Urban Heat Islands

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A Stevenson screen or instrument shelter is a
screen to shield meteorological instruments
against precipitation and direct heat radiation
from outside sources, while still allowing air to
circulate freely around them. It forms part of a
standard weather station.
Six's thermometer is a thermometer, also known as
a Maximum minimum thermometer, which can measure
the maximum and minimum temperature during a
given time It was invented by James Six in 1782,
and named after him. It consists of a U-shaped
capillary tube with two separate temperature
readings, one for the maximum temperature and one
for the minimum temperature. There are bulbs at
the top of each arm of the U-shaped tube. The one
at the top of the minimum reading scale contains
alcohol, the other contains a vacuum or low
pressure alcohol vapour. In the bend of the U is
a section of mercury which is pushed around the
tube by the expansion and contraction of the
alcohol in the first bulb. It is the alcohol
which measures the temperature, the mercury
indicates the temperature reading on both
scales. As the mercury moves it pushes 2 small
steel markers which are sprung into the tube.
They record the furthest point reached by the
mercury in each arm of the tube. When the
temperature reverses and the mercury is moved in
the opposite direction by the expansion or
contraction of the alcohol, the sprung markers
remain in the tube at the furthest position they
have been pushed by the mercury. They thus record
the extremes of temperature experienced by the
device since it was last reset. The markers are
reset by using a small magnet which can drag the
markers along the tube so that they again rest on
the surface of the mercury.
Schematic drawing of a simple mercury barometer
with vertical mercury column and reservoir at
base
Hygrometers are instruments used for measuring
humidity. A simple form of a hygrometer is
specifically known as a psychrometer and consists
of two thermometers, one of which includes a dry
bulb and the other of which includes a bulb that
is kept wet to measure wet-bulb temperature. In a
psychrometer, there are two thermometers, one
with a dry bulb and the other with a wet bulb.
Evaporation from the wet bulb lowers the
temperature, so that the wet-bulb thermometer
usually shows a lower temperature than that of
the dry-bulb thermometer, which measures dry-bulb
temperature. When the air temperature is below
freezing, however, the wet bulb is covered with a
thin coating of ice and yet may be warmer than
the dry bulb. Relative humidity is computed from
the ambient temperature as shown by the dry-bulb
thermometer and the difference in temperatures as
shown by the wet-bulb and dry-bulb thermometers.
Relative humidity can also be determined by
locating the intersection of the wet- and
dry-bulb temperatures on a psychrometric chart.
An anemometer is a device that is used for
measuring wind speed, and is one instrument used
in a weather station.
An aneroid barometer uses a small, flexible metal
box called an aneroid cell. This aneroid capsule
(cell) is made from an alloy of beryllium and
copper. The evacuated capsule (or usually more
capsules) is prevented from collapsing by a
strong spring. Small changes in external air
pressure cause the cell to expand or contract.
This expansion and contraction drives mechanical
levers such that the tiny movements of the
capsule are amplified and displayed on the face
of the aneroid barometer. Many models include a
manually set needle which is used to mark the
current measurement so a change can be seen. In
addition, the mechanism is made deliberately
'stiff' so that tapping the barometer reveals
whether the pressure is rising or falling as the
pointer moves.
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Objectives

• You should know what a microclimate is
• You should be able to explain how urban areas
  modify their climates for a range of weather
  characteristics
• You should be able to quote examples of places
  where urban areas have modified their climates.

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• What is a microclimate?

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Produce a mind map on all of the ways that urban
areas could influence their climates
Explain each point, add case studies with
detailed information to your map using the Geo
Fact sheet and Waugh
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Describe and explain the patterns opposite. How
might this vary?
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Describe and explain the patterns in the chart
Weather characteristic Variation in Urban Area
Sunshine duration 5 to 15 less
Annual mean temperature 0.5-1.0 C higher
Winter maximum temperatures 1 to 2 C higher
Occurrence of frosts 2 to 3 weeks fewer
Relative humidity in winter 2 lower
Relative humidity in summer 8 to 10 lower
Total precipitation 5 to 10 more
Number of rain days 10 more
Number of days with snow 14 fewer
Cloud cover 5 to 10 more
Occurrence of fog in winter 100 more
Amount of condensation nuclei 10 times more
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Objectives rate yourself out of 10 for each

• You should know what a microclimate is
• You should be able to explain how urban areas
  modify their climates for a range of weather
  characteristics
• You should be able to quote examples of places
  where urban areas have modified their climates.