Chapter-4: Fronts and Jets Flashcards
Define Synoptic
seen at the same time/together “simultaneous observations”
Midlatitude is also known as
Extra-tropics
Define midlatitude
between 23o27’ N and 66o33’N and between 23o27’ S and 66o33’ S latitude, or, the earth’s temperate zones between the tropics and the arctic and antarctic polar regions
important weather in the midlatitude
fronts and extratropical cyclones
front is defined as
a transition zone between two air masses of different temperatures/densities.
These atmospheric fronts are
hree-dimensional zones
These atmospheric fronts are three-dimensional zones that represent:
- a rapid transition of the thermal field
- marked shift in the wind direction
- a significant transition in the moisture field
- an increase in the magnitude of the wind
The thermal gradient is usually largest at ………………………………..and weakens as the …………………………….
the Earth’s surface
zone slopes upward
The transition zone may extend over a distance of………………………………….., and a typical ……………………………………..ratio is ……………….
1000 km or more along the Earth’s surface
cross-front to along-front
1:10
a front is:
…………………… and …………………… (………km)
long and wide (100 km)
frontal zones:
Polar front
Arctic front
mP stands for:
Maritime polar
mT stands for:
Maritime tropical
warm, moist
cT stands for:
Continental tropical
hot, dry
cA stands for:
Continental arctic
very cold, dry
cP stands for:
Continental Polar
cold, dry
The polar front boundary:
eparates warm, humid air to the south from cold polar air to the north, extends upward to over 5 km.
arctic front:
separates cold air from extremely cold arctic air, is much more shallow than the polar front and only extends upward to an altitude of about one or two kilometers.
Fronts are observed at ………………….in ……………………………………….and are associated with………………………………………………..
all longitudes
the extratropical latitude belt
most of the significant weather events.
A jet is
an intense, narrow, quasihorizontal or horizontal current of wind that is associated with strong vertical shear.
Atmospheric jet streams dimention
thousands of kilometers long, a few hundred kilometers wide, and only a few kilometers thick.
Wind speeds in the central core of a jet stream often
exceed 100 knots and occasionally exceed 200 knots.
Jet streams are usually found at the ……………………………at elevations between ………………………….., although they may occur at …………………………………….
tropopause
10 and 15 km
both higher and lower altitudes.
subtropical jet stream
the jet stream situated near 30° latitude at about 13 km above the subtropical high
polar front jet stream is also known as
polar jet stream
polar front jet stream
jet stream situated at about 10 km (33,000 ft) near the polar front
Both fronts and jets are usually marked by
a concentration of isotherms (strong temperature gradient) and strong vertical wind shear
Fronts and jets are …………………..phenomenon because………………………………………….
hybrid
each is characterized by two different horizontal scales that differ by as much as an order of magnitude.
If the length of a front or jet is on the order of …………………….., then the Rossby number is usually reasonably small for flow along the front or jet so that ……………………………………………
1000 km
geostrophic balance is approximately maintained across but not along the front or jet.
…………………………………………………………………are associated with jets and jet streaks through the ………………………………………, and hence a dynamical explanation of fronts also explains jets.
Long, narrow zones of strong temperature gradient
thermal wind relation
Long, narrow zones of strong temperature gradient are associated with jets and jet streaks through the thermal wind relation, and hence a dynamical explanation of fronts also explains jets.
This, of course, may not be true for
smaller scale fronts and jets
a theory that explains fronts may also explain jets. ……………………. and………………………are often discussed together as……………………………….
Upper-level fronts and jets
jet-front systems
Types of Fronts
- cold front
- warm front
- Quasi-stationery front
A front is classified based on
the movement relative to the warm and cold air masses
Cold Front
- A cold front is one that moves in a direction in which cold air displaces warm air at the surface.
- In other words the cold (or cooler) air mass is moving toward a warmer air mass.
- The cooler, denser air is sliding under the warmer, less dense air displacing it upward.
Draw “cold front”
Warm Front
- A warm front is one along which warmer air replaces colder air.
- In this case, a warmer air mass is moving toward a cooler retreating air mass.
