Modern Day Satellite PhotosGeostationary Operational Environmental

Satellite (GOES) ProgramOperated by National Weather Service (NWS)

Weather Service began as part of the War Department

September 1, 1872

http://weather.msfc.nasa.gov/GOES/goeseastconuswv.html

http://www.cdc.noaa.gov/cgi-bin/db_search/SearchMenus.pl

Cloud charts, Thermometers, Wet Bulb shoelaces, Humidity charts. pinwheels

Density falls off with altitude; pressure is caused by impacts, dense air has more impacts

Buoyancy Lifting

Start Local Heat Air Expands Dense AirJiggles

Underneath(heated atoms

speed uppaddle board

analogy)forces warm

air up

Atoms close together (dense, high pressure) “fall into” the empty (low pressure) area

Pressure differences cause WIND

Wind is caused by differences in pressureWind is caused by differences in pressure

Pressure and Density of air are two aspects Pressure and Density of air are two aspects of the same thingof the same thing

Atoms close together (in dense, high Atoms close together (in dense, high pressure areas ) “fall into” the empty (low pressure areas ) “fall into” the empty (low pressure) areapressure) area

WINDWIND

The greater the contrast in pressure the faster the wind.One atmosphere (pressure at surface) is about 1000 millibars These red isobars are lines of equal pressure

HL

Pressure Gradients

Coriolis turning

Initially wind flows from high to lowbut Coriolis turns it nearly parallel to lines of equal pressure (isobars)

Winds blowing parallel to isobars are called geostrophic windsThis occurs well above the surface where there is no friction

Notice pressure gradient force always from high to low pressure, but Coriolis perpendicular to actual flow direction

Winds blowing parallel to isobars are called geostrophic winds

Winds Aloft, maybe 3 km upWind speed feathers on the “from” end. Winds named for the “from” direction

Friction turns surface winds back toward the pressure gradient.Near the surface, winds almost move from High to Low pressureThey spiral counterclockwise into a Low in Northern Hemisphere

Near the surface, winds almost move from High to Low pressure

They spiral counterclockwise into a Low in Northern Hemisphere

L

L lowest local pressure

Blue arrows PGF

Red arrows Coriolis

Black arrows wind path

Ferrel Cell

Polar Cell

Hadley Cell

Since earth’s rotation turns air, flows are broken into 3 cells per hemisphere

Horizontal temperature differences

Temperature effects density and pressure:

500 mb

700 mb

850 mb

Warm Cold Psurface

If you heat something it expands and gets less denseA 500 millibar pressure level is much higher in hot air. Hotter air has lower density and greater volume

1000 mb

P = R’ T so T = P /R’

Polar Jet FormationPolar Jet Formation

Steep gradients of Steep gradients of Pressure cause Pressure cause higher velocity higher velocity geostrophic geostrophic winds.winds.

This is the trigger This is the trigger for jet stream for jet stream flow.flow.

LOW _____________________HIGH

For the Polar Jet, the eddy is in the Ferrel cell on the upper polar side, and so air flows from the west to the east, the “Westerlies”

Since the pressure difference is great at the boundary, the jet is a very fast wind

Polar air is denser, so it wedges under the low density warm air. Rotation causes an eddy to form

Air MassesAir Masses

An air mass develops when the atmosphere An air mass develops when the atmosphere is located above a relatively uniform land or is located above a relatively uniform land or water surface for several days. water surface for several days.

The lower atmosphere assimilates some of The lower atmosphere assimilates some of the properties of the underlying surface.the properties of the underlying surface.

Air MassesAir Masses

Large regions (1,000s kmLarge regions (1,000s km22) of the lower ) of the lower troposphere with uniform characteristics troposphere with uniform characteristics (temperature, moisture content) originally (temperature, moisture content) originally defined by a source area.defined by a source area.

Labels refer to temperature (arctic vs. Labels refer to temperature (arctic vs. polar vs. tropical) and source area polar vs. tropical) and source area (continental vs. maritime). The source (continental vs. maritime). The source area determines the moisture content.area determines the moisture content.

Labels around here: cT cP mT mPLabels around here: cT cP mT mP

http://www.met.tamu.edu/class/Metr304/Dir-surface/surface.html~

Polar air enlarges and moves further south during Polar air enlarges and moves further south during winter and retreats northward during summerwinter and retreats northward during summer..

