Study Guide Flashcards
Pressure
The force per unit area (Force / Area)
Density
mass per unit volume (mass / volume)
Hydrostatic balance
The change in pressure with height is equal to the weight of the fluid
How do liquid barometers work?
As the atmospheric pressure increases, more force is placed on the reservoir of mercury, which in turn forces the mercury to a higher level of the tube.
Along the length of the barometer tube are millimeter markings, allowing for an easy reading of the pressure at any given time
Geostrophic balance
The most fundamental horizontal force-balance arises when the PGF is counterbalanced by the Coriolis force
Balance exists because the Earth turns and causes the Coriolis force
PGF always pushes from ______________________
higher toward lower pressure
Gradient balance
The three-way balance of horizontal pressure gradient, Coriolis force, and centrifugal force
Guldberg-Mohn balance
The ability of friction to slow the wind and therefore weaken the Coriolis force
Divergence
Surface pressure drops when there is divergence of the wind in the column aof air above the low
the horizontal spreading out of air
Convergence
Occurs when air near the surface flows together from different directions.
When the air near the round converges, or is squeezed together, it causes upward motion
Lifted index
A stability index
Index starts with an air parcel from the surface, lifting and cooling it dry adiabatically to saturation and then lifting and cooling it moist adiabiatically to 500 millibars.
The temperature of the parcel at 500 =mb is subtracted from the environment’s temperature at 500mb. If the observed 500-mb temperature is colder than the lifted air parcel, then the parcel is unstable and will be able to keep on rising and form a tall cumulonimmbus cloud => negative
Negative values of the LI can be related to the potential for thunderstorm severity
0 => -3 air is marginally unstable and unlikely to lead to severe thunderstorms
- 3 => -6 moderately unstable conditions
- 6 => -9 very unstable regions
LI values less than -9 reflect extreme instability
* >0 => thunderstorms unlikely without strong lifting mechnamism
Eveolution of a single cell thunderstorm
Cumulus stage - initial stage of a thunderstorm; warm air near the ground rises and cools initially at the dry adiabatic lapse rate. The rising air parcel approaches saturation as the relative humidity increases, until condensation occurs
Mature stage - begins when precipitation starts to fall from the cloud. The thunderstorm produces the most lightning, rain, and even small hail. The updrafts in the cumulonimbus become organized and strong, providing the vertical motion needed for coud-droplet growth
Dissipating stage - Occurs when the updraft, which provides the required moisture for cloud development, beings to weaken and collapse. Downdraft dominates the updraft and the cumuonimbus begins to disappear
Multicell Thunderstorms
Composed of several individual single-cell storms, each one at a different stage of development: cumulus, mature, and dissipating
Key difference: prescence of moderate amounts of vertical wind shear
Supercell Thunerstorms
A large single-cell storm
Produce one or more of the following: strong wind gusts, large hail, dangerous lightning, and tornadoes
Development of a supercell requires a very unstable atompshere and strong vertical wind shear
Often, wind direction at the surface is southerly, whereas the winds aloft are much stronger and from the west
Vertical wind shear in multicell thunderstorms
Multicell storm has a moderate amount of vertical wind shear
This shear tilts the thunderstorm and prevents the precipitation from falling into the updraft and quenching it, as happens in the single-cell thunderstorm. This allows the updraft and downdraft to co-exist.
The dense, cold air of the downdraft forms the gust front, which helps to lift the warm, moist air flowing toward the storm and then form new cells
Vetical wind shear in Supercell thunderstorms
Supercell requires a strong vertical wind shear
Vertical wind shear causes supercell thunderstorms to rotate around a vertical axis
The vertical updraft inside the thunderstorm then tilts this spinning air so that it spins in the vertical
Types of Severe Weather
Tornadoes, lightning, flooding, hail, and high winds
Conditions necessary for tornado formation
Develop underneath supercell thunderstorms
Sea Breeze
Steady wind blowing in from the water that is a result of the uneven heating during the daytime between the land and the adjacent water.
