Download Lecture PowerPoint

Fronts
Definition (Glossary of Meteorology)
Front: …generally the interface or transition zone between two
air masses of different density
Air Mass: …a widespread body of air that is approximately
homogeneous in its horizontal extent, particularly with reference
to temperature and moisture distribution…
Where’s the front?
(easy question)
What kind of front
is on this map?
(tougher question)
How can you tell
from the data
what kind of front
it is? (really tough
question – or maybe
not so tough)
This map has every major kind of front analyzed somewhere on it.
What are the frontal types and their symbols?
Example of a very powerful front. What makes it
strong? (Use the data and the definition : a “transition zone between two
air masses of different density”)
Where is the front on
this one?
Why does this
warm front have
gaps?
Moderate
cold front
Typical symbols on
a weather map.
Note the cross
section goes
through the cold
front.
Viewed along the
cross section,
these are typical
clouds and precip
types.
At a station, this is
the sequence you’d
expect with this
cold frontal
passage (front moving
east to west)
Similar to the cold front,
this is a typical weather
map for a warm front.
Warm front cross
sections have stratoform
clouds and steady precip.
Here’s a typical weather
sequence if the front is
moving from south to
north.
(Show time lapse FromClrtoLowovcst
Doesn’t exist in the real atmosphere
Much more realistic
All fronts are located in troughs of low pressure. This is because of the
converging winds necessary to create the air mass change.
Because of their convergent winds, Lows will have fronts. Highs will not. You
can have Lows without fronts and fronts without Lows.
The dashed orange lines are pressure troughs. They could become fronts
if they strengthen.
Spectacular front passes Oneonta 18Z on Mar 31, 1987.
Here’s the temperature
drop.
At what time did that cold front reach Oneonta?
Dewpoint trace
Same time, of course
Before you see the pressure trace, what do you
expect to see, given what you know now about
fronts?
Pressure falls
before front arrives
Rapid pressure rise after front passes
This is not as powerful a front as Mar 13, 1987. It’s typical of a strong situation.
At 00Z there is a
squall line
preceding the
front. Note the
high temperatures
and south winds.
At 06Z, frontal
passage is
occurring in
central NY with
showers (on radar)
but temperatures
haven’t fallen yet.
By 12Z, winds
have shifted and
the temperature
is falling.
Frontolysis as the front gets near the Gulf of Mexico. Step through this.
This front undergoes frontolysis and becomes a
pressure trough.
The Norwegian Cyclone model
didn’t account for this but it
happens and has effects on the
weather
We can calculate the strength of the fronts using the logic of
Margules. For that we need to look at the weather observations.
The slope of a
front is one way to
assess its strength
There is obviously a front here. To calculate its slope, you need the data
plotted at the stations. I added Oneonta using our observations.
Here’s a small part of the 18Z surface map from Sept 11, 2014.
Consider Albany to be in the light air and Oneonta to be just across
the front in the dense air.
Here is Margules’ formula:
Assume the mean latitude is 42.5°N and g = 9.8 m/s. Use the wind shown at
Oneonta (4 kts) as an estimate of the Geostrophic wind normal to the front.
Estimate anything else you need from the observations. Calculate the angle this
front made with the ground.
Front
We know f = 2 sin() =
2 x 7.292 x 10-5 s-1 x sin(42.5°)
So f = 9.85 x 10-5 s-1
g = 9.8 m s-2
Estimate the mean temperature to be 69.57°F = 20.87 °C = 294.0 K
(no ° sign for Kelvins)
Estimate TD – TL = -1°F = -0.56°K (don’t convert as this is a temperature difference)
Estimate the wind difference to be 4 knots = 2 m s-1 (Why knows why?)
So, Tan  =
- (9.85 x 10-5 s-1 x 294.0 K) x
9.8 m s-2
So  = .605°
( 2 m s-1 )
-0.56K
Does that seem small?
What could one do with that number?
=.0105543 (no units)
(Based, in part, on Margules)
Next: Stability