Transcript Section 05 Pressure Systems Lesson 10

Section 05
Pressure Systems
Lesson 14/15/16
Pressure Systems
• The air pressure may vary considerably between different places
on earth.
• These pressure differences are of decisive importance to the Earth’s
weather and winds.
• On the charts of the weather service we find the pressure pattern,
delineated by the isobars, enclosing areas of different pressure, e.g.
– high pressures (anticyclones) and ridges,
– low pressures (cyclones) and troughs.
– cols
Anticyclones, Ridges
• A high pressure system is:
– an area enclosed by isobars,
– pressure values of which decrease with distance from the centre of the high or axis of
the ridge.
• vertical airflow in a high pressure cell comprises;
–
–
–
–
–
mass convergence at height and
divergence at low levels creating a
descending air movement or subsidence within the core of the anticyclone with an
outflow at low level with a
subsidence inversion.
Anticyclonic Subsidence
Subsidence Inversion
Cloud Formation
High Pressure Types
• Warm Anticyclones/Ridges
– Subtropical high pressure belt at 30° N/S
• Cold Anticyclones/Ridges
– Siberian High
– Cold ridge ahead of a warm front
Warm Anticyclone
• High pressure at height sustained relative to
surroundings
• Convergence aloft fed by subtropical jet stream.
Subtropical High
•
•
•
•
•
Permanent high pressure system
Situated at 30° N/S
Subsidence from aloft
The centres consist of warm air.
The subsidence inversion in these cells is sometimes called
Trade Wind inversion.
• Large areas of ST and SC on polar side
• Trade wind CU in the equatorial side.
• Source region for Tropical Maritime air.
Cold Anticyclones
• Pressure decreases with height
COLD
Cold Anticyclones/Ridges
• Form over cold continental land masses in winter
– Siberian High, Canadian High.
•
•
•
•
•
•
Usually temporary
Shallow in depth, max depth up to 700 mb.
Turn into thermal lows with summer heating.
Very stable air.
Strong surface inversion.
Source region for Polar Continental air.
Cold High/Ridge with Travelling
Depression
Low Pressure System
Cold Core Low (e.g. Icelandic Low)
• Low pressure intensifies with height
• Strong convergence at low level
• CU/CB/RASH
Warm Core Low
• Forms over land in summer or,
• Forms over warm water in winter.
• Thermal low/Asiatic Monsoon Low.
Thermal Low
Origins of Non Frontal Low Pressure Areas
• Orographic Lows (with mountain ranges)
• Thermal Lows
•
•
•
•
Instability Lows/Polar lows
Summer lows over land/Monsoon Low
Winter Lows over a warm sea(Mediterranean Low/Baltic Low)
Equatorial Low/Trough (Intertropical Convergence Zone)
Origins of Non Frontal Low Pressure
Areas, cont’d
• Cold Air Pool
• Tropical Revolving Storms (see Chapter 17)
• Easterly Waves
• Tornadoes (see Chapter 16)
• Whirlwinds/Dust Devils/Water spouts
Types of NFD
• Not all depressions are frontal
• Over the UK most depressions are frontal.
• However world-wide most depressions are non-frontal.
Types of NFD
• Most of these are thermal in origin, and arise from surface
heating and the release of latent heat.
• Some depressions are orographic such as the Genoa Low
• The consequent cooling and condensation produce the
associated bad weather.
Orographic Lows
• Wind flowing across a mountain barrier tends to curve
anticyclonically over the high ground and cyclonically to its lee.
• This leads to a ridge of high pressure over the mountain or hills and a
trough on the lee side.
• The inertial effect of air piling up on the windward side of high
ground reinforces the pressure contrast, and closed circulations may
develop on the lee side.
Orographic/Lee Depression
• Sometimes
associated
with Föhn
wind
Cold
Warm
Orographic Occlusion
• Occlusion
formed on
wave
Orographic Lows Cont.
• Lee lows are not common in the U.K., though lee troughing may be
observed.
• High mountains favour the development of lee orographic lows.
• Strong N.W. flows over the Alps lead to the formation of the "Genoa
Low" in N. Italy.
Orographic Lows Cont.
