Weather and Climate

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Weather and Climate
Weather: the state of the atmosphere at a given time and place
Climate: the average weather conditions of a place of an area over a period of 30 years or more
Atmosphere: the mixture of gases, predominantly nitrogen, oxygen, argon, carbon dioxide and
water vapour, that surrounds the earth
Humidity: a measure of the amount of moisture in the air. Absolute humidity tells us how much
moisture is in the air (g/m3). Relative humidity expresses this amount as a percentage of the
maximum that air of a certain temperature can hold
The Structure of the Atmosphere
Troposphere: this is the zone closest to the earth where most weather processes take place. It has
the highest temperatures as solar radiation warms the earth's surface which, in turn, warms the air
directly above it by conduction, convection and radiation. However, this effect decreases rapidly
with distance from the earth's surface with temperature dropping by 6.4°C with every 1000m
gained in height. Wind speeds also increase with increasing altitude as frictional drag decreases with
distance from the surface. The troposphere is the most unstable layer containing mostly water
vapour and particulate matter. The end of the troposphere is marked by the tropopause which is an
isothermal layer where temperature remains constant as altitude increases, it marks the upper limit
of this zone.
Stratosphere: this zone is characterised by a steady increase in temperature (temperature inversion)
as a result of absorption of solar radiation by the ozone layer which absorbs much of the incoming
ultraviolet radiation that would be harmful to humans otherwise. The atmosphere is thinner in this
zone as pressure decreases with height and there is a lack of vapour and dust. Wind speeds increase
with height towards the stratopause, another isothermal layer.
Mesosphere: temperature declines rapidly to -90°C in this zone as there is no water vapour or dust
to absorb radiation. It is characterised by very strong winds (3000km/h) and culminates in the
Thermosphere: there is an increase in temperature from the absorption of UV by atomic oxygen
found at this altitude.
Why does temperature decline with altitude throughout the troposphere?
At lower altitudes there is more air molecules meaning there is more friction between them which
leads to higher temperatures. However at higher altitudes where there are less air molecules there
is less friction and therefore temperatures are much lower.
Environmental Lapse Rate: the rate of decrease with height for an atmospheric variable
(temperature unless otherwise specified). On average the temperature lapse rate is 6.49°C/1000m
from sea level to 11km and from 11km to 20km the constant temperature is -56.5°C.

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The Global Energy Budget
The interaction between solar energy, the atmosphere and the earth's surface
Open energy system
Energy input = Energy output on annual and global scales
There are latitudinal and local imbalances which drive the climate system (Quasiequalibrium)
Latitudinal Variation in radiation balance:
Tropics have a constant energy surplus (net gain)
Poles have a constant energy deficit (net loss)
Latitude 36° has a balance
Why is there an energy surplus at the equator and a deficit at the Poles?
At the equator the…read more

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Why don't the Poles get increasingly colder and the equator increasingly hotter?
Heat transfers - Horizontal
80% of heat is transferred away from the equator horizontally by winds (jet streams,
hurricanes and depressions)
20% is transferred by Ocean Currents e.g the North Atlantic Drift
Heat Transfers - Vertical
Heat is transferred vertically away from the equator by radiation, conduction, convection and
latent heat.…read more

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Coastal areas are therefore milder than inland areas in winter and cooler than locations at the centre
of the continent in summer. Also, areas close to the sea have a smaller annual temperature range
than areas at the centre of continents. E.…read more

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Hadley's Cell: shows that warm high pressure air rises into the atmosphere moving through the jet
stream where cool high pressure air will fall over tropical areas.
Inter-tropical Convergence Zone (ITCZ): a result of the heating of part of the Earth's surface,
caused by the concentrated insolation from the overhead sun. This leads to heating of the air lying on
that surface. This heated air becomes less dense and rises. This draws in cooler air which flows across
the surface to replace the rising air.…read more

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Convectional rainfall/thunderstorms in cities such as London and Birmingham due to extreme
over heating of the ground, occurs when the air over the ground is warmed, becomes less
dense than surrounding air and rises.
Relief rainfall in most parts of Britain - occurs when moist air has been travelling over the sea
is forced to rise over upland areas. As it rises and cools, the air reaches dew point (becomes
saturated), condensation occurs leading to rainfall.…read more

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Tropical Maritime (south-westerly)- very mild and wet during winter with
temperatures of 11°C in Janunary as formed over low latitudes. Often causes heavy
stratus cloud cover giving hill and coastal fog. Poor visibility but no frost is also
common in winter. Warm in summer but not hot at 18°C as hard to break through
cloud cover. Lower air is stable but if forced to rise over hill the upper layers can
become unstable leading to thundery showers. Winds usually moderate to fresh.
e.…read more

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The passages of anti-cyclones tend to be much slower
than with depressions.
Summer Anticyclones
Characterised by clear skies, high temperatures of over 25°C during the day time, however
significant radiation loss by night can lead to short-term temperature inversion and the formation of
dew and mist which clear quickly in the morning.…read more

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Monsoon Climate and it is responsible for the highly
successful agricultural system which has lasted 5000 years. 25% of the Earth's surface is affected by
the monsoon climate and it is extremely fragile as a small change in year-to-year climate can be
Monsoon Climate has 3 different seasons:
Oct-Mar: the only dry season with temperatures of 21-25°C, little rainfall and winds blowing
from the north east called the `rising Monsoon winds' - land to sea breezes.…read more

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Hurricanes use energy from the warm ocean water to grow stronger and as long as it has
warm water a hurricane will continue to grow.
Case Study: Hurricane Katrina
Sixth strongest hurricane ever recorded
Started as category 5 but weakened to category 3 when it hit land along the Gulf of Mexico
on 29th August 2005
Caused catastrophic damage along the coastlines.…read more


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