.

Trade Winds

Polar Easterlies- Surface air flowing out of the polar highs moves into the subpolar lows,

forming a belt of weak winds.

Westerlies- Zones of westerly winds flowing between the STHs and the subpolar lows.

Upper Level Trough – valleys of relatively low heights of a given pressure, with height

contours bending equator-ward.

Upper Level Ridge – upward bulges in heights of a given pressure, with height contours

bending pole-ward.

ROSSBY WAVES

- Normally move from east to west

- 3-7 may be found encircling the globe at a time

- WINTER- fewer, longer wavelengths, stronger winds

- SUMMER- more, shorter wavelengths, weaker winds

Summer Isotherms

High pressure troughs over NH oceans

Low pressure ridges over NH continents

High pressure ridges over SH oceans

Low pressure troughs over SH continents

Winter Isotherms

High pressure troughs over NH continents

Low pressure ridges over NH oceans

High pressure ridges over SH continents (but not greatly)

Low pressure troughs over SH Oceans

Zonal Flow – primarily westerly directions

(Flow pattern is flat).

Meridional Flow – pronounced troughs and ridges

(Large or high amplitudes in waves).

OCEAN CURRENTS

Horizontal movements of surface water, often found along the rims of major ocean basins.

-Warm or Cold currents may influence onshore air masses and bring warm or cold weather to

unlikely areas.

“UPWELLING” – offshore winds may push warmer surface water out to sea, allowing colder

water from below to rise upward.

WIND SYSTEMS

MONSOONS – seasonal reversal of surface winds. Classic in South Asia during summer.

‘Wet’ surface air flow is from the southern Himalayas towards the southwest, offshore into the

Indian Ocean. In winter, the flow reverses and the dry season starts.

FOEHN (Downslope) WINDS – ‘Alp term’, synoptic scale winds that flow down mountains

slopes and are warmed by compression, bringing warm, dry, clear conditions to the adjacent low

lands. Classic Foehn wind is created from a cyclone.

-In North America called “Chinooks”

-In California called “Santa Ana Winds”, unique to start fires

KATABATIC (Drainage) WINDS – consist of cold, dense air that flows downslope under the

influence of gravity. Originate over high elevation plateaus, which are ice caped/sheets.

-Classic in Antarctica, Greenland, and the Balkans

SEA (Lake and Land) BREEZE – local winds found along sea coasts and shores of large lakes.

Day – land heats up faster than the water, resulting Night- land cools up faster than the

in winds from water to land. water, resulting in winds from land

to water.

VALLEY & MOUNTAIN BREEZES – local winds associated with mountain sides & valleys.

DAY- mountain sides oriented to the sun, NIGHT- mountains cool off and air

create a weak ‘LOW PRESSURE’ as air expands. sinks into the valley.

AIR MASSES & FRONTS

Air Mass – a large body of air having little horizontal variation in temperature and moisture.

Typically several 1,000’s of km. apart. Temperature and humidity stay fairly constant

horizontally. Forms as it takes on characteristics imparted from surface weather conditions.

“SOURCE REGIONS”- produce air masses (high & low altitudes)

AIR MASS CLASSIFICATION:

Based on Moisture:

-Continental (Dry, Low RH) lowercase c (c)

-Maritime (Moist, High RH) lowercase m (m)

Based on Temperature:

-Tropical (Warm) uppercase t (T)

-Equatorial (Extremely Warm) uppercase e (E)

-Polar (Cold) uppercase p (P)

-Arctic (Extremely Cold) uppercase a (A)

Continental Polar (cP) – forms over large, high

latitude land masses such as northern Canada or Siberia.

Typically very stable, few clouds, and dry.

Arctic (A) – forms over arctic regions such as extreme

northern Canada & the Arctic Ocean in winter.

Typically cooler than cP air, dry, stable, and clear conditions.

Maritime Polar (mP) – forms over areas of high

latitude oceans, Northern Pacific & Atlantic Ocean.

Typically more moderate temperatures and more moist.

Continental Tropical (cT) – originates in summer over hot,

Low-latitude land areas, such as the SW U.S.A & N. Mexico.

Typically very hot & dry, often unstable and cloud free.

Maritime Tropical (mT) – develops over warm, tropical waters,

west tropical Atlantic & east tropical Pacific. Typically warm moist,

and unstable, clouds & precipitation.

FRONT – boundary between two differing air masses. Comes from the WWII-era,

zones of air masses clashing. Will often bring changes in weather, fairly drastic.

Provides uplift, which makes clouds / precipitations.

Cold Front – marks boundary between relatively cold air masses as it advances and displaces a

relatively warm air mass.

Warm Front – marks the boundary between relatively warm air as it advances into relatively

cold air.

