Meteorology Test 2

Stability, Forces, Fronts, Surface Maps & Isoplething, Upper Air Maps, Severe Weather Safety

parcels

independent bubble of air in the atmosphere

Behavior of parcels as they rise

Pressure changes?

Temperature changes?

Moisture changes?

DALR vs. MALR

Dry adiabatic lapse rate (DALR)

Rising unsaturated air parcels cool at the dry adiabatic lapse rate

9.8 °C/km

(= 5.5 °F/1,000 ft)

Moist adiabatic lapse rate (MALR)

Rising saturated air parcels cool at the moist adiabatic lapse rate

~6 °C/km

(= ~3.3 °F/1,000 ft)

Varies throughout the troposphere

Stability

the environment determines what behavior an air parcel exhibits

motion is dependent on the temperature of the parcel (Tp) compared to the temperature of the surrounding environment (Te)

Stable

Tp < Te

Parcel is more dense than surrounding environment (negatively buoyant)

Parcel will sink, clouds do not form

In the atmosphere, stable environments

cause air parcels to return to their

original starting elevation after being

lifted up by some force

often lead to clear blue skies

Unstable

Tp> Te

Parcel is less dense than surrounding environment (positively buoyant)

Parcel will continue to rise and accelerate upward, clouds do form

In the atmosphere, unstable

environments cause air parcels to

continue to move/accelerate upwards

after a lifting force is applied to them

This is the scenario that forms clouds!

Neutral

Tp = Te

Parcel has the same density as the surrounding environment

Parcel will not move, clouds do not form if not formed already

In the atmosphere, neutrally stable

environments allow for air parcels to halt

their motion and remain where they are

in elevation when the lifting force is no

longer applied

So, they don’t sink back to where they were they just stop

Adiabatic

closed

parcels are treated as closed systems

lapse rate

the rate at which temperature decreases with increasing height

lapse rates are positive when they indicate decreasing temperatures

Lifting Condensation Level (LCL)

the altitude at which temperature and dew point are equal (T = Td)

Air parcel becomes saturated

Above this altitude clouds form

Cloud bases mark the location of the LCL

why does MALR ≠ DALR

MALR < DALR because of latent heat release

When water vapor condenses into liquid

water, the molecules release some

energy as heat warming the air, and

counteracting the cooling due to rising

motion

This causes the air parcels cool less rapidly

moisture’s affect on density

For a constant (same) temperature, moist air is less dense than dry air

molecules are displaced from the air by the H2O that replaces them, moist air is less dense than dry air

air that is both warm and moist is the least dense type of air

air that is both cold and dry is the densest type of air

temperature inversion

when temperature warms with height

troposphere changes with height

on average troposphere temperature decreases with height but temperature is highly variable and can actually warm with height; this is called

a temperature inversion

weather balloons

released twice daily by the nws (morning and evening)

make rapid measurements of the atmosphere as they ascend through the troposphere (and even stratosphere) before the balloon pops

measures how temperature, pressure,

moisture, and other variables behave from

the surface as far up as the balloon rises

pushed along by the wind

Balloons can travel almost 200 miles from where they

are launched

Rawinsondes

attach to weather balloons

measure:

• thermometers (temperature)

• Hygrometers (humidity)

• Barometers (pressure)

• Anemometers (wind)

soundings

plots of atmospheric variables

with height

synthesized and plotted from weather balloon data

inversion layer stability

typically stable layers

Sometimes called a “cap” since they prevent air parcels from rising further

As the parcel rises and cools, it must stay

warmer than its environment in order to keep

rising

inversion inhibits cloud formation

weather stays in troposphere because stratosphere is typically stable layer

where do inversions form

at the top of the boundary layer during the daytime

atmospheric boundary layer

lowest ~1 km of the atmosphere The layer in closest contact with the ground

Prevents air parcels from rising high

enough/cooling enough to form clouds

Can also trap pollution

force

an interaction that causes a

substance (including the air) to change

speed and/or direction

a force will cause an object to

accelerate

types of forces

magnetic

gravitational

electrical

frictional

pressure gradient

coriolis

magnetic force

causes magnetized objects to repel or

attract one another

gravitational force

causes objects to fall down towards the center of the Earth

gradient

change ( increase or a decrease) of some quantity

over a distance

what a causes wind

Changes in the horizontal pressure

pressure gradients

Winds blow outward from high pressure, and

inward toward low pressure

High Pressure Center (H)

