B) Weather Instruments

1) barometer: measures air pressure.

Air has pressure because it consists of gasses such as nitrogen (78%) and oxygen (21%) which have mass and therefore exert pressure on the earth's surface.

There are different kinds of barometers including aneroid, mercury, and barographs.

High air pressure is consistently associated with dry conditions while low air pressure is consistently associated with clouds and precipitation. This occurs because with low air pressure, the air molecules are rising and leaving the earth's surface reducing the pressure on it. Once the air begins to rise, it's temperature cools, and the water vapour in it begins to condense into water droplets. These droplets combine to form clouds and possibly precipitation. The questions then is, what causes air to rise? That will be answered later in the unit under Processes of Precipitation.

With high air pressure, the opposite occurs. The air is sinking and putting greater pressure on the earth's surface. Air that is sinking is generally warming and evaporating moisture; hence making it difficult for clouds to form.

One may ask, how does one tell the difference between low and high air pressure? The dividing line between these two categories is called Standard Sea Level Pressure. Air pressure is measured in units called kilopascals (kPa) or millibars (mb). Some old home barometers may also measure it in inches. Standard Sea Level Pressure is 29.92 inches or 101.3 kPa or 1013 mb. To convert from kilopascals to miillibars, simply multiply by 10. Standard Sea Level pressure is the air pressure one can expect on a fair day with partly cloudy skies.

2) thermometer: measures air temperature. When using a thermometer, always remember to take a reading in the shade as you want the instrument to reflect the temperature of the air around it, not the temperature of itself being warmed by direct sunlight.

maximum / minimum thermometer: a special thermometer that measures high and low temperatures over a 24 hour period.

3) wind vane, wind rose or weather cock: measures wind direction.

Note: A wind is always named for the direction it is coming from.

4) anemometer: measures wind speed.

5) hygrometer or sling psychrometer with wet bulb and dry bulb thermometers: measures relative humidity, expressed as a %, compared to saturation or 100% .

Relative humidity is the amount of water vapor in a given volume of air divided by the maximum amount of water vapor that volume of air could contain at a given temperature before it would begin to condense into water droplets.

The dew point is the temperature to which the air must be cooled in order to reach saturation or 100% relative humidity. Foggy conditions, drizzle, or heavy rainfall are associated with 100% relative humidity.

6) rain gauge: measures precipitation totals expressed in millimeters (mm). Snow can be melted down and added or the snow depth can be roughly converted into a liquid equivalent. In general, 25 cm of snow is equivalent to approximately 25 mm of rainfall. This varies though depending upon the type of snow that has fallen (i.e. dry snow versus wet snow).

7) Campbell-Stokes Sunshine Recorder: records hours of sunshine.

8) Stevenson Screen: a compartment for housing a max. / min. thermometer & wet / dry bulb hygrometer; approximately 121 cm off the ground with louvered sides.

D) General Rules For Predicting Weather Changes