“Overrunning” - Less-dense warm air will flow up over more-dense colder air which creates a

gently-sloping, wedge.

Stationary Front – a frontal boundary that is not moving, or moving very slowly, boundary will

consist of warm air over cold.

Occluded Front – a frontal boundary where warm air has been lifted off the ground. (Moving)

MID-LATITUDE WAVE CYCLONES (MLWC’s)

- Consist of 3 parts

1.) A Low Pressure

2.) A Trailing Cold Front (typically southwest)

3.) A leading warm front

Based on : Cyclogenesis, “Vlihelm Bjerknes” of the Norwegian Geographical Institute

ESSAY QUESTION

1.) Process begins along polar front, which separates the cold air of the Polar Easterlies from the

warmer air of the Westerlies.

2.) A minor “kink” in the front develops.Allows cold air to move south and warm air to move north.

Sets up a CCW rotational flow in the NH (CW in the SH), around a weak low pressure system (i.e.

a cyclone).

3.) With a little time, the system becomes better developed as low pressure deepens and distinct

warm and cold fronts emerge from original polar front. Clouds begin forming along frontal

boundaries in response to uplift and convergence. This is the mature stage of a midlatitude

wave cyclone (a low pressure center, a trailing cold front and a leading warm front).

4.) In the mature stage, cumuliform clouds will run along and ahead of cold front, created by

displacement of warm air by denser, colder air. Precipitation along cold front will be intense but

short-lived and generally increases towards the center of low (more convergence). As cold front

passes, wind velocity will increase and wind direction will shift from south (in the warm sector)

to north or northwest. Air temperatures and relative humidity will also drop after cold front has

passed.

Occlusion - Occlusion will typically occur in

later stages of life cycle of the MLWC.

Upper end of cold front catches the upper end

of warm front causing an occluded front to form.

This usually signals the end of the MLWC.

The entire life-cycle may last from one to two weeks.

PROCESSES OF THE MIDDLE AND UPPER TROPOSPHERE

Relative Vorticity – turning of air relative to the surface.

Earth Vorticity – imparted by earth’s rotation about its axis.

Absolute Vorticity – a combination of Earth & Relative Vorticity.

POSITIVE VORTICITY- counter-clockwise rotating, its relative vorticity results in higher

absolute vorticity.

NEGATIVE VORTICITY- clockwise rotating, its relative vorticity results in lower absolute

vorticity.

Rossby Waves account for;

- Formation

- Intensification

- Dissipation of MLWC’s

Convergence :(i.e. coming together), which results in downward (sinking) movement of air. On

the surface, air will be pushed downward and out, which results in an anticyclone at surface,

which is a warm core, or dynamic high (as opposed to a thermal high that results from sinking

cold air).

Divergence: (i.e. turning slowly = slowing down), air will be diverging (i.e. spreading out),

which draws air up from surface. On surface, air will be pulled upward, which results in a

surface cyclone (This is the low from the MLWC!), which is a dynamic, or cold core, low (as

opposed to a thermal low that results from increase heating).

MLWC’s 3 Major Conveyor Belts

A Warm Conveyor Belt originates near the surface in the warm sector of the MLWC. It

flows toward and over the warm front wedge, which leads to cloud formation and

precipitation. Eventually, the warm belt turns to the right (under the influence of Coriolis)

and joins the upper level westerly flow.

A Cold Conveyor Belt lies ahead (or north) of the warm front in the MLWC. It flows

from the east (moving to the west) along the surface toward the center of the cyclone.

Eventually, it ascends and turns CW (anticyclonically) and becomes incorporated into the

westerly flow aloft.

A Dry Conveyor Belt originates in the upper troposphere as part of the westerly flow

aloft. It brings the coldest air into the cyclone and maintains the strong temperature

contrast across the cold front. It also separates the cloud bands from the warm and cold

conveyor belts, which gives the clouds of the MLWC their distinctive comma- like shape.

LIGHTNING, THUNDER STORMS, and TORNADOES

LIGHTING – a discharge of electricity, which arcs between positive & negative electrical

charges that have become separated = Charge Separation.

- 80% of lighting occurs in clouds

- 20% of lighting is cloud to ground

1.) Positive charges (+) move toward the cooler TOP part of the cloud.

2.) After charge separation, a branch of negatively charged (-) air called a “STEPPED

LEADER” extends out of the base of the cloud. Moves 50 meters per microsecond.

3.) As the stepped-leader approaches the ground, a spark will surge upward from the surface

to the leader. When the two connect it completes the pathway for the flow of electrons,

which initiates a sequence of bright return strokes. It is 54,000oF

(5X the heat of the sun)

4.) Almost instantly another leader, called a “Dart Leader”, will emerge from the cloud and a

second stroke will quickly follow.

TYPE’s OF LIGHTNING

-Ball Lighting – round, glowing mass of electrified air.

