Geoscience Reference
In-Depth Information
night and day, so in exposed coastal districts
showers may occur at any time.
In the Southern Hemisphere, polar low
mesocyclones appear to be most frequent in the
transition seasons, as these are the times of
strongest meridional temperature and pressure
gradients. In addition, over the Southern Ocean
the patterns of occurrence and movement are
more zonally distributed than in the Northern
Hemisphere.
Siberia. They probably form as the result of strong
vertical motion and adiabatic cooling in occluding
baroclinic lows along the Arctic coastal margins.
Such lows are especially important during the
Arctic winter in that they bring large amounts of
medium and high cloud, which offsets radiational
cooling of the surface. Otherwise, they usually
cause no 'weather' in the Arctic during this season.
It is important to emphasize that tropospheric
cold lows may be linked with either low or high
pressure cells at the surface.
In mid-latitudes, cold lows may form during
periods of low-index circulation pattern (see
Figure 6.27
) by the cutting-off of polar air from
the main body of cold air to the north (these are
sometimes referred to as
cut-off lows
). This gives
rise to weather of polar air-mass type, although
rather weak fronts may also be present. Such lows
are commonly slow-moving and give persistent
unsettled weather with thunder in summer. Heavy
4 The cold low
The cold low (or
cold pool
) is usually most evident
in the circulation and temperature fields of the
middle troposphere. Characteristically, it displays
symmetrical isotherms around the low center.
Surface charts may show little or no sign of
these persistent systems, which are frequent over
the northeastern North America and northeast
10
8
Year
Molecular
diffusion
10
7
Month
Jet stream
10
6
Week
4 days
Anticyclone
2 days
10
5
Hurricane
Day
Local winds
10
4
Thunderstorm
Hour
Large cumulus
10
3
Tornado
10
2
Small cumulus
Minute
Sound
waves
10
1
Dust devil
Small-scale
turbulence
10
0
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
10
8
Distance scale (m)
Figure 9.26
The spatial scale and lifespans of mesoscale and other meteorological systems.
