Weather Climate

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					Weather & Climate
….the atmospheric
 conditions at a certain place
 at a certain time.
Weather Components
 Temperature – how hot or cold the air is. Measured with a thermometer.
  Temperatures may be shown on a weather map by isotherms (a line
  connecting areas of equal temperature).
 Wind – horizontal movement of air. Wind blows from high to low
  pressure. Direction measured with a wind vane (blowing from) –
  measured with an anemometer – recorded in knots or m/s and Beaufort
 Clouds – water droplets or ice held in the atmosphere. Measured by
  observation, recorded in oktas (eighths of the sky)
 Precipetation – deposition of water from the atmosphere as rain, hail,
  snow etc. Measured with a rain gauge in mm.
 Air pressure – weight of the atmosphere. Measured with a barometer,
  recorded in millibars (mb).
Warm air rising = low pressure
Cool air sinking = high pressure
 What is Pressure?

 pressure                     Low
is cold air                pressure
  sinking                  is warm
                           air rising

              HIGH   LOW
How Wind is Formed
                        As air cools it can no longer rise
Cold air sinks

                                                                 Ground heats air
                                           Air rises and
                                           cools in the

                                WIND moves
                                   between                                          Sun heats
                            high and low pressure                                    ground

                 HIGH                                      LOW
  The Three Causes of Rain
The General Process of Forming Rain
 1 - The sun is the driving force behind the
 process of convection which is the main
 process behind the creation of rain.
 2 - The sun heats up the sea and this in turn
 heats up the air. The sun cannot directly heat
 the air. When the water is heated, it turns to
 water vapour
 3 - As the air is warm, it rises up into the
 atmosphere, taking the water vapour with it.
 As it does so, it cools as it gets further away
 from its heat source (the sea).
 4 - Eventually, the air and vapour cool to the
 point where the vapour condenses, making it
 visible as clouds.
 5 - Water droplets within the clouds collide
 and join together to form rain drops, which
 fall back down to the sea / ground.
Relief Rain / Orographic Rain

                   1 - Air is forced to rise over a hill or
                   mountain. As air rises away from the
                   heat source (the ground heated up by
                   the sun) the air cools by 1°C per 100m.
                   2 - As the water vapour in the air
                   condenses, it forms clouds and rains. Air
                   that has cooled cannot hold as much
                   water as warmer air.
                   3 - The air starts to descend the hill or
                   mountain and begins to warm up again.
                   4 - As air warms up, it can hold more
                   water and as it has precipitated, the air
                   has less water overall. Therefore clouds
                   disappear and rain stops. This side of the
                   relief  feature    is   known      as    a
Frontal Rain

               1 - The colder air mass is more dense
               than the warmer air mass, so carries
               on pushing forward along the ground.
               2 - The warmer air mass is lighter
               than the colder one so is forced to rise
               over the advancing cold air mass.
               3 - As the air is forced to rise, it cools
               and water vapour in the air condenses
               to form clouds.
               4 - Further forcing of the air (now with
               clouds) upwards causes precipitation
               to occur.
Convectional Rain
 3. Air cools and water vapour
   condenses to form storm
   clouds. (Cumulo-Nimbus)

                           2. Warm air
1. Sun                        rises
 heats                                     4. Heavy
ground                                   precipitation
Air masses affecting the UK

