cannock chase - u3asites.org.uk · 6 sources of air mass as in picture. weather “fronts” are...
TRANSCRIPT
Weather Science:What causes weather
Understanding weather charts
13th July 2017
CANNOCK CHASE U3A
SCIENCE & TECHNOLOGY GROUP
THE DIFFERENCE BETWEEN WEATHER
AND CLIMATE IS…
�The climate is what you expect;
�The weather is what you get.
� Frequently attributed to Mark Twain.
� So, why do we get what we get?
� There are loads of factors at play…
TEMPERATURE
� The Tropics are absorbing more energy than they are
losing, whilst the North and South Poles are losing more
energy than they are absorbing.
� This would mean that the Tropics would be always
warming, and the Polar regions always cooling.
� However this isn’t the case, something else must be
transporting heat from the Tropics to the poles – Winds.
PRESSURE & WINDPressure is how much air
there is above your head
Tropopause acts as a lid
Zero mb
1000 mb
� Temperature makes hot air rise,
with cooler air replacing it, causing a
flow of air or wind.
� If more air diverges at the top than
converges (friction), we end up with
less air in the column and the air
pressure falls (less weight).
� If there is more pressure in one
place, and less elsewhere, then the
air moves from high to low pressure.
� The bigger the difference between
high and low pressure, the faster
winds that blow.
� The units of pressure are mb or hPa
or inches of Hg, where 1mb = 1 hPa.Air weighs 1.225 kg/m3
Water is 1000kg/m3
Pressure is shown on a barometer and can indicate the weather approaching.
WHAT ARE ALL THE
WEATHER MAP
MARKINGS?
� Isobar contours are lines of equal pressure, usually ovals at 4mb
intervals. Pressures decrease towards centre of (L)ows & viceversa
� (H)igh and (L)ow pressure centres marked with a cross.
� Close together isobars means pressure rising rapidly, so wind
speed is higher and blows almost parallel to isobar lines.
� Spread out isobars mean little wind with slower pressure changes.
� Low pressure usually means cloud rain & wind (cyclone).
� High pressure usually means fine weather (anticyclone).
� Temperature, Pressure
contours, Wind & Fronts all
appear on various charts.
� Pressure is usually in the
range of 950-1050mb, UK,
marked by lines (isobars).
AIRMASSES AND FRONTS
ON CHARTS
� 6 sources of air mass as in picture.
� Weather “Fronts” are the boundaries
of two Air Masses.
� Northerly winds can bring cold air.
� Southerly winds can bring warm air.
� Warm fronts on charts are red
semicircles and mean rising air,
higher pressure (higher column of
air).
� Cold fronts are blue triangles and
mean falling air, falling pressure.
� Air masses/Fronts meeting will clash
at occluded front in purple, causing
bad storms.
[-------Cold showery weather-------]
THE CORIOLIS EFFECT &
WIND
� Normally the wind
tries to blow from high
to low pressure, but the
Earth’s spin acts on it,
diverting it.
� Wind then follows the
pressure contours and
blows around low
pressure in an
anticlockwise direction.
� And around high
pressure in a clockwise
direction.
� (True only for the
Northern Hemisphere).
Remember that we name winds by the direction they are blowing from
Video
DEPRESSION, FRONTS,
CLOUDS & RAIN
� 4 stages of a depression
Cold fronts are marked with triangles
(reminiscent of icicles) and warm fronts
with semi-circles (reminiscent of the
warm sun).
The lines are the boundaries of the wave
of warm air.
The occluded front (purple) occurs when
the wave breaks, and the cold front
catches up with the warm front.
The grey shading shows the
region of continuous cloud
cover.
WHAT HAPPENS ATWARM & COLD
FRONTS
APPROACHING FRONT:
DEPRESSION PASSING
OVERHEAD
1. The first thing that reaches you is the
warm front, far above your head. The
clouds are on the front, so you will see
high clouds, typically wispy, ice clouds
called cirrus, then Cumulonimbus
rising.
2. As the front moves towards you, it gets lower
and similarly the clouds get lower and heavier.
You’re going to need your umbrella.
3. On the warm front, you’ll typically see
featureless stratus or ‘sheets’ of cloud, becoming
nimbostratus or ‘rainy sheets’ with a prolonged
spell of rain when the front is near the ground.
