Collecting Weather Data (Cambridge (CIE) O Level Geography)

Weather Instruments

Stevenson screen

• This is a wooden box standing on four legs at a height of 121 cm to avoid heat radiated from the ground and to have the thermometer bulbs at a standard heigh of 125 cm

• They are painted white to reflect the sun's rays

• Made of wood to avoid the conduction of heat into it

• The sides are slatted (louvered) to allow free movement of air

• The rood is made of a double layer of wood with airspace between for insulation

• The screen is usually placed on a grass-covered surface, which reduces radiating ground heat

• Readings must be taken at the same time each day

• These days, meteorological stations use automated digital recording instruments that transmit data to the Met. Office's computers

• Instruments found inside include:

• Maximum-minimum thermometer (Six's thermometer)

• Wet and dry bulb thermometer - hygrometer

• Instruments found outside include

  • Rain gauge

  • Wind vane

  • Anemometer 

• Weather station layout is:

  • Barometers and barographs are kept away from strong air movements, direct sunlight and heat sources

  • Thermometers are kept away from buildings that may radiate heat

  • Wind vanes and anemometers are positioned in the open, away from trees or buildings, and away from the nearest obstacle by at least three times the height 

  • The rain gauge must be in an open space with a distance from the nearest object twice its height

Wind direction

• Wind direction is measured with a wind vane

• Direction is the compass point from where the wind is blowing - south, north, north-east etc. 

• The unit of measurement for wind direction is compass direction

Wind speed

• Wind speed is measured with an anemometer

• The unit of measurement for wind speed is m/s or km/hr

• The anemometer consists of 3 or 4 cups fixed on metal arms that rotate freely on a 10m vertical shaft

• The stronger the wind, the faster the cups rotate, and more rotations are recorded on the counter

• The digital handheld anemometers need to be held into the oncoming wind and as the fan rotates, the number is shown on the screen

• Many anemometers are digital that transmit data directly to apps and computers to show readings directly

• Wind vanes and anemometers are placed well away from any buildings or trees that can interfere with air movement

• Buildings can create wind tunnels or slow airflow and therefore, affect the accuracy of any reading

Precipitation

• Precipitation is any water that falls to earth - hail, mist, rain, sleet, or snow

• Unit of measure is millimetres (mm)

• A rain gauge is used to measure precipitation

• At the same time each day, any water that has collected is poured into the tapered measuring cylinder

• Measuring cylinder needs to be on a flat surface

• The water level is then read with the eye at the same level as the lowest part of the meniscus of the water

• Measurements are then recorded; too small a reading and it is recorded as 'trace'

Temperature

• Unit of measurement is Celsius ° C, or Fahrenheit ° F depending on where you are in the world

• Temperature is measured using a thermometer or thermograph

• Shade temperature is measured, as air temperature is variable due to direct insolation and cloud cover

• Several types of thermometers can be used, but the most traditional is a Six's thermometer, which houses a maximum and minimum thermometer in a U shape

• The following measurements can be done:

  • Daily minimum temperature

  • Daily maximum temperature

  • Average temperature calculations for one day

  • Temperature range for 24 hrs - called the diurnal temperature range  

  • Mean monthly temperature is found by recording daily mean temperature for one month and then dividing by the number of days in that month

  • The sum of the mean monthly temperatures divided by 12 gives the mean annual temperature

• A thermograph has an exposed bimetallic strip which deforms with a change in temperature

• This change is transmitted to amplifying levers which trace a curve on a roll of graph paper

• A vertical movement of 1 mm is equivalent to about 1°C

Humidity

• Humidity is the amount of water vapour in a given volume of air

• Wet and dry bulb thermometers are used to measure relative humidity and are called a hygrometer

• Relative humidity is a measure of how much water vapour the air is holding in relation to the maximum amount of water vapour it could hold at a specific temperature

• Warm air can hold more water vapour than cold air

• When the air is holding as much moisture as it can, it’s said to be saturated

• If the air is not saturated, water vapour evaporates from the wet bulb muslin, this evaporation cools the bulb and the mercury contracts and registers a lower temperature 

• If the air is saturated with vapour, then evaporation is not possible, so both thermometers show the same temperature

