Weather and Climate
Unit 1B
Key Question 1
Elements and measurements of
the weather
What is the difference between
weather and climate ?
WEATHER
is about day to day
changes in the
atmosphere. It includes
changes in temperature,
rainfall, sunshine & wind)
CLIMATE
is the average or typical
weather of a place worked
out over a long period of
time.
We know that in Britain it gets cold in the winter and
warm in the summer. This is climate.
But we don't know if it is going to be warm
tomorrow or not. This is the weather.
Weather Elements
1. Temperature
2. Precipitation
3. Wind Speed
4. Wind Direction
5. Cloud Cover
6. Cloud Type
7. Visibility
8. Air Pressure
KQ 1- Measuring the weather
Cloud Cover Chart
RIGHT Types of Cloud
What type of cloud is this ?
Found at high altitudes - CIRRUS
What type of cloud is this ?
They rise to very great heights and are
associated with thunderstorms CUMULONIMBUS
Name all of the following
instruments
The Stevenson Screen
Characteristics
–
–
–
–
–
It is white
It has slatted sides
It has a double roof
It is on stilts
Its door opens
northwards
Other Weather Stations
Key Question 2
The Influence of AIR MASSES
over the British Isles
Air Masses
Blackwood Comprehensive School.
What is an air mass?
• An air mass is a large body of air with
similar temperature and/or humidity.
• Air masses form in stable “source regions”
such as in the sub tropics or near to the
poles.
Air Mass
types
Five
main air
masses
affect
Europe
Air mass classification
• Tropical Maritime (Tm) - mild and moist
• Tropical Continental (Tc) - warm and dry
• Polar Maritime (Pm) - cool and moist
• Polar Continental (Pc) - cold and dry
• Arctic Maritime (Am) - cold and moist
Tropical
Maritime
Tm air masses come from the south west and
originate over the Azores or the Caribbean. They bring
mild, damp, cloudy weather.
Tropical
Continental
Tc air masses come from the south and originate over
dry northern Africa . They bring hot, dry weather and
summer heatwaves.
Polar Maritime
Pm air masses come from the north-west and
originate over the north Atlantic. They bring cool, moist
conditions.
Polar
Continental
Pc air masses come from the east and originate over
Scandinavia and Russia . They bring clear dry conditions
- cold in winter, warm in summer.
Key Question 3
Depressions and Anticyclones
Air stops rising when it
meets air of equal
density, then diverges at
high level to produce
more wind which
eventually sinks
elsewhere to complete
the circulation cell
Pressure, Wind and Weather
Systems
WINDS are horizontal flows of air;
winds blow from areas of high
pressure to areas of low pressure
(nature tries to equalise pressure)
�
PRESSURE describes the
tendency of the air to rise or to sink
at any given place or time.
�
Air tends to rise or sink as a result
of its density.
�
Insolation
Air heated by
contact with ground
expands; becomes
less dense and rises
Denser air
drawn in at
low level to
replace rising,
less dense air
Air density varies with altitude
but, at the ground level, air density
is governed by its temperature.
�
Thus, variations in radiation and
temperature control pressure and
wind.
�
LOW
PRESSURE
Sun heats up ground
Denser air drawn in at low
level to replace rising, less
dense air
WIND DIRECTION & STRENGTH
Wind strength depends on
the difference in pressure
between the high and low
Farmers plant trees to protect orchards,
pressure systems, and the
houses, stock or prevent soil erosion
distance between them.
� This is called the PRESSURE
GRADIENT; it is a similar
concept to the physical slope
between two places, shown on
Locally, wind is channelled
a contour map. Pressure is
down streets (wind canyons).
shown by ISOBARS on a
weather map.
Strong winds also occur
� Pressure difference
in low latitudes due to
essentially depends on the
stronger heating and
temperature difference
steeper presure gradients.
between the two places.
Hurricanes and
A steep pressure gradient results
tornadoes are both
from a large pressure difference
Strong polar winds due to low
tropical phenomena.
or short distance between places
friction
and causes strong wind.
�
Hurricane in Florida
Tornado in USA
Beach windbreaks reduce windsped
by increasing friction
HIGH PRESSURE
High Pressure means that air tends to sink. Sinking air is
compressed, warms up as a result and its relative humidity
falls below saturation. Any clouds evaporate. Rainfall is
unlikely, apart from occasional short, intense convectional
storms due to insolation with lack of clouds in daytime.
LITTLE
WIND
CLEAR SKIES
In Britain, high pressure systems have clear skies, little or
no wind, little rainfall and tend to be stable and slow moving.
� Visibility is intially good, but rapidly deteriorates as dust is
trapped by sinking air and is not washed out by rainfall.
� Cloud cover is slight, resulting in a high diurnal ranges of
temperature (hot days, cold nights). Due to the trapped dust
particles and cold nights, dew, frost, fog or smog are
common.
� Air quality is low as all forms of pollution are retained in
the lower atmosphere.
�
VISIBILITY IS
POOR
FEW CLOUDS
CLEAR SKIES CAUSE FROST
VISIBLITY REMAINS BETTER IN
MOUNTAINS - LESS POLLUTION
FOG & SMOG IS
COMMON
MID-LATITUDE LOW PRESSURE SYSTEMS
Mid-latitude low pressure
systems are called
depressions in Britain. They
also involve rising air, clouds,
strong winds and rainfall
and are fast moving.
