Collecting Weather Data (Cambridge (CIE) IGCSE Geography)

Weather Instruments

Stevenson screen

• This wooden box stands on four legs at a height of 121 cm to avoid ground heat affecting the thermometer bulbs at 125 cm

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

• Made of wood to prevent heat transfer

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

• The wood is double-layered with airspace for insulation

• The screen is placed on grass to reduce ground heat

• Daily readings are taken at the same time

• Today, meteorological stations send data to the Met Office's computers using automated digital recording equipment

• The instruments inside include:

  • Maximum-minimum thermometer (Six's thermometer)

  • Hygrometer with wet and dry bulb

• Instruments found outside include

  • Rain gauge

  • Wind vane

  • Anemometer 

• The weather station layout is:

  • Barometers and barographs are protected from severe winds, direct sunshine, and heat

  • Heat-radiating buildings are avoided for thermometers

  • Wind vanes and anemometers should be placed outside, away from trees and buildings, and at least three times the height of the nearest obstacle

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

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

• Anemometers measure wind speed

• Wind speed is measured in m/s or km/hr

• The anemometer has 3 or 4 cups on metal arms that spin freely on a 10 m vertical shaft

• The cups rotate quicker and the counter records more rotations with stronger winds

• Hold digital portable anemometers into the wind, and the fan revolves to display the number on the screen

• Many digital anemometers send data directly to apps and computers for readings

• Wind vanes and anemometers are far from buildings and trees that could block airflow

• Buildings can form wind tunnels or delay airflow, affecting reading accuracy

  

Precipitation

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

• The 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 a tapered measuring cylinder 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

• Measurements are in Celsius ° C or Fahrenheit ° F, depending on location

• 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  

  • Recording the daily mean temperature for a month and dividing it by the number of days in that month provides the mean monthly temperature

  • The sum of the mean monthly temperatures divided by 12, which 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

• Hygrometers measure relative humidity with wet and dry bulb thermometers

• Relative humidity measures the amount of water vapour in the air compared to its maximum capacity at a given temperature

• Warm air holds 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 wet bulb muslin, cooling the bulb and lowering the mercury temperature

• If the air is saturated with vapour, evaporation is impossible, 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

• The 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

• A mercury barometer is a hollow tube with all the air extracted 

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

• The atmospheric pressure on the mercury in the bath forces it up the tube

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

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

  • It rises as air pressure increases

  • It drops as air pressure falls

• An 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

• A flexible arm tracks changes in pressure on a rotating cylinder

• Paper with 2-hour vertical lines divided across it covers the cylinder

Sunshine hours

• A Cambell-Stokes sphere calculates the hours and minutes of sunshine that a location receives

• The recorder is a metal-framed glass sphere

• The sphere focuses sunlight on a recording card below the focal point

• The rays leave a trace on the card, indicating the length of the sunshine at that point

• 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 

  • On the 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 an eighth of the sky that is cloud-covered

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

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

  • Stratus means layers

  • Nimbus 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

• As the particles grow too big and heavy to be supported in the air, they 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

Cirrus

• Level: high above 6 km

• Description: narrow wisps, white and made of ice crystals

• Weather: fine

Cirrostratus

• Level: high above 6 km

• Description: thin, white layers made of ice crystals with a wide horizontal spread often covers whole sky

• Weather: fine

Cirrocumulus

• Level: high above 6 km

• Description: thin, white, heaped cloud with ice crystals

• Weather: fine

Altostratus

• Level: medium 2-6 km

• Description: can be thin and white or grey and thick with layer of water droplets

• Weather: fine

Altocumulus

• Level: medium 2-6 km

• Description: thick white, or grey-looking, heaped cloud of water droplets

• Weather: fine

Stratus

• Level: low 0-2 km

• Description: thin, uniform, grey sheet of small water droplets with a fairly flat base

• Weather: fine drizzle

Cumulus

• Level: low 0-2 km

• Description: white with a darker, flat base and a billowy/globular upper surface. Made of water droplets and can be compact or have height

• Weather: sunny by day, fine weather

Stratocumulus

• Level: low 0-2 km

• Description: white and grey partly heaped cloud made of water droplets

• Weather: fine

Nimbostratus

• Level: base can be low or above 2 km

• Description: thick, dark grey layers of water droplets

• Weather: steady rain or drizzle

Cumulonimbus

• Level: low base, but cloud usually extends to high levels

• Description: dense, dark grey with vertical height, storm clouds. Form from cumulous clouds and have a billowy head. The head flattens as it reaches the tropopause and will then spread out in an anvil shape. Composed of ice crystals at the higher levels and water droplets at the lower levels

• Weather: very heavy rain, snow showers or hail with thunder and lighting

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

• High winds, floods, and storm surges all cause damage

• 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 (252 km) 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