                    •Tropical maritime - warm
                    and moist

                    •Tropical continental - warm
                    and dry

                    •Polar maritime - cold and
                    (fairly) moist

                    • Returning polar maritime

                    •Polar continental - cold and
                                  High Pressure

                                     Cool air
   Anticyclones are air masses     sinks and
    of HIGH pressure                  warms
   Air sinks – as it sinks it
    warms – warm air holds
    more water vapour – clouds
    unlikely to form.
   Summer anticyclones = light
    winds sunshine and high
   Winter anticyclones = light
    winds, sunshine, low
    temperatures and frost.
   Clock wise winds
                                                 Low pressure
   Depressions are areas of LOW
    pressure formed when a warm
    air mass and cool air mass
   Warm air rises over cool air to
    form a warm front.                              Warm
                                      Cool air                      Cool air
   Cool air undercuts warm air                      air
                                       mass                          mass
    from behind to form a cold                      mass
                                           movement of depression
   Warm air rises along both
    fronts, cools, condenses and
    forms rain.
   Air rises at the centre of a
    depression and draws in
    anticlockwise winds. The lower
    the pressure the stronger the
   Clouds form when rising air cools to a point where it can
    no longer hold its water vapour. The vapour condenses
    in the form of tiny droplets.
   Air is lifted because of:
   Widespread ascent i.e. up a warm/cold front
   Encountering orographic barriers - mountains and hills
   Convection - when air is heated at low levels by contact
    with sun warmed ground - creating rising bubbles of air
    i.e. thermals.
How do we describe clouds?
 Clouds can be cirrus (wispy), cumulus (heaped) and
 stratus (in layers). They are classified into 10 main
 cloud types according to height and shape. Not all
 clouds bring rain, some are signs of fine weather.
 The word ‘nimbus’ added to the beginning or end of
 a cloud type, e.g. cumulonimbus or nimbostratus,
 means that the cloud is a rain cloud and will usually
 appear to be dark grey. Generally cumulonimbus
 are the most dangerous clouds associated with hail,
 lightning, tornadoes, downdraughts, downbursts and
Depressions and Anticyclones
 Weather in mid and high latitudes is controlled by systems called
 depressions and anticyclones. Anticyclones, or 'highs', usually
 bring fine, settled weather. Depressions, or 'lows', bring rain,
 strong winds and changeable conditions. Changeable weather is
 a feature of British weather, depressions are responsible for
 much of this. In Britain, most depressions cross the country from
 west to east, starting in the Atlantic Ocean. As a depression
 passes there are distinct changes in weather conditions. Air
 moves in clockwise and anti-clockwise whirls like water in a rocky
 stream. These whirls move around warm and cold air in the form
 of air masses. (with uniform temperature and humidity
 characteristics often from the region in which they formed).
 Fronts develop at the boundaries of the air masses because
 masses of air at different temperatures do not mix but move up
 and over each other.
Acid Rain

         The Increase in Acid Rain over Europe

  This was the acid
   rain problem in

  This was the acid
   rain problem in

  Imagine what it is
  like now in 2005!
What is Acid Rain?

 Acid rain is in fact rain which has been turned acidic
 (acid is a pH of 6.0 or below) when water in clouds
 chemically reacts with natural CO2 or human pollution.

 NATURAL Causes:
 CARBON DIOXIDE (CO2) in the atmosphere.
 HUMAN Causes:
 OXIDES (NOx) from cars and factories.
What is Climate?
The climate of the Earth varies because of a number
of factors. Key to this variability is the relationship
between the Earth and the Sun. Temperatures on
Earth depend on energy from the Sun. This energy
is not distributed evenly across the Earth’s surface
and varies with latitude. Away from the equator, the
Earth’s curved surface means that each beam of
sunlight is spread over a greater area and must pass
through a greater thickness of atmosphere. So the
solar heat energy reaching the Earth is more intense
at the equator than it is at the poles.
Global Distribution of Climates
Five Factors Affecting Climate

   Latitude
   Altitude
   Distance form sea
   Prevailing Wind
   Ocean Currents

  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 as
  the rays have to travel further.

Latitude is how far North or South a place is from the Equator – a major influence on
temperature and precipitation.
Density of atmosphere decreases with altitude
and absorbs less heat.

                                                 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.

Temperature decreases by 6 deg C every 1000m of height gained.
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

        Sea Winds – bring precipitation
        Land Winds – bring dry weather
        Polar Winds – bring cold weather
        Tropical Winds – bring warm or hot weather
Distance From Sea

   Places that are influenced by sea
    temperature have a maritime climate – wet
    with a small temperature range.
   Places that are not influenced by sea
    temperatures have a continental climate – dry
    with a large temperature range.
Ocean Currents
   Warm ocean currents
    flowing from the tropics
    towards the poles warm
    the surrounding area,
    especially in winter.