• Lee lows also form to the lee of Greenland and Scandinavia.
• Inactive due to descending air in the lee of the high ground. This
causes the weather to be more like that found in an anticyclone,
rather than a depression.
Thermal Depressions
• Expansion of the air due to warming over a warm surface leads to a
rise in pressure aloft.
• The greater the heating, the greater the pressure change, level for
level.
• The rise in pressure at the level above the surface leads to an
outflow of air aloft.
• This causes a fall in surface pressure and convergence.
• Cyclonic circulation exists at the surface with divergence aloft.
• Such thermal lows tend to be shallow.
Thermal Depressions, cont’d
• Thermal depressions are often sub-divided
into:
– Monsoon low
– Summer lows over land
– Equatorial low or trough
Formation of Thermal Low
Formation of Thermal Low
Air flows away aloft
Rising Air
Pressure falls at the surface
Formation of Thermal Low
Horizontal
divergence
Horizontal
convergence
Polar/Instability Low
• Polar lows may
form in Pm air
which is unstable
• They usually form
between the
southern tip of
Greenland and the
Norwegian coast
• They quickly
dissipate over land
Polar lows
• Cold arctic or polar air (-20°C to -40°C) moving out from Northern
Canada or Greenland is warmed by the relatively warm sea (+01°C).
• The temperature contrast exists by night as well as day so that these
polar lows tend to persist if the air is unstable.
• The clouds are mainly CB.
• They track SE in the Polar airstream.
• If a polar low is close to the polar front, a wave may develop on the
front, bringing tropical air into the polar low and greatly invigorating
it.
Polar Lows Along Norwegian Coast
• Cyclonic airflow
develops between two
highs
• When cold air reaches
the warmer sea,
instability lows can
develop
Baltic Sea Low
• Energy is released
into the existing
low from warm
waters
• Heavy
precipitation and
squalls may
develop
Cold Pool Formation
• Unstable meridional extension of upper air
flow.
• Wave collapses leaving a closed circulation
of cold unstable air aloft.
Cold Pool
– A region of low mean temperature in an isobaric layer aloft.
– Sometimes cold air outbreaks, cut off from the main stream,
generate a pool of cold air at height in a position far south of the
normal Polar front.
– This cold pool can remain for several days constituting a potential
area of instability at height.
Cold Pool
– In the summer warm lows form over the continents, and
sometimes these may develop into instability lows.
Cold Pool
– This happens when cold air is carried in over the low (by the
upper airflow) or when a cold pool already exists at height.
– In these conditions the atmosphere becomes unstable, and a
major area of thunderstorms may develop.
– These thunderstorms interfere with aviation in the area, since it
may be difficult to fly round them.
• Waterspout on the Solent (Hampshire/Isle of
Wight), on July 7, 1966 (O. Thomas)
Water Spouts
– There are two types of waterspouts.
– The first is developed downwards from a convective cloud
formed over warm water surfaces where ordinary towering
cumulus clouds are developing.
– The second type is formed by a rapidly converging airflow over a
warm water surface and builds upward from the surface just like a
whirlwind.
Water Spouts
– These columns of rotating air are known as waterspouts.
– Southern Florida in summer.
– Autumn waterspouts can be seen over the northern Baltic
Sea.
Tornadoes, Whirlwinds &
Dust Devils
• Tornadoes form inside cloud & come down
• Dust devils develop at the surface and go
upwards
• Tornado (USA) – dia. 500m – associated with
Cb/Ts
• Violent whirlwinds – dia. 50 – 250m
• Small whirlwinds (Dust Devils) – dia. 10m max.
20kt
• In arid areas – dia. 30m height 300ft max. 50kt
Average Global Surface Pressure - January
Lows - January
• Icelandic Low
• Aleutian Islands Low
• Thermal Lows
– Southern Hemisphere
Highs - January
• Sub-Tropical High
• Siberian High
• N. American High
Average Global Surface Pressure - July
Lows - July
• Icelandic Low
• Asiatic/Baluchistan Low
• N. American Low
Asiatic Monsoon Low
Highs - July
• Azores High
• N. Pacific High
• Sub-Tropical High