Nimbostratus
Cirrus
4. As the front crosses you on
the ground, the
temperature rises as there
is a shift from cold to warm
air, typically by a few °C.
5. The air pressure falls
steadily ahead of and
during the passage of the
warm front, but then rises
slowly after its passage.
THE PHYSICS
� PV / T = constant
� So, if the pressure falls, the gas must
cool. As the rising air cools, the rate of
evaporation becomes less than the rate
of condensation until eventually cloud
droplets form.
Cumulus clouds are indicative of
convection. They can billow upwards as the
air rises within them.
� As water vapour condenses into liquid water, latent heat
is released providing the energy source for the developing
storm.
� The latent heat generated is significant. For example,
there is enough heat released in a small cumulus cloud to
power an average home for 17 years.
� As the storm continues to develop, some of the cloud
droplets will become large enough to fall as rain.
THE JET STREAMYet another force at play.
The polar front jet stream is a
belt of westerly winds in the
upper troposphere, with wind
speeds up to 250mph. It sits at a
around 10km, the height at
which aeroplanes fly. Trans-
Atlantic pilots make use of it.
The jet stream can split up warm
& cold fronts.
Summer 2012 UK Jet Stream
Normal Summer UK
Jet Stream
Powerful example of wind flow driven by
pressure & equator/pole temperature
differences, but shaped by earth’s spin.
FRONTS & FEATURES ONWEATHERMAPS
� To sum up so far:
� Let’s look at some features of a
weather map
� ANYONE FOR TEA?
�Next after tea…
�Will it snow?
�How high is a cloud?
�Making a cloud
WILL IT SNOW?
� To have snow, we need a
combination of two factors:
� A weather front, convection
clouds (cumulous) or mountains
need to be causing precipitation.
� The atmosphere needs to be cold
enough for precipitation to fall as
snow.
� Are there any cold fronts or air masses on Met Office
surface pressure charts bringing rain?
(www.metoffice.gov.uk)
� Arctic maritime or polar continental air masses can
bring snow in winter to the UK. Look for “528”
contours.
24 Dec 2010
1. ARE THERE COLD FRONTS? LIKELY TO RAIN?
LOOK ATMETOFFICE CHARTS. SEE 528 LINE (DOTTED)
SURFACE PRESSURE AT WWW.METOFFICE.GOV.UK
2. WILL IT SNOW?
� 500-1000mb is the bottom half mass of atmosphere.
� Height to 500mb pressure – Look for 528 lines = 5280m.
� 5280m = height below 500mb, snow is possible.
�Weather forecast charts from netweather.tv (charts/gfs)
� Select ‘HGT 500-1000’
© Netweather.tv
� Look at thickness of bottom
half of atmosphere.
� Hot air rises.
� Colder more dense air sinks to
bottom half.
� So smaller height = colder air.
2. Is the atmosphere cold
enough for precipitation to
fall as snow?
netweather.tv
500mb
1000mb
3. WILL IT SNOW?
� Check whether the atmosphere really is cold
enough throughout its entire depth:
� Go to wetter3.de …Select ‘Vertikalschnitte’
(Vertical section) and select latitude & longitude.
� wetter3.de
Saturday
Feb 11th
2017
Above
London
(00)
It
snowed!
EXTRA TIME…
�Measure the height of a cloud
�Making a cloud
MEASURE THE HEIGHT OF CLOUDS
� Temperature change from the ground:
� Approx -6 degrees C per km (-1.8C per 1000ft)
� Use an Infra Red non-contact temperature sensor.
� Point IR sensor to clouds for temp
measurement (eg -16C).
� Point to ground for ground temp
(eg +5C).
� Temp difference, say 5C – (-16C) =
21C
� Height = 21/6 km = 3.5km
�Making a cloud video
� [Copyright Reading University]
� THAT’S IT FOLKS
� See you next time
� References thanks to Reading University
RAIN, WHEN?
What causes precipitation (rain and snow)?
Precipitation forms when cloud droplets (or ice particles) in clouds grow and combine to become so large that their fall speed exceeds the updraft speed in the cloud, and they then fall out of the cloud. If these large water drops or ice particles do not re-evaporate as they fall farther below the cloud, they
reach the ground as precipitation.
CLOUDS