- The warmer, less dense air moves only toward and replaces the colder, denser air if the colder air mass is also moving.
Draw “warm front”
Quasi-Stationery Front
- This type front is one along which no one air mass appreciably replace the other.
- These fronts are stationary or nearly so (speed under 5 knots). They can move or undulate toward either the cold or warm air mass.
- If the air masses on both sides of the boundary line are moving parallel to each other, the front will not move.
Draw “Quasi-Stationery Front”
Occluded Front
one where a cold front overtakes a warm front, forcing the warm air upward.
The occluded front may be
either a warm front or a cold front type
warm front type
one in which the colder air behind the cold front overrides the cold air in advance of the warm front, resulting in a cold front aloft.
Cold front type
one in which the cold air behind the cold front under rides the warm front, resulting in a warm front aloft.
Draw “occluded front”
To describe the frontal general characteristics, we consider
the horizontal and vertical distribution of three weather elements (temperature, wind, and pressure) in a frontal zone.
Typical fronts always consist of
warm air above cold air
The frontal zone slope ………………………………………………………………………………..
upwards over colder air mass as a relatively narrow layer where the normal decrease of temperature with height, is reversed (temperature inversion).
The frontal zone slope upwards over colder air mass as a relatively narrow layer where the normal decrease of temperature with height, is reversed (temperature inversion).
This temperature inversion is called
frontal inversion
A cold front generally shows a ………………………………… than a …………………………..
stronger inversion
warm front.
Draw a graph of the frontal characteristics
Frontal characteristics
- temperature
- winds
- veering
- backing
- pressure
describe frontal characteristic “wind”
A front is associated with wind shifts in a cyclonic direction (veering) at the surface. This is true with the passage of all frontal types.
The wind direction also shifts in the vertical with ………………….in two ways:
height
Veering and backing
Veering:
- Veering is a change in wind direction, clockwise in the Northern Hemisphere. Ex: wind changes from southwest to northwest.
- Veering generally occurs through a warm front.
Backing
- If the wind direction changes in anticlockwise direction, it is called backing. Ex: wind changes from northwest to southwest.
- Backing occurs with height through a cold front.
One of the important characteristics of all fronts is that (pressure)
on both sides of a front the pressure is higher than at the front.
Fronts are associated with
troughs of low pressure
A trough is
an elongated area of relatively low pressure. A trough may have U-shaped or V-shaped isobars.
Friction causes the wind near the ground to
drift across the isobars toward lower pressure.
Friction causes the wind near the ground to drift across the isobars toward lower pressure.
This causes a
cross-isobaric flow towards the front from both sides, resulting in convergence and rising motion in the region of a front.
This causes a cross-isobaric flow towards the front from both sides, resulting in convergence and rising motion in the region of a front.
This is an important characteristic of fronts, since
the lifting of the air causes condensation, clouds, and weather.
Draw Vg after friction force
Draw wind veering at the surface of a cold and warm front
Another important characteristic of fronts is their
slope
Another important characteristic of fronts is their slope, with which the ………………………………………….. are associated
weather, intensity of weather and the movement of fronts
The slope is the
ratio of the vertical rise to horizontal distance
A slope of 1:50 (1 mile vertically for every 50 miles horizontally) would be
considered a
steep slope
and a slope of 1:300 a
gradual slope
Factors favoring a steep slope are:
- a large wind velocity difference between air masses
- large temperature difference and
- high latitude
Because cold air tends to run under warm air, the steeper the slope, the
more intense the lifting and vertical motion of the warm air and, therefore
the more intense the weather
Across the frontal zone, there is a
stronger horizontal temperature gradient
Across the frontal zone, there is a stronger horizontal temperature
gradient, which is strongest near
the surface, and weakens with increasing altitude
According to the thermal-wind relationship
the geostrophic wind will increase with height in strong horizontal temperature gradients.
If the frontal zone extends vertically over a large portion of the troposphere, then the wind speed
will continue to increase with height, reaching a maximum near the tropopause
jet streams are associated with
frontal zones.
The jet blows parallel to the
frontal zone
The jet blows parallel to the frontal zone, with greatest wind speeds on
the warm side of the frontal zone