Stability
A measure of the liklihood that a physical system will remain unchanged after it is perturbed
Stability of the Atmosphere
Refers to the liklihood that a parcel will:
- return to its origin (stable)
- accelerate away from its origin (unstable)
- be at equilibrium with its environment (neutral)
Stability of the Atmosphere key:
Compare a parcel’s temperature with the temperature of the environment into which it’s lifted
If the parcel is unsaturated, then as it is lifted, it will be _______________
colder than its surroundings
Absolutely stable
Environmental lapse rate is less than 6 degrees C/1000m
Absolutely Unstable
Environmental Lapse Rate is greater than 10 degrees Celsius / 1000m
Conditionally Unstable
Enviornmental Lapse Rate is less than 10 degrees C/1000m but greater than 6 degrees C/1000m
Stabilty depends on where the parcel is
Condionally Unstable
If the parcel is unsaturated, then as it is lifted it will be __________ than its surroundings
colder
Conditionally unstable
If the parcel is saturated, then as it is lifted it will be _____________ than its surroundings
warmers
Stable
Few clouds, light cumulus humilus, clear skies
Often no level of free convection (LFC)
Shallow to moderately deep layer of conditional unstability
Cumulus congestus, patchy clouds, and sometimes breezy
Deep layer of conditional instability
Cumulonimbus, thunderstorms possible, possible severe weather
Single-cell thunderstorm has 3 stages
- Cumulus - parcels ascend in the updraft and get saturated at the lifting condensation level, LCL, which marks cloud base
- Mature - begins when precipitation starts to fall
- time of most lightning, rain, small hail
- a downdraft develops with cooling due to evaporation precipitation - Dissipating - updraft weakens, downdraft dominates
Lifting Condensation Level
As a parcel of air rises, it cools.
The LCL, or cloud base, occurs where the actual and dewpoint temperatures of that parcel are equal
Level of free convection
When parcel’s temperature becomes warmer than the enviornment, it freely convects: rises on its own
Level of Free Convection
level at which parcel’s temperature first becomes warmer than its enviornment
Above 6km, the moist adiabatic lapse rate is closer to ___________
8 degrees/km,
Parcel continues to cool as it rises, and eventually becomes colder than the environment
Cloud Top
Level at which parcel becomes cooler than its surroundings
LI =
T(enviornment) - T(air parcel) at 500mb
Lifted Index
A way to describe stability with one number
Is a difference in temperature at 500 mb
Negative values are unstable
3 stages of a single-cell thunderstorm
- cumulus stage
- mature stage
- dissipating stage
How can a thunderstorm grow in intensity?
Need to cancel the “shutoff” mechanism for a single-cell thunderstorm
Get rid of evaporation (saturated enviornment)
- This sometimes occurs in hurricane genesis
Move the precipitation away from the updraft
- vertical wind shear
Vertical Wind Shear
More severe thunderstorms can develop when there is vertical shear of the horizontal wind
- The westerly (west to east) part of the wind increasing as height increases
- Clockwise turning of the direction from which the wind blows as height increases
Air-mass/ordinary single-cell
“Popcorn” in visible satellite image
Small vertical wind shear
Chance of severe weather: unlikely
Multicell
MCC: state-sized circular cloud in infared sattellite image
Squall line: line of thunderstorms in radar or satellite images
Vertical Wind Shear: Small, moderate
Chance of Severe Weather: Likely,
Supercell
Hook echo in radar reflectivity image
Vertical Wind Shear: Large
Chance of Severe Weather: very large
Multicell Thunderstorms
Composed of several individual single-cell storms, each one at a different stage of development
- can last several hours
- can produce severe weather
Moderate amount of vertical wind shear
- Wind shear moves downdraft away from updraft
- new cells originate along gust front
Supercell Thunderstorm Characteristics
Strong wind gusts, large hail, dangerous lightning and tornado
Require a very unstable atmosphere
Requires strong wind shear
Microbursts
Develop when rain falling from a thunderstorm evaporates underneath the cloud, cooling the air beneath
– Cold heavy air plunges to the surface and splashes
against the ground
– Air then rushes sideways and swirls upward as a
result of the pressure gradient between the cold
air and the warm surroundings
*can do as much damage as a tornado
Characteristics of a tornado
Around very narow regions of low pressure beneath a thunderstorm
Visible because of condensation, dust, and debris
If the circulation does not hit the ground, called a funnel cloud
Usualy < 1.6km across
Velocity
magnitude and direction of motion
Speed
magnitude of velocity
Acceleration
Chance in velocity
*can be a change in speed and/or direction
Net Force
= Mass x Acceleration
Sum of all forces acting on an object
Gravity
Mass x gravitational acceleration
*always directed downward
**equals mass times 9.8 m/s^2
Friction
Acts opposite to direction of motion
Strength is proportional to speed
Pressure Gradient Force
Change in pressure over a distance
- always directed toward lower pressure
Why doesnt air fall down?