-St. Elmo’s Fire – ionization of air surrounding tall surface objects (churches/Ship masts)

-Sprites – large, but short lived electrical burts that rise from tops of some thunderstorms. (1% of

all lighting events)

-Blue Jets – blue-colored, upward-moving lighting electrical ejections shot from the top of

clouds.

-Elves – an expolsive ring of lighting lasting a thousandth of a second. (300 miles across)

Thunder – when lightning occurs, the rapid increase in temperature causes air pathway to

expand explosively, which sends out bursts of sound waves.

Self-Extinguishing Thunderstorms – will extinguish themselves out as downdrafts, cutting off

the supply of moisture to the cloud.

Self-Propagating Thunderstorms - downdrafts will cause the storms to intensify and

propagate, creating severe thunderstorms.

3 STAGES of a THUNDERSTORM

CUMULUS – unstable air rises forming

cumulus clouds, starting vertical growth.

MATURE – begins as heavy rain, gives

downdrafts, produces cooling, produces

lighting and thunder.

DISSIPATED – downdrafts dominate clouds,

soon cutting moisture, and clear skies appear.

Mesoscale Convective Complexes (MCC’s) - Mesoscale Convective

Systems may appear as oval or roughly circular clusters of several

thunderstorms that are part of a single organized system.

Common to the U.S. & Canada, their downdrafts can lead to the

formation of new storms.

Squall Line – Mesoscale Convective Systems will occur as linear

bands. Reach 180 - 300 miles long. Found ahead and parallel of

a cold front in a linear band.

Supercell Storms – single, extremely power cell lasting

2 – 4 hours and as long as 12 – 30 miles. Occurs in isolated

storms (shown as a hook shape on radar).

Downburst – strong, potentially deadly downdraft from T-Storms which may exceed 165 mph.

Microburst – downdrafts that are less than 2.5 miles in area.

DISTRUBUTION of THUNDERSTORMS

Most common in;

‘ Florida’ & Gulf Coast

Rockies in Colorada, Wyoming, and New Mexico

TORNADOES – caused by rotation as a result of large vortices (mesocyclones) created by wind

shear that causes a rolling motion. Mainly develop in North America because of the perfect setup

of the north-south running mountains that allow cold and warm air masses to meet unhindered.

- Florida has the highest amount of Tornadoes.

- Mostly a Spring time event in the NH

- Tornado Ally (Mid West)

FUJITA SCALE

F-0 F-1 F-2 F-3 F-4 F-5 F-6

MHP <73 73-112 113-157 158-206 207-260 261-318 319<

HURRICANES

HURRICANES – (Atlantic & Eastern Pacific)

TYPHOONS – (Western Pacific)

CYCLONES – (Indian Ocean & Australia)

Western Pacific generates the most Hurricanes (Typhoons).

Form on the west side of ocean basins during late Summer & early Fall.

Most powerful of all storms

Sustained winds have to be atleast 74 MPH

Absence of strong vertical wind shear

Need unstable conditions

Need Coriolis force to move cyclonic rotation

Typically 350 miles wide & move 90 MPH

Fueled by deep warm ocean water (81oF

)

Increased sea surface temperature, may cause stronger hurricanes

Trade Wind Inversion - Sinking air heats adiabatically, but does not reach ground, therefore

creating a situation of warm air over cool air.

Eye = 12-30 miles diameter (Can reach 60 miles in diameter)

Eye Wall can produce 100 inches of rain a day / 4 inches per. Hour

Stronger on the right side in the NH, because the hurricane experiences less friction and more

time over water.

Tornadoes are also found, most likely in the right forward quadrant

1.) TROPICAL DISTURBANCE – start on the east sides of Ocean Basins as clusters of

disorganized thunderstorms. (Only 10 % make it as a Tropical Depression)

- EASTERLY WAVES - large undulations or ripples in normal trade winds.

- STREAMLINES – easterly waves, poleward bends in wind directions,

divergence aloft & convergence at surface of east axis.

2.) TROPICAL DEPRESSION – winds under 37 MPH

3.) TROPICAL STORM – winds 37 - 74 MPH

4.) HURRICANE – winds above 74 MPH

National Hurricane Center (NHC):

issues a WARNING within 24 hours

of a hurricane approaching land.

Storm Surge - A wall of water that moves inland, causing flooding in low-lying coastal areas,

complete inundation of buildings, and sometimes reversal of river flow. As much as 7 meters

high (23 feet)!

Caused by;

– Surface winds, which pile up and force water ahead of the storm and also generate

high waves.

– Low pressure within storm, which allows sea level to rise.

2005 Hurricane Season

27 storms, 15 full blown hurricanes, 7 major storms, Hurricane WILMA Category 5 (RECORD)