Download Weather Systems - Red Hook Central Schools

High and Low Pressure
• High Pressure
– Colder denser air
– Drier air
– Fair weather
High and Low Pressure
• Low Pressure
– Warmer less dense air
– humid air (more water vapor)
– Wet, cloudy weather
High and Low pressure systems appear like this
on a weather map, but what are they really like?
The spacing of the isobars determines the gradient
HIGH Pressure Systems are associated with
the following weather characteristics:
1. cooler denser air
2. dry, low moisture content air
3. a clockwise flow or spin (anticyclone)
4. clear skies and fair weather
5. diverging air at ground level
LOW Pressure Systems are associated with
the following weather characteristics:
1. warmer air
2. humid, high moisture content air
3. a counterclockwise flow or spin (cyclone)
4. cloudy skies and wet weather
5. converging air at ground level
Highs and
Lows
Lows spin
counterclockwise
Highs spin
clockwise
Coriolis Effect
causes winds to
turn to the right
in the northern
hemishere
H
X
When highs are located
north or west of our area
it pushes cool dry air over us
Red Hook
Note the wind flow in our
region for a high located
above the Great Lakes
This is known as
A Bermuda high
H
When highs are located
south and east of us, it
pushes warm humid air
over us
Lows are a bit more complex and are often associated
with fronts
Also, most highs and lows are
started by bends in the upper air
flow known as the jet stream
H
L
An upward bend or ridge
produces anticyclonic
flow or a high
A dip or trough in the jet
stream produces cyclonic
flow or a low
Let’s now look at how a typical low pressure system (cyclone)
develops in North America
Stage 1 – a stationary front
Stage 2 – a wave develops from trough in jet stream
Stage 3 – Cyclonic circulation established
A distinctive warm and cold front have formed
Warm sector
Stage 4 – Occlusion begins and warm sector starts to get
pinched off (zipper effect)
Stage 5 - Occluded front develops – warm sector nearly gone
Occluded front side view
Stage 6 – cyclone dissipates
The mixing of warm and cold air is complete
Wind flow shown for
Tuesday only
L
Wed
L
Usually highest snow fall
for Red Hook with this track
Tuesday
L
L
Sunday
Nor-easter Track
Monday
Day 3
Why would this track
bring rain or mixed
precipitation to
Red Hook in winter?
L
Day 2
Day 1
L
L
Wind flow
shown day 3
only
On this track the
nor-easter goes
out to sea and
Red Hook gets no
precipitation
L
Day 3
L
Day 2
L
Day 1
To understand why storms usually track in a
northeast direction we need to look at our old
friend convection
Convection is what drives the global winds, which
is caused by the unequal heating of the Earth’s
surface
Recall that the equator is a heat source and the
poles are a heat sink
high
low
(convergence)
high
(divergence)
Low (convergence)
high (divergence)
low
high
(convergence)