Net force must be zero or air would accelerate upwards or downwars
Hydrostatic balance
Gravitational force balances Pressure Gradient Force
Pressure Gradient Force
Equals change in pressure over a given distance (height)
Always pushes toward lower pressure
Hydrostatic equation:
= -p x g x ∆z
Pressure changes _______________ in the vertical than in the horizontal direction
much more
Trough
Elongated region of low height
Ridge
Elongated region of high heights
Winds:
Tend to blow parallel to the lines of constant height
Pressure Gradient Force
The force that results from pressure differences over distances in a fluid
PGF always directed from high to low pressure
= pressure change / distance
The PGF:
Points (locally) towards lower pressure (from high toward low pressure)
Is larger where isobars (contour lines) are closely spaced
Is perpendicular to contours of constant pressure / height
Friction
Coriolis Force
Apparent force that exists because we live on a rotating reference frame
- Deflects wind to the right in the NH, to the left in the SH
- is the strongest at the poles, and zero at the equator
- is stronger for stronger winds, and weaker for weaker winds
- is 0 for calm, it cannot start wind
Winds aloft (above about 1km)
Pressure Gradient Force
Coriolis Force
No friction
Winds near the surface
Pressure Gradient Force
Coriolis force
friction
Air spirals ____________ around a low in the northern hemisphere
Counter-clockwise
Cyclonic
counter clockwise movement
Anticyclonic
Clockwise motion
Divergence
The spreading out of air / fluid
Convergence
The piling up of air / fluid
Trough
Cyclonic (PGF > CF)
Ridge:
Anticyclonic (CF > PGF)
Jet Streams
Thin “Rivers” of air with winds exceeding 100kt
(sometimes 200kt) found at ~ 10-15km elevation
Thermal Wind
A horizontal change in temperature causes
wind speed to increase with height.
Thermal Wind
Relates temperature and winds to each other
The winds are more westerly as you go up wherever
it’s colder toward the poles
Sea Breeze
Daytime circulation
Depends on differenAal heaAng at the surface
between land and water
Has the warmer, rising air column over the land,
which absorbs more incoming solar radiaAon
• Has the cooler, sinking air column over the water,
which absorbs less radiaAon
Monsoons
Monsoons are weather features driven by
seasonal differences in the heating of land
and ocean
– Indian summer monsoon has cooler air over
water, heated air over land, upslope onshore
wind and generation of clouds and
precipitation—wet season
– Indian summer monsoon has return flow aloft
from land to water and sinking air over the
Arabian Sea and the Bay of Bengal
General circulation
what conditions (e.g. winds, pressure, precipitation, temperature) look like if we average over long periods of time, and over large areas
Air mass
A large body of air whose properties of temperature and moisture content (humidity) are similar in any horizontal direction
Source regions have similar characteristics
Light winds
A uniform surface over a large region
Not coastline
Air mases can:
move away from their source regions
Polar (P)
Formed poleward of 60 degrees
Cold or cool
Arctic (A)
Formed over the arctic
very cold
Tropical (T)
Formed within about 30 degrees of the equator
hot or warm
Continental (C)
Formed over large land masses
Dry
Maritime (m)
Formed over the oceans
moist
Continental polar (Cp)
Winter: very cold and dry
Summer: Cool and dry
Maritime tropical
Winter: warm and humid
Summer: warn and humid
cP
Cold, dry days
“Polar front” (sits over us during winter)
Cold surges
Lake effect snows
Lake Effect Snow
• cP air (cold, dry) blows over relatively warm water
• Air gains moisture / heat => more buoyant near
surface => unstable
• Downstream, clouds and snow (hills and
convergence on leeward side help with lifting)
mT: Maritime Tropical
mT: Warm, moist air from tropical regions. Pacific (Hawaii), Gulf of Mexico
Midwest: low pressure stalls over central / western US =>
warm moist flow from Gulf => warm, humid conditions
Front
Transition zone between two air masses of different
densities (usually due to temperature differences)
Cold Fronts
Denoted by blue line with triangles pointing in direction of
movement
Cold, dry air is replacing (relatively) warm, moist air
Cold Fronts
Sharp temperature gradient across front
Changes in moisture content (dewpoint)
Wind shift / pressure minimum
Clouds andPrecipitation
Warn fronts
Denoted by red lines with semi-circles pointing in
direction of movement.