Pressure

• Air has weight and exerts pressure on the Earth's surface

• Sea level pressure is approximately 1.03 kg/cm²

• Pressure varies with altitude and temperature

• Unit of measurement is millibars (mb)

• Lines of pressure on a map are called isobars

• A barometer measures air pressure of which there are 3 types:

  • Mercury barometer

  • Aneroid barometer

  • A barograph

• Mercury barometer is a hollow tube with all the air extracted 

• The open end is then placed in a bath of mercury

• Mercury is forced up the tube by atmospheric pressure on the mercury in the bath

• When the two pressures equalise, mercury will stop rising in the tube

• The height of the column of mercury will change with air pressure:

  • Rising as air pressure rises

  • Dropping as air pressure falls

• Aneroid barometer has a partly vacuumed, corrugated metal chamber inside

• There is a strong metal spring within the chamber that prevents it from collapsing

• The spring will expand and contract with changes in atmospheric pressure

• Levers magnify these changes, and a pointer moves across a calibrated scale to show atmospheric pressure at that time

• A barograph is a continuous reading of atmospheric pressure for one week

• Changes in pressures are traced on a rotating cylinder by a flexible arm

• The cylinder is covered by paper divided by 2-hour vertical lines

Sunshine hours

• The amount of sunshine a place receives is measured by a Cambell-Stokes sphere in hours and minutes

• The recorder is a glass sphere partly surrounded by a metal frame

• Sunlight is concentrated through the sphere onto a recording card placed beneath the focal point

• The rays burn a trace on the card

• The length of the trace shows the sunshine duration at that location

• At day's end, the card is replaced 

How it is used

• Sunshine recorder is placed in an open space, south-facing in northern hemisphere or north-facing in southern hemisphere

• Make sure the recorder is outside, unaffected by shade and has direct exposure to sun’s rays 

  • E.g. on roof of a building

  • On a pedestal or stand

  • Where the sun shines all day

• Make sure the paper (card or sheet) is inside

• The sun’s rays will scorch/burn the card (paper or sheet) 

• At the end of the day, measure the length of the burn line, then convert to hours and finally calculate the time it was sunny

• Record every 24 hours at the same time every day and stop at sunset to record for the day

• Remove and replace the card (paper or sheet) each day into sunshine recorder 

Clouds

• Cloud cover is measured in units of oktas

• Each okta represents one eighth of the sky covered by cloud

• 0 oktas = clear sky, 8 oktas = total coverage

• Clouds are categorised according to shape and height using Latin terms

  • Stratus which means layers

  • Nimbus which means rainy cloud etc.

• Clouds consist of tiny water droplets or ice particles that are too light to fall to Earth

• Clouds will form when air rises, cools and condenses into water droplets or ice crystals if cold enough

• The tallest clouds form in the tropical regions, as the tropopause is at its highest and clouds do not form beyond it

• Clouds only produce precipitation if they have enough water or ice particles that can collide and join together

• The particles will then grow too big and heavy to be supported in the air and will fall through the rising air currents

• Cumulonimbus and stratus are the only clouds capable of producing precipitation

  • Stratus clouds are just thick enough to produce drizzle

  • Cumulonimbus clouds have strong rising air currents, vertical height and thickness

Tropical storms 

• Tropical storm is the collective name for deep, low-pressure systems with spirals of strong air

• They are known as:

  • Typhoons in the South China Sea and west Pacific Ocean

  • Hurricanes in the Gulf of Mexico, Caribbean Sea and west coast of Mexico

  • Cyclones in the Bay of Bengal, Indian Ocean and northern Australia

• Damage is caused by high winds, floods and storm surges

• Tropical storms are rated on the five-point Saffir-Simpson scale based on wind speeds

• Tropical storms are considered major when they reach category 3 and have wind speeds between 111-129 miles (178-208 kilometres) per hour

• A category 5 storm can deliver wind speeds of more than 157 miles (252km) an hour

• The path of a hurricane can be erratic, so landfall is not easy to predict, and this makes evacuation times short

• Tropical storms develop as intense low-pressure systems over the warm tropical oceans

• Winds spiral rapidly around a calm central area known as the eye

• Tropical storms can be as much as 800 km in diameter, but winds are not constant across that, they vary with the strongest and most destructive winds being found within the eyewall