Basic Instruments &Tools for Weather Prediction
While the UK Meteorological Office & National Weather Service uses sophisticated equipment such as Doppler radar and high-altitude balloons to collect data, you can use many of the same tools they use.
Barometers measure the air pressure, which is sometimes referred to as barometric pressure. The pressure of the air on the pool of mercury in the barometer causes the mercury to rise in a tube. We measure the height of mercury in the tube in inches. Therefore, air pressure is often stated in inches of mercury.
More common are aneroid barometers, which don't contain mercury but have a small box inside instead. The air pressure on this box causes it to change shape, moving a needle on a gauge that indicates the air pressure. Normal air pressure readings vary from 950 to 1050 millibars. Quick changes in air pressure often mean a change in the weather is about to occur. That's why you'll often hear and read about barometric pressure during local weather reports.
Meteorologists use anemometers to measure wind speed, but you can estimate wind speed just by looking around. Watch how smoke rises in chimneys, how leaves move in trees, and how flags wave in the wind. Sailors and other people sometimes rate their observations of wind speed according to the Beaufort scale.
If you don't have an anemometer to measure wind speeds, you can get a good idea of how fast the wind is blowing just by looking at objects around you. In 1805, the British Admiral Sir Francis Beaufort devised an observation scale for measuring winds at sea. The Beaufort Scale measures winds by observing their effects on sailing ships and waves. Beaufort's scale was later adapted for use on land and is still used today by many weather stations.
While anemometers measure how fast the wind is blowing, wind vanes tell you from which direction the wind is blowing. And knowing where the wind is coming from might give you clues to the temperature and the amount of water in the air moving into an area. For example, winds from the south are often warmer and carry more moisture than winds from the north.
Psychrometers, or wet bulb thermometers, measure relative humidity. A psychrometer uses two thermometers, one bulb of which is covered with a wet cloth. As the cloth dries, the cooling effect of evaporation lowers the temperature on that thermometer. Then the temperatures on the two thermometers are compared on a special chart to find the relative humidity. Often, the relative humidity is the weather condition that makes people the most uncomfortable.
Thermometers measure the air temperature via the expansion or contraction of a liquid or a metal as the air temperature changes. Some thermometers contain red-coloured alcohol, others contain mercury, while still others have a bimetal coil attached to a gauge.
Most temperature scales today are expressed in degrees Celsius (°C), although one will sometime see Fahrenheit (°F) in use, particularly in the United States. The Celsius scale is fixed by two points, the freezing and boiling point of water, which at normal atmospheric pressure are 0°C and 100°C respectively. The scale is then divided into 100 units. 0°C is equivalent to 32°F and 100°C to 212°F. The Kelvin temperature scale is the absolute temperature scale. Absolute zero, the coldest temperature possible in the universe is 0K or -273°C. Because one Kelvin is equivalent to one degree Celsius, 0°C is the same as 273K. 15°C is the same as 288K.
Special thermometers are used to indicate the maximum and minimum temperatures reached over a period, usually one day. For the amateur, a popular combined maximum and minimum thermometer is the U-shaped thermometer. Thermometers are also used to measure the temperature of the ground at night, which may fall several degrees below that of the air above, and to calculate the humidity of air.
Rain gauges are very simple instruments used to measure the amount of liquid precipitation. Any open container with a flat bottom and straight sides will work just by adding a scale of inches to it. Other kinds of precipitation are usually recorded by collecting the precipitation in a similar instrument, then letting the precipitation melt to find out the liquid equivalent. But at home, you might just want to use a ruler or yardstick to measure a deep snowfall! Then you can use the average conversion of 10 inches of snow equals 1 inch of rain to find out how much water fell.
Records of daily weather conditions have of course been
kept for 200 years and more but, traditionally, have always required a
diligent and dedicated human observer to record readings from manual
instruments at a fixed time, without fail, every single day. And to
analyse the daily data collected over months and years, more painstaking
paperwork was called for.
All current weather readings can be seen from indoors, at a glance and at any time, Routine daily maintenance chores (e.g. emptying the rain gauge) are done automatically. AWS stations can automatically record maximum and minimum values for a range of weather parameters through each day and keep track of total monthly and yearly rainfall.
A data logger and PC can be readily linked to the station
so that all weather data is automatically logged, This means that, automated systems can run for weeks and months without
attention whilst continuously recording all details of the weather.
Much greater detail is available e.g. the complete pattern of wind speed &
direction through the day can be logged.
Comprehensive statistics can be automatically calculated and analysed,
impressive visual graphics can be displayed.
Detailed weather conditions may be viewed at any distance from the station
itself, for example over the Internet.
The pressure chart shows the distribution of atmospheric pressure. Pressure systems - depressions (LOW pressure regions) and anticyclones (HIGH pressure) are marked and Isobars are drawn on the chart to link areas with the same pressure. Isobar lines are drawn at 4mB interval (4 HPa) and weather frontal systems are marked using standard symbols.
Wind direction and some indication of strength can be deduced from the pressure chart. In the Northern Hemisphere, winds blow in an anti-clockwise direction around a depression (LOW) and in a clockwise direction around an anticyclone (HIGH). The closer the isobars are together, then the greater the pressure gradient and the higher will be the wind strength.
Pressure charts are a useful help in interpreting satellite images. The satellite image shows the pattern of cloud cover and with the help of the pressure chart, frontal systems can be identified and tracked over a period of time. Typically, rain will be associated with the passage of a front - identifying and tracking the fronts can allow the forecast of rain, changes of temperature, wind direction and speed etc.
A cold front marks the leading edge of an advancing cold air mass. On a synoptic chart a cold front appear as a blue line with triangles. The direction in which the triangles point is the direction in which the front is moving.
A warm front marks the leading edge of an advancing warm air mass. On a synoptic chart a warm front appears as a red line with semi-circles. The direction in which the semi-circles point is the direction in which the front is moving.
Occlusion (or occluded front)
Occlusions form when the cold front of a depression catches up with the warm front, lifting the warm air between the fronts into a narrow wedge above the surface. On a synoptic chart an occluded front appears as a purple line with a combination of triangles and semi-circles. The direction in which the symbols point is the direction in which the front is moving.
Fronts describe thermal characteristics. They also happen to be where there is significant precipitation. However, precipitation is not confined to fronts. Drizzle in warm sectors or showers in cold air occur fairly randomly, but occasionally, lines of more organised precipitation can develop. These are called troughs.
Isobars are lines joining places with equal mean sea-level pressures (MSLP).