anticyclones

highest Mean Sea-Level Pressure

Low Pressure Center (L)

cyclones

lowest Mean Sea-Level Pressure

pressure gradient force (PGF)

the force directed from high pressure toward low

pressure in the presence of a pressure gradient

PGF is the fundamental driver of wind—it is

what causes the wind to blow

PGF is proportional to the difference in

pressure

low pressure

partial vacuum due to lack of air molecules

high pressure

abundance of air molecules

Air from high pressure centers flow towards

low pressure centers

what changes the wind

Coriolis force and frictional force

Coriolis effect causes the wind to deflect which way

in the NH wind deflects right

on the SH wind deflects left

caused by earth’s west to east (left to right) rotation

Coriolis effect

Earth’s west-to-east rotation causes

fluids (including the air) to deflect from

their original straight paths

frictional force

always acts opposite to an object’s direction of motion

acts to slow down the wind

where does frictional force act

primarily acts upon winds in the boundary layer

Friction force is too small in the free

atmosphere (above the boundary layer) to noticeably affect the wind

The lack of structures, buildings, landforms,

etc. allows the wind to flow free

which forces act on wind in the boundary layer

PGF: creates the wind

COR: deflects the wind

FR: slows the wind

frictional force decreases the magnitude of the Coriolis

force, but has no effect on the magnitude of

the PGF.

Circulation is not perfectly circular like it is in

the free atmosphere

which forces act on wind in the free atmosphere

PGF and COR

geostrophic balance

the balance between PGF and COR

leads to perfectly clockwise (“anticyclonic”; H) and

counter-clockwise (“cyclonic”; L) wind pattern

in the NH boundary layer, the balance of

PGF, COR, and FR creates surface winds

that flow:

Clockwise and outward from H

Counter-clockwise and in to L

convergence

wind blowing inward

Causes a net inflow of air

occurs at the surface as winds rush in towards a low pressure center

Air molecules pile up and have to go

somewhere

Can’t push downward into the Earth’s crust,

so they move upward instead

divergence

wind blowing outward

Causes a net outflow of air

occurs at the surface as winds approach a high pressure center:

◦ Air molecules go outward, and must be

replaced.

◦ This causes air to sink in from above

convergence causes

Rising motion through the depth of the

troposphere that happens above all surface

low pressure centers

Rising motion --> cooling and moistening -->

clouds and precipitation

divergence causes

Sinking motion through the depth of

the troposphere happens above all

surface high pressure centers

Sinking motion --> warming and drying -->

clear skies

Low pressure areas: where surface winds

converge, and air is rising

ITCZ, polar front

High pressure areas: where surface winds

diverge and air is sinking

Horse latitudes, at the poles

why is displacement limited to the troposphere

Rising air does not penetrate into the stratosphere because the stratosphere is very stable

air does not sink from the stratosphere above a surface low pressure center

jet stream

a long, narrow corridor of extremely strong winds near the top of the troposphere

◦ Length: 1000-3000 miles

◦ Width: 200-300 miles

◦ Depth: 1-3 miles

• speeds: 55-250mph

encircle the globe

Winds must be AT LEAST 55 mph in order to be considered a jet stream

exist due to strong temperature gradients

Warm air to the south is separated from the cold air to the north

where do jet streams form

above the boundaries between warm and cold air masses

These boundaries are most pronounced

between the Horse Latitudes and Polar Fronts

two primary jet streams

Polar jet stream: above 60 °N and S

Subtropical jet stream: above 30 °N and S

where are jet speed streams the strongest

where the surface temperature gradients are the strongest

temperature gradient between Ferrel and

Polar cells > temperature gradient between

Ferrel and Hadley cells.

Polar jet is usually much stronger than

subtropical jet

why are jet streams not straight

they meander north and south to line up with the greatest surface temperature contrast

In the NH, tend to occur further south in the

winter and further north in the summer

where is divergence aloft typically strongest

on the eastern side of a southward “dip” in the jet stream

Influences the creation of surface low pressure centers in these locations

how is the jet stream connected to the location of pressure centers

The winds in the jet stream converge above

high pressure centers.

The winds in the jet stream diverge above low

pressure centers

The winds in the jet stream converge above

high pressure centers

• Jet stream winds slow down in certain locations.

• Air converges where the slowing occurs, and must go somewhere.

• This causes the air to sink downward

The winds in the jet stream diverge above low

pressure centers

• Jet stream winds speed up in certain locations.

• Air diverges where the increase in speed occurs, and must be replaced from below.

• This forces the air to rise

Low pressure is “attached” to the

enhanced divergence aloft in the jet

stream, so it moves west-to-east with it

advection

the transport of any atmospheric property (e.g., temperature, moisture, pollution) by the wind

Advected atmospheric property moves in the

same direction as the wind

Surface winds advect air masses away from

their source regions, creating the weather

Northerly and northwesterly surface winds

advect cold, dry cP air southward from Canada

Westerly and southwesterly surface winds

advect warm, dry cT air from the Rocky Mountains, Desert Southwest, and Mexico

Southerly and southeasterly surface winds

advect warm, moist mT air northward from

the Gulf

easterly winds

Air mass (mT) in the Atlantic is blocked by the

Appalachians.

winds are rarely ever easterly.

advection in the nothern hemisphere

Surface winds blow clockwise and outward around high pressure centers

Surface winds blow counterclockwise

and inward around low pressure centers

These winds also advect air masses into

regions

fronts

boundaries between air masses of different temperatures

Areas of large temperature gradients

how do fronts form

air masses are advected away from their source regions and come close to one another

where are fronts attached

attached to low pressure centers, and not attached to

high pressure centers

Surface winds diverge outward from high

pressure centers, push air masses away from

each other. This decreases the temperature

gradient.

Surface winds converge into low pressure

centers, which works to push air masses

toward each other. This enhances the

gradients and creates the frontal boundaries

The type of front is determined by

wind direction

Cold front: wind blowing from cold to warm

Warm front: wind blowing from warm to cold

cold front

cold air mass advancing toward warm air mass.

Front marks the boundary between the cold and warm air

Location noted as a blue line with blue triangles pointing toward warmer air.

Cold fronts typically extend N-S, and are

associated with westerly or northerly

winds behind the front

warm front

warm air mass advancing toward cold air mass.

Front marks the boundary between the warm

and cold air.

Depicted as a red line with red half circles pointing toward colder air.

Warm fronts typically extend E-W, and

are associated with southerly winds

behind the front

warm front charcacteristics

Warm air is less dense than cold air, and

rises above the cold air.

Rising motion is very gradual.

Gradual rising motion leads to widespread,

light precipitation.

Precipitation occurs on the cold side of the front.

Storms can form along warm fronts, but are

less likely.

signs warm front has passed overhead

Increase in T and Td

Wind shift to southerly winds

Skies change from cloudy to clear

signs cold front has passed overhead

Decrease in T and Td

Wind shift to westerly or northerly winds

Clouds thicken overhead, possibly in a linear formation with embedded storms

cold front characteristics

Cold air is more dense than warm air, and

pushes the warm air up and over it.

Rising motion is rapid.

Sudden rising motion leads to heavy rain

and intense storms.

Precipitation occurs on the warm side of the

front.

Lines of heavy storms (squall lines) are very

common.

stationary front

a motionless boundary that persists between warm and cold air

Neither warm nor cold air is advancing

Noted by a blue and red dashed line,

with blue triangles pointing toward

warmer air on the blue dashes and red

circles toward colder air on the red

dashes.

Precipitation can still form here!

Warm air still rising up and over the cold air.

Midlatitude Cyclone

weather system that covers thousands of miles and can bring the midlatitudes hazardous/impactful weather

Parent system to snowstorms and severe

thunderstorms

Tend to form along the polar jet, which then

steers the cyclone from west to east

parent driver of fronts

midlatitude cyclones

midlatitude cyclone formation

The winds associated with low pressure systems drive the frontal boundaries around the low. This causes the front to move with the system.

The enhanced temperature contrasts in close proximity to the linked surface and upper level

systems intensify the low pressure.

This feedback creates a Midlatitude Cyclone

sectors of midlatitude cyclone system

Cold sector

Warm sector

Cool sector

Entire system can be over 1,000 miles across

Span a large portion of the continent

cold sector

the cold air mass behind the cold front

Contains the coldest air in the entire midlatitude cyclone

air is usually cold and dry( cP)

warm sector

the warm air mass in between cold and warm fronts.