Warm, moist air is overriding more stationary, cold, dry air
Occluded fronts
Denoted by purple linewith alternating triangles and semicircles pointing in direction of movement
Cold front “catches up” with a warm front
Extra-tropical (Mid-latitude) Cyclones:
Weather phenomenon characterized by surface low,
~1000-2000 km horizontal scale, warm / cold fronts, and
associated weather.
Extra-tropical (Mid-latitude) Cyclones:
(i) Begin as a disturbance, some times on a
stationary front
(ii) Develop into “open wave”
(iii) Intensify, begin to occlude (maximum intensity)
(iv) Dissipates (dies off) after occlusion
Key ingredients for cyclogenesis
– Surface temperature gradients, a front
– A strong jet stream, helps the low deepen and the
fronts intensify
– Presence of mountains or other surface
boundaries like a coastline near a warm ocean
current
– Winds blowing across temperature gradients
Alberta Clipper
– Develop over western Canada, move southeastward to
Great Lakes
Nor-easter
Develop over Gulf Stream – pummel N.E. U.S
If a system tilts westward with height =>
it will strengten
Temperature advection
– Cold advection under an upper level trough causes the
trough to intensify.
– Warm advection under an upper level ridge causes the
ridge to intensify.
Upper level divergence / convergence
– Divergence above a surface low causes the low to deepen
– Convergence above a surface high causes the high to
strengthen
Total Effect
Cold advection beneath trough => trough intensifies
Warm advection downstream of trough => ridge develops
NET EFFECT: TROUGH CURVATURE INCREASES
Vertically stacked cyclone
low pressure aloft is directly above low pressure at the surface
Divergence aloft (above a low)
Loss of mass (above a low)
Low pressure deepens
Air tends to __________ upstream of a trough
converge
Air tends to ___________ downstream of a trough
diverge
Cold advection beneath trough =>
trough intensifies
Warm advection downstream of trough =>
ridge develops
If divergence aloft > surface convergence
Loss of mass
Low intensifies
If convergence aloft > surface divergence
Accumulation of mass
High intensifies
Vorticity
‘spin’ of air around its vertical axies
Stretching:
increases relative vorticity
Squashing
Decreases relative vorticity
Absolute vorticity =
Relative + Planetary
Cold advection under an upper level trough causes:
trough to intensify
Warm advection under an upper level ridge causes:
the ridge to intensify
Sea Level Pressure
The atmospheric pressure at mean sea level
All surface barometric pressure readings are adjusted to sea level to remove the effect of altitude on pressure
Geostrophic balance
An equilibrium achieved when the horizontal pressure gradient and Coriolis forces push equallly in opposite directions
westerly wind
A wind that is coming from the west
Eastward ocean current
A current that moves toward the east
Cp
Winter: very cold and dry
Summer: cool and dry
mT
Winter: warm and humid
Summer: warn amd humid
Lake-effect snow is more common in the __________
fall
*as cold cP air mas moves over a warm body of water, there is a rapid exchange of heat and moisture. The lowest layer of the air mass warms and moistens, increasing the instability of the air mass. If the temperature difference between the air and water is large, rapid evaporation occurs
