German Barometers

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:

1. Biological contaminants including bacteria, viruses, fungi, and mites.
2. Pathogens causing respiratory problems including allergic rhinitis and asthma.
3. 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.

Comparison method

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.

You asked???

How does a Gallileo Thermometer work?

A Galileo thermometer, (Galilean thermometer named after Italian physicist Galileo Galilei), or thermoscope is a thermometer made of a sealed glass cylinder containing a clear liquid. Suspended in the liquid are a number of weights. Commonly those weights are themselves sealed glass bulbs containing coloured liquid for an attractive effect. As the liquid in the cylinder changes temperature its density changes and those bulbs which are free to move, rise or fall to reach a position where their density is either equal to that of the surrounding liquid or where they are brought to a halt by other bulbs. If the bulbs differ in density by a very small amount and are ordered such that the least dense is at the top and most dense at the bottom, they can form a temperature scale.

The temperature is typically read from an engraved metal disc on each bulb. Usually a gap would separate the top bulbs from the bottom bulbs and then the temperature would be between the tag readings on either side of the gap. If a bulb is free-floating in the gap, then its tag reading would be closest to the ambient temperature. To achieve this requires manufacturing the weights to a tolerance of less than 1/1000 of a gram (1 milligram).

The Galileo thermometer works due to the principle of buoyancy. Buoyancy determines whether objects float or sink in a liquid, and is responsible for the fact that even boats made of steel can float (of course, a solid bar of steel by itself will sink). The only factor that determines whether a large object will float or sink in a particular liquid relates the object's density to the density of the liquid in which it is placed. Small objects, such as a pin, can float through surface tension. If the object's mass is greater than the mass of liquid displaced, the object will sink. If the object's mass is less than the mass of liquid displaced, the object will float.

Galileo discovered that the density of a liquid is a function of its temperature. This is the key to how the Galileo thermometer works. (As the temperature of water increases or decreases from 4 degrees Celcius, its density decreases.)

In the Galileo thermometer, the small glass bulbs are partly filled with a different (coloured) liquid. Once the hand-blown bulbs have been sealed, their effective densities are adjusted by means of the metal tags hanging from beneath them. Even though these bulbs expand and contract with changing temperatures, the effect on their density is negligible. The heating and cooling of the coloured liquid and air gap inside the bulbs will not affect their density. The clear liquid in which the bulbs are submerged is not water, but some inert hydrocarbon (probably chosen because its density varies with temperature more than water does).

These Galileo thermometers make ideal gifts. They are a decorative talking piece! For those with poor eyesight, reading the temperature is easy. The internal bulbs are coloured and range up to 27 degrees celcius. These quality items are made in Germany and suit our Australian conditions. When its over 27 degrees we know it is hot! Watch the bulb levels change when you switch on the airconditioner or heater!

A great gift for those who have everything!