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.
Schematic drawing
of a simple mercury
barometer with
vertical mercury
column and
reservoir at base
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.
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.
An anemometer is a
device that is used for
measuring wind speed,
and is one instrument
used in a weather station.
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 wetbulb 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 wetand dry-bulb temperatures on a psychrometric
chart.
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.
• What is a microclimate?
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
Describe
and explain
the patterns
opposite.
How might
this vary?
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
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.