2.3.1 Weather

Weather Instruments

1. Rain gauge: Is a fixed diameter so that they collect the same amount of water & so comparisons can be made.

• Made of a hollow cylinder (C) containing:

• Funnel (A) to collect the water.

• A container to collect the water. This might be graduated or water can be poured into a measuring cylinder. It is emptied once every 24hrs at the same time. Rain is measured in millimetres.

• Is sunk into the ground, but not level with the surface so that splashes or surface water cannot get into it.

2. Maximum-minimum thermometer: Records max. & min. temps. over a 24hr period.

• Maximum thermometer contains mercury the minimum contains alcohol.

• As temperature rises mercury expands & pushes up a metal index when it cools & mercury contracts the index is left in place at highest temp.

• As temperature falls alcohol contracts & pulls metal index with it, but as the alcohol expands it flows passed the index leaving it in place at the lowest temp.

• Both indexes are read once every 24hrs from the bottom of the index.

3. Wet and dry bulb thermometer (hygrometer): Dry bulb is a normal mercury thermometer it measures actual air temp.

• Wet bulb is the same but the bulb is covered with a fine cloth which is connected to a reservoir of water.

Water evaporates from the cloth & cools the temperature so it reads a few degrees lower than air temp.

• Both the wet & dry bulb temperatures are read.

4. Barometer: Measure air pressure.

• An aneroid barometer has a corrugated chamber containing a vacuum.

• As air pressure rises & falls the chamber contracts & expands.

• Levers conduct this movement to a spindle which moves the pointer on

the dial which records the air pressure in mm of mercury.

5. Anemometer: Measures wind speed.

• Three light rotating cups are blown around by the wind the revolutions are counted & converted into; metres per sec.; km per hour; knots etc.

6. Wind vane: Records wind direction.

• The fang is blown by the wind so that the arrow head points into the wind.

• Both are mounted on a high pole.

Stevenson’s Screen: Contains the thermometers.

• Painted white to reflect the sun with a double lid for insulation.

• Slatted sides to let the air circulate, but slanted downwards to prevent light getting in.

• Legs 1m long to prevent heating from ground.

• On short grass so it’s standardised i.e. same amount of reflectivity.

Cloud types:

Cumulus: These clouds usually have flat bases lumpy tops & distinctive boundaries.

Cumulonimbus: The cloud type associated with a thunderstorm & heavy rain, (at high levels cooled water droplets convert to ice crystals).

Altocumulus: These clouds are a good indicator of medium level instability and high moisture content -and often the precursor for widespread thunder activity within the following 24 to 48 hours.

Stratocumulus: Low-level cloud type, varying from thin, well broken layers with little impact for general weather, to deep, sometimes unstable character, and a risk of moderate turbulence & moderate icing. Little rain.

Nimbostratus: Heavy cloud layer, often dark in appearance. Often combined with continuous falling rain/drizzle or snow

Stratus: Are thin-layered clouds

• Are low to the earth’s surface

• Look like stripes or streaks in the sky.

• No rain.

Cirrus: Thin because they form in the higher levels of the atmosphere where little water vapor is present. No rain. Made of ice crystals

Locating weather stations

• They should be on short grass. (Concrete reflects heat, tarmac absorbs it.)
• They should be in an open area away from obstacles like buildings & trees so there is no shade or protection from the wind or rain

Weather symbols

Cloud cover is measured in eighths: Hold the sheet directly above your head & estimate the cloud cover directly above you.

Warm and cold front symbols: