MEASURING OF AIR PRESSURE - Barometers
The air in its state of rest exerts a pressure directed perpendicular to any orientated surface. The static pressure acting on the surface of the earth is called atmospheric pressure or air pressure. It is caused by the weight of a column of air reaching from the earth's surface up to the top of the atmosphere. Atmospheric pressure is measured in hectopascals (hPa) or as the lengths of a column of mercury (mmHg) at zero degrees celcius and altitude 45 degrees. Under normal conditions the following is valid:
1 mmHg = 1 Torr = 1,3332 hPa.
To measure atmospheric pressure, aneroid barometers use the deformation of a flat circular metal box exhausted of air with faces corrugated for strength. Variations of the external atmospheric pressure cause changes in the displacement of the walls of the box as long as a new equilibrium position is reached so that there is equilibrium between the flexible forces of the springy walls or the spring respectively and the atmospheric pressure. The reading of the aneroid barometer is completely independent of gravity so that there isn't any gravity correction necessary in contrast to mercury barometers. The atmospheric pressure is next to air temperature, humidity, wind, cloud and precipitation one the most important elements of the weather. However as the development in weather is mainly determined by changes of the atmospheric pressure the aneroid barometer offers a good chance for meteorological observation and a local weather forecast.
MEASURING HUMIDITY - Hygrometers
Humidity refers to the concentration of water vapour in the air. A relative humidity of 100 per cent is when the air has as much water vapour as it can hold at a particular temperature, and mists or fogs form. High humidity can be uncomfortable and may bring on a feeling of impending rain or storms. It can be unpleasant especially during hot, humid weather, as the perspiration on your skin does not evaporate as quickly, hampering your body’s efforts to cool down.
Monitoring the humidity is important because it may affect our health. Hygrometers measure the amount of moisture in the air. Your house may be moist during the hot humid summer until you switch on your airconditioner. In winter, heating the air in your home will reduce the amount of moisture. This dry air in our homes and offices not only dries our skin, but also robs the delicate membranes of the nose and throat of their normal moisture. Low humidity may make us more uncomfortable or even subject to various respiratory problems.
However, there may be more effects of both high and low humidity than just discomfort. Studies show that humidity may affect three groups of factors with respect to health:
- Biological contaminants including bacteria, viruses, fungi, and mites.
- Pathogens causing respiratory problems including allergic rhinitis and asthma.
- Chemical interactions including ozone production. Some of these factors may thrive at low levels of RH while others may prefer high levels of RH.
INSTRUCTIONS FOR SETTING OF BAROMETERS
It is well known that air pressure decreases the greater the height above sea level. For that reason meteorologists base all their measurements of air pressure on sea level height, so as to have a constant value for weather maps and weather forecasts. We recommend to adjust the aneroid barometer in the same way so that your readings will be based on sea level air pressure.
The barometer is set by the manufacturer for a determined height above sea level which is shown on the box. Because this altitude is usually not the same as the height at destination it is suggested that you adjust your barometer after you purchase it.
A simple method for the adjustment of barometers is the comparison method. For this method there is no need to know the local altitude.
The weather forecasts usually give the air pressure values in millibars, inches or millimetres of mercury. The barometer dials are divided in these pressure units and you may adjust the barometer to the value which is given in the weather forecast. It is advisable to check the adjustment several times. The comparison of air pressure is at its most reliable if the weather forecast reports a constant high pressure zone.
Numerical calculation of the altitude correction
If you know the altitude at your destination and the altitude for which the barometer has been set by the manufacturer you can calculate the correction numerically. You remember that air pressure decreases about 0.1 inch of mercury per 91.5 feet of increase in altitude (1 mbar per 8 m). If the ultimate destination is situated lower than the altitude for which the barometer is adjusted, move the pointer anticlockwise to the lower pressure which you have calculated.
Example: Altitude for which barometer has been set: 660 feet (200 m)
Local altitude. 290 feet ( 90 m)
370 feet (110 m)
The pointer has to be moved by 370 :91.5 = 0.4 in Hg (110:8=14 mbar) to the lower pressure.
If you have calculated the value and direction of the correction adjust the pointer to the calculated air pressure value by turning the brass screw which you see In an opening of the back cover. Please check the adjustment by tapping on the glass.
Want to see the latest air pressures in Australia? Visit www.weatherchannel.com.au click on WEATHER MAPS and select SYNOPTIC CHART. There are all sorts of interesting weather facts available at this site. Another interesting website is the Australian Government's Bureau of Meteorolgy at www.bom.gov.au
General Rules for Weather Forecasts
1. At air pressures if 1020 mbar (30.1 in Hg) or higher, you can expect calm and dry weather . The higher the pressure the less changeable is the weather. In summer we have hot weather, in winter clear frosts and fog is also possible depending on your location.
2 At pressures between 1005 and 1020 mbar (29.7 – 30.1 in Hg) the weather is changeable.
3 At pressures below 1000 (29.5 Hg) the sky is mostly cloudy and the weather tends to rain. If the pressure falls considerably below 1000 mbar you can usually expect storms.
4. Slow and constant increases in air pressure is characteristic of a tendency to better weather, whereas steadily decreasing pressure generally indicates deteriorating weather
5. A rapid increase in air pressure followed by a drop in pressure is characteristic of changeable weather
6. In summer a sudden rapid fall indicates the approach of a thunderstorm
Temperatures and Humidity in the House
Often barometers are combined with a thermometer and hygrometer. Thermometers and hygrometers are measuring instruments for the climate inside your house. You feel comfortable at room temperatures between 18 and 22 degrees Celsius with humidity between 45 and 70 percent.
The relative humidity outdoors changes frequently. Indoors the humidity remains almost constant and changes only with the seasons. In summer we have 50 – 70% relative humidity, but in winter only 25 – 40% because in winter there is a great difference between the indoor and outdoor temperatures. Temperature and relative humidity will differ greatly in Australia – it depends on your location.