   Cold currents have less
    effect but may lower the
    temperature causing
Climate Change - Key Points

   Global temperatures have risen by over 0.7C in the last 300 years - climate change is therefore already
    taking place. 0.5C of this warming occurred during the 20th century. Most of the warming was from 1910
    to 1940 and from 1976 onwards
   Four out of five of the warmest years ever recorded were in the 1990's
   The 1990's was the warmest decade of the last millennium with 1998 being the warmest year globally
    since records began in 1861
   1999 was the warmest year on record in the UK
   January-June 2002 was the warmest start to a year in the northern hemisphere
   The total number of cold days (where the average temperature was under 0C) has fallen from between
    15 and 20 per year prior to the twentieth century, to around 10 per year in recent years
   1995 saw the most hot days in 225 years of daily measurements - 26 days above 20C
   Sea levels are rising globally, arctic sea ice is thinning and rainfall is becoming heavier in some parts of
    the World. Average global sea levels have increased by between 0.1 and 0.2 metres over the last 100
   Snow cover in the Northern Hemisphere has declined since the late 1960's by around 10%
   Mountain glaciers in non polar regions have retreated significantly during the 20th century
   There is evidence of more precipitation in large parts of the World - an increase of 0.5-1% per decade in
    many mid and high level areas of the northern hemisphere
   In the same area of the World there has been a 2-4% increase in the frequency of heavy rainfall events
   In Asia and Africa there has been an increased frequency and intensity of droughts in the last few
Global Climate Change
   The Intergovernmental Panel on Climate Change (IPCC) was set up in 1988 to asses
   information on climate change and its impact. It's Third Assessment Report predicts global
   temperature rises by the end of the century of between 1.4C and 5.8C. Although the issue
   of the changing climate is very complex and some changes are uncertain, temperature
   rises are expected to affect countries throughout the World and have a knock on effect with
   precipitation and sea level rises.Scientists have argued about whether temperature rises
   are due to human activities or due to natural changes in our environment. The IPCC
   announced in 2001 that

"most of the warming observed over the last 50 years is likely to be attributable to
   human activities".

   This was a more forceful statement than in 1996 when the Second Assessment Report
   stated that there was a "discernible human influence on the climate" which was the first
   time they had concluded such a link.Many experts believe the faster the climate changes,
   the greater the risk will be.Scientists in the UK are playing a major role in predictions for
   our future climate. The UK's most recent climate scenarios (published by UKCIP) details
   projections for the UK's climates which fit in with the IPCC's view of world changes.

   A microclimate is the distinctive climate of a small-scale area, such
    as a garden, park, valley or part of a city. The weather variables in a
    microclimate, such as temperature, rainfall, wind or humidity, may
    be both subtly different to the conditions prevailing over the area as
    a whole and to those that might be reasonably expected under
    certain types of pressure or cloud cover. Indeed, it is the amalgam of
    many, slightly different microclimates that actually makes up the
    climate for a town, city or wood.
   It is these subtle differences and exceptions to the rule that make
    microclimates so fascinating to study, and these notes help to both
    identify and explain the key differences which can be noticed by
    ground-level observations.
Heat Islands
Factors Affecting
   The formation of a heat island is the result of the interaction of the following factors.
   The release (and reflection) of heat from industrial and domestic buildings.
   The absorption by concrete, brick and tarmac of heat during the day, and its release into
    the lower atmosphere at night.
   The reflection of solar radiation by glass buildings and windows. The central business
    districts of some urban areas can therefore have quite high albedo rates.
   The emission of hygroscopic pollutants from cars and heavy industry - these act as
    condensation nuclei, leading to the formation of cloud and smog, which can trap radiation.
    In some cases, a pollution dome can also build up.
   Recent research on London's heat island has shown that the pollution domes can also filter
    incoming solar radiation, thereby reducing the build up of heat during the day. At night, the
    dome may trap some of the heat from the day, so these domes might be reducing the
    sharp differences between urban and rural areas.
   The relative absence of water in urban areas means that less energy is used for
    evapotranspiration and more is available to heat the lower atmosphere.
   The absence of strong winds to both disperse the heat and bring in cooler air from rural
    and suburban areas. Indeed, urban heat islands are often most clearly defined on calm
    summer evenings, often under blocking anticyclones.
Key points
projections for climate change globally:
   By the second half of the 21st century, wintertime precipitation in the
    northern mid to high latitudes and Antarctica will rise
   By the same time, Australasia, Central America and sourthern Africa is
    likely to see decreases in winter precipitation
   In the tropics, it's thought some land areas will see more rainfall and
    others will see less
   It is thought the West Antarctic ice sheet is unlikely to collapse this
    century. If it does fall apart, sea level rises would be enormous
   Global average temperatures are predicted to rise by between 1.4C and
    5.8C by 2100
   Maximum and minimum temperatures are expected to rise
   More hot days over land areas and fewer cold days and frost
   More intense precipitation events