Contains the warmest air in the entire midlatitude cyclone

air is usually warm and moist (mT)

cool sector

the cool or cold air mass north of the

warm front

Air is cold, and can be moist (mP) or dry (cP)

wind shift

abrupt change in wind direction over a short distance

surface winds converge along a front

Cold front: change from S/SW to N/NW as

you go east to west

Warm front: winds change from E to S/SE as

you go north to south

sector behavior

Cold air is dense, so it advances more rapidly

toward the warm air than the warm air moves in

the warm sector

o Cold front quickly progresses counterclockwise

around the low

o Cold front “catches up to” the warm front

◦ Lifts some of the less dense warm air upward

◦ Warm air loses contact with the ground

◦ Forms occluded front

◦ Occluded front: purple line with purple half circles

and triangles

o Low pressure center can no longer access the

warm, moist air in the warm sector

o The midlatitude cyclone decays

What forms between air masses of different moisture characteristics

drylines

They are still areas of enhanced rising

motion

The air masses they separate still have very

different densities

Areas of clouds, precipitation, and storms

dryline

boundary between a warm and

moist air mass and a warm and dry air mass

Because drylines don’t separate air masses of

different temperatures, they are not classified as

fronts

represented by orange line with closely

spaced half circles pointing towards

moist air

The passage of a dryline is marked by

Decrease in Td (and therefore, RH)

Wind shift from southerly to westerly

triple point

point where the dryline intersects any component of a midlatitude cyclone

Cold front, warm front, occluded front, low

pressure center...

Most commonly the cold front

Low level jet (LLJ)

narrow tube of extremely strong winds

Unlike the polar and subtropical jets, the LLJ

forms near the top of the boundary layer

o Winds are typically southerly (rather than

westerly)

◦ From the Gulf into the Southern Plains

◦ Advects warm, moist air

o As with other jets, the LLJ separates air

masses

◦ Cold air on its left, warm air on its right

Most commonly forms in the warmer

months of spring and summer

o Warm, moist air advected northward

rises and saturates easily to form clouds,

precipitation, and storms

LLJ most commonly forms in the

Southern Plains and extends through the

Central Plains

◦ Occasionally reaches up into the Northern

Plains

o Typically forms overnight

◦ Begins to form around sunset

◦ Reaches maximum strength around 2 AM LT

◦ Weakens and disappears around sunrise

Sources: map of continental US

Although the LLJ forms overnight, can

be enhanced and even sustained into the

daytime by the low pressure center in a

midlatitude cyclone

◦ PGF draws winds towards the low pressure

center, enhancing/sustaining the LLJ

◦ LLJ typically flows into the warm sector just

ahead of the cold front

◦ Warm air to the right, cold air to the left

◦ Advects warm, moist air into the warm sector

◦ This low density air can be forced to rise extremely

quickly by the cold front

◦ Storms!

Surface weather stations measure

measure weather conditions at the surface

-Temperature

- Pressure

- Humidity

- Wind speed and direction

- Cloud cover

- Current weather

- Precipitation type

- Amount of rain that has fallen

surface plots

compact, standardized symbols used to report surface weather observations

surface plot symbols

Upper right: mean sea level pressure (MSLP)

Upper left: temperature (°F)

Middle: cloud cover

Middle left: weather symbol

Lower left: dewpoint (°F)

wind barb

If surface MSLP starts with a 0, 1, 2, or 3 (0-3)

Put a 10 in front of the entire number

◦ Put a decimal point in front of the last number

◦ Units: mb

◦ Example: a reading of “018” would be 1001.8 mb

If surface MSLP starts with any other number

Put a 9 in front of the entire number

◦ Put a decimal point in front of the last number

◦ Units: mb

◦ Example” a reading of “979” would be 997.9 mb

The shading of the circle in the middle

denotes

the fraction of cloud cover

◦ Usually referred to as “octas” meaning

“eighths.”

◦ Symbol on the right: 6/8 octas, or the sky is ¾

covered by clouds

symbol between temperature and dewpoint

Current type of weather

Sometimes this information is given,

sometimes it is not—it depends on whether or

not the current weather was reported