Depressions result from
the convergence of warm air
from the tropical high
pressure belt with cold air
from the poles along the
Polar Front.
� The energy of the
depresion is a result of the
difference in temperature
and humidity between the
two air masses.
� This contrast varies with
the exact origin of the air
mass, the season and the
nature of the surface over
which they have passed.
POLAR
MARITIME
from
Greenland
Cool, humid.
ARCTIC
MARITIME
from Arctic Ocean
Cold, humid.
POLAR CONTINENTAL
from E.Europe
Cold, dry in winter
Warm, dry in summer.
�
POLAR
MARITIME
RETURN
Coolish, very
humid.
TROPICAL
MARITIME
from Atlantic near
tropic
Warm, humid
TROPICAL
CONTINENTAL
From N.Africa
Hot, dry
POLAR
FRONT
this shifts
polewards in
summer and
equatorward
s in winter,
hence British
seasonal
contrasts.
MID-LATITUDE LOW PRESSURE SYSTEMS
Mid-latitude low pressure
systems are called depressions in
Britain. They also involve rising
air, clouds, strong winds and
rainfall and are fast moving.
Depressions (L)
over Europe
showing FRONTS
OCCLUDED FRONT
Cold & warm fronts meet
COLD
FRONT
Depressions over
NW Europe
POLAR MARITIME
AIR
WARM
FRONT
COLD
FRONT
WARM
FRONT
COLD
FRONT
TROPICAL
MARTIME AIR
Depressions result from the convergence of warm air from the
tropical high pressure belt with cold air from the poles along the
Polar Front. The systems move rapidly across the Atlantic before
filling and drifting north-eastwards to Scandinavia from Britain.
� The energy of the depresion is a result of the difference in
temperature and humidity between the two air masses.
� This contrast varies with the exact origin of the air mass, the
season and the nature of the surface over which they have passed.
�
POLAR
MARITIME
COLD FRONTS
TROPICAL MARITIME
As the depression moves
eastwards, the warm
tropical air continues to be
forced upwards by the
colder, denser polar air
mass.
The cold front is steeper, also due to ground level
friction slowing the lower air, so uplift is more rapid than
along the warm front.
� This causes cumulo-nimbus clouds and possible
thunderstorms rather than thick stratus cloud.
� Eventually, the two fronts meet, forcing the warm air
off the ground. This is an OCCLUDED FRONT
(occlusion), and happens to all depressions as they ‘fill’.
� The whole system takes about 24 hours to pass.
�
OCCLUSION
Tm air forced up
Pm air
meets at
ground level
A FRONT is the boundary betwen two air masses. A depression
has two, a warm (the front of the warm air) and a cold.
WARM FRONTS
1
TROPICAL MARITIME AIR
POLAR MARITIME AIR
The warm front is
angled gently due to
ground level friction
which slows the air at
low level as the whole
system moves
eastwards.
As the warm tropical maritime air moves eastwards towards Britain, it is forced upwards by colder,
denser polar maritime air. The speed of uplift depends on the relative temperature of the two air
masses. Uplift causes expansion, cooling, falling relative humidity until dew point temperature is
reached when condensation starts to occur on particles. The amount of precipitation depends on the
hunidity and temperature of the warm air mass, and the particles available.
2
MAINLY STRATUS
CLOUDS
The Passage of a Depression
An example of a LOW PRESSUE System
Key Question 4
Factors that affect Climate
Climate
Factors Affecting Climate
What do you need to know today?
1. There are many factors affecting climate.
2. Weather patterns recorded over long
periods of time can lead to the construction
of climatic belts.
Factors Affecting Climate
1 - Latitude
This is the most important factor. All climate stems
from the equator. This is the hottest place on earth.
As you go north or south from this line it becomes
colder as you travel towards the North and South
Poles as the sun’s rays are more spread out.
Factors Affecting Climate
2 - Altitude
This is the effect of
height
on
temperature - the
higher you go, the
colder it gets. The
sun heats up the
earth’s surface and
then heat is radiated
back up into the
atmosphere.
The
higher
up
the
radiated heat goes,
the colder it gets.
Factors Affecting Climate
3 - Prevailing Winds
The movement of the earth’s winds starts at
the equator, where it is hottest.
There are certain set patterns of winds called
prevailing winds, which means the direction
winds travels in most in a particular place.
Two things affect the direction - the earth’s tilt
(on it’s axis) and the revolution (revolving) of
the earth.
Wind affects the climate when it has travelled:
· over a particular feature (land or sea)
· and from a particular direction
Factors Affecting Climate
4 – Relief
Relief or orographic rain is formed when air is forced to cool when it rises
over relief features in the landscape such as hills or mountains. As it rises it
cools, condenses and forms rain. The highest rainfall totals of over 1,600
mm per year occur in the mountain areas along the west coast. There is
often a rain shadow effect whereby the leeward (downwind) slope receives a
relatively small amount of rain.
Factors Affecting Climate
5 – Aspect
North Facing = Cold
South Facing = sunny and warmer
(UK example)
Factors Affecting Climate
6 – Continentality
Air temperature is greatly
affected by the location of a
place relative to a large body
of water. The impact of
continental location on
weather and climate
characteristics of a place is
called "continentality". Air
temperature near or over
bodies of water is much
different from that over land
due to differences in the way
water and land heat and cool.
Key Question 5
Weather hazards and
management