Heating (cP air moving south) will lead to instability (bouyancy) as warms.

Cooling (mT air movingnorth) has the opposite effect, because colder air cannot rise

Orographic Lifting forces maritime mP and mT air upward over mountain ranges in the western U.S., leading to condensation and precipitation that converts the formerly humid air to a much dryer air mass.

Typical Air Mass Changes

Air Mass Interactions

• Weather in any region is influenced by the interactions between air masses. Recall that the boundaries between contrasting air masses are called FRONTS

cP

mT

cPmT

Heavy rains can result from the interaction between the continental polar cP air mass and the maritime

tropical mT air mass

ExtraTropical Cyclones are up to ExtraTropical Cyclones are up to 2,000 km across & 2,000 km across & control the weather control the weather for three days to a week.for three days to a week.

Winds circulate counter-clockwise (CCW) Winds circulate counter-clockwise (CCW) around the Low in Northern Hemis.around the Low in Northern Hemis.

Mid-latitude Cyclones

L

Def: Synoptic Scale: space and timescales of

mid-latitude depressions i.e. several thousand kilometers and

timescales of several days

0. Maximumheat from

Sun

7. Mid-Latitude Cyclones are dominant where Ferrel meets Polar

1. ITCZLOW

HIGH

LOW

HIGH

2.

3. to E

4. Ferrel CellMid-Latitude Cyclones northDry south

5. 6. Gyre

8. Cold air sinks

Polar Cell

Dishpan Experiment

Camera rotates with dishCold Center hot edgeLarge scale waves and eddies formThey flow CW & CCW

Rossby Waves

Why are the boundaries between cold and hot not straight?Cold mass is denser and forces its way under the warmer mass

Mid-Latitude Cyclones

This clash between cP and mT air masses is the most common source of frontal systems in the U.S.

Weather conditions, and cloud types, change in a predictable sequence as warm and cold fronts pass over an area.

A front is a transition from one air mass air mass to another

http://www.met.tamu.edu/class/Metr304/Dir-surface/surface.html~

cP mT

DBZ is Decibels received back

cP

mT

Map Symbols These show surface positions

Warm Front• Ahead of a warm front, warm, humid air is

transported upward over a distance of approximately 1,000 km (625 miles).

• Rain may last longer than for a cold front because the warm front moves slowly and extends over a larger area.

Dangerous storms if cold air is very cold

Warm Front Clouds and Winds• First sign of warm front is sequence of clouds

(cirrus, cirrostratus, altostratus). • If winds are right, up to 12 hours after the cirrus,

the higher clouds will be replaced by lower nimbostratus with associated light to moderate precipitation.

• Temperatures and humidity rise and winds typically shift direction (first from the south or southeast, then from the southwest) with the passage of the warm front.

Warm Front Changes

Expect increasing Temperature and Humidity as the warm front passes

Cold Frontchanges

Cold, dense continental polar air cP replaces moist, warm maritime tropical air mT across a cold front

Expect decreasing temperature and humidity and increasing atmospheric pressure with the passage of the cold front.

Warm Air Rises

Warm wet air has lowest density, so it will always rise over cooler air.

Both the cold and warm fronts are inclined toward the warm air mass.

Warm air is pushed up and over the advancing cold front, causingrelatively rapid cooling and condensation that results in thedevelopment of tall cumulonimbus clouds. They host heavy butrelatively short-lived precipitation

Approaching Cold Frontis behind Thunderstorms

Cold Front cloud bands are narrowbecause cold front wedges are steep

http://www.emc.ncep.noaa.gov/mmb/gmanikin/nas125/tstorm/squall1.gif

Cold Fronts are narrowCold Fronts are narrowbecause the edge of the because the edge of the cold air mass is steepcold air mass is steep

A Cold Front Squall Line

Rapidly advancing cold fronts may be marked by the growth of a squall line of thunderclouds

Cyclogenesis 1 – Stationary Front A small scale wave forms

mT

cP

Mid-Latitude Cyclones start as Stationary Fronts.

These persist if the air masses have equal pressureThen many small waves form, and storms are very frequent

Cyclogenesis 2 – Warm and Cold Fronts

cP

mTIn the Ferrell cell, winds have a strong westerly component, and storms move East.As they mature and move, Warm Fronts pass over, followed by Cold Fronts.

Cyclogenesis 3 – Occluded Front forms

Cold front catches warm, forcing warm air aloft. Broad precipitation area results.

cP

mT

Where the cold front catches up, they are called Occluded Fronts. Note symbol.

Occluded Fronts• The cold front is faster than a warm front and will

eventually close the gap between the fronts, forcing the intervening warm air upward generating additional rain

Nimbostratus

cP

mT

Rain then covers a wide area

Storm evolution

1. Surface map, so winds crossIsobars and spiral into the LOW

2. Wide precipitation bandmT

cP cPmT

Along the occluded front warm moist air is force aloft, resulting in a broad band of rain

Thunderstorms

Thunderstorms form where warm, humid air is forced upward to altitudes of up to 15 km (20 for supercells).

Condensation occurs as the air cools, releasing latent heat and ensuring that the rising air remains unstable (warmer so less dense than surrounding air).

Rising Warm air overshoots into stratosphere

Prevailing Wind Aloft

Freezing Line

CondensationLine

Surface

All downdraftsDroplets coalesce

15 minutesOnly updrafts.

No rain.

Rain and Ice too heavy for updrafts Lasts 15 to 30 minutes

Rain, plus gusty winds caused by downdrafts

Type 1: Isolated Thunderstorms from unstable (hot, moist, low density) air

Lightning

• Most dangerous frequently encountered weather hazard that people commonly experience each year.

• Second most frequent weather killer in the United States with nearly 100 deaths and 500 injuries each year,

after floods and flash floods

http://www.nssl.noaa.gov

+ marks concentrations

of rising and falling ice

Ice often positive

Lightning equalizes large charge differences between storm levels and the ground

2. Synoptically Forced Thunderstorms

Most of the cloud formation associated with a cold front is actually in the warm moist air mass

Supercells Severe thunderstorms, or supercells, are

associated with frontal lifting along the cold front between the continental polar and maritime tropical air masses in mid-latitude cyclones. The lower portion of these storms, the mesocyclone, rotates. They often contain severe hail and sometimes tornadoes.

Most common during spring and early

summer, when the contrast in temperatures and moisture between air masses is greatest.

Tropopause

Tropopause

2. Strong updrafts pull in more air from below.

3. The strong updrafts lift falling ice and it gets covered with another layer of ice. If this happens enough times, large hailstones are the result.

Hail Formation

1. Condensation heats the moist air, which accelerates upwards

Supercell Hail

Tornadoes Funnel clouds that rotate at speeds

of up to 500 km/hr beneath supercells.

Ranked from F0 (weakest) to F5 (strongest) using the Fujita Intensity scale.

Most move to the east or northeast at an average speed of approximately 50 km/hr.

Develop in association with mesocyclones

Wind Shear (different wind directions at different altitudes)causes rolling

Updrafts can pick up the roll

Jet Stream winds result from large pressure gradients at the boundary between mT and cP air, where a large difference in Tropopause height exists.

N

The jet stream and tornadoes: pressure differences in the extreme

The Jet Stream

A strong tornado often is associated with rapid

removal of updraft air by the Jet Stream aloft

Tornados are narrow areas of extremely

fast updrafts

The Dry LineThe Dry LinecT

cP

mT

Tornado Conditions

Jet Stream Aloft

Potential for wind shear with hot dry unstable cTSimilar temp but different density than mT which is moist and therefore “light”

Dry, so dense

Multiple vortices in an F5

May 3, 1999

Stecker

Supercell tops overshoot into stratosphere

A Stecker, Oklahoma Home

http://www.nssl.noaa.gov/teams/swat/Cases/990503/A_images/stecker.gif

“Only” F3

May 4th, 1999

A Perfect Hook

Hurricanes &Typhoons (Tropical Cyclones)

When extremely hot ocean surface temperatures (>26oC) cause hot, moist surface air, huge clusters of thunderstorms develop at sea. If uplift gets extreme, these can organize into a gigantic Low with spiral storm lines, and winds exceeding 74 mph, a Tropical Cyclone, aka Hurricane

Hurricanes are fueled by Latent Heat of Condensation release.One day equals the energy production of US for a year

Jeanne

Hurricanes need hot Hurricanes need hot moist air as fuel. This moist air as fuel. This

is why they weaken is why they weaken over landover land

Katrina

Storm Surge

Trapped in House, swept away

Storm Surge

Freshwater (rain) floods cause most

fatalities