Dry bulb temperature refers basically to the ambient air temperature. It is called dry bulb because it is measured with a standard thermometer whose bulb is not wet - if it were wet, the evaporation of moisture from its surface would affect the reading and give something closer to the wet bulb temperature. In weather data terms, dry bulb temperature refers to the outdoor air temperature.

It is usually given in degrees Celsius (°C) or degrees Fahrenheit (°F), however its true SI unit is Kelvin (°K). On the Kelvin scale, 0°K equals -273°C.

Figure 1 - Where the air is cold...

Lines of constant dry bulb temperature run vertically up the Psychrometric Chart, as shown above in Figure 2. This makes it the primary scale along the X-axis. Dry bulb temperatures in Earth normally range from around -20°C at the poles up to around 46°C on a very hot day in a desert. Conditions outside this range can occur, but they make for environments that are far from habitable.

Figure 2 - Dry bulb temperature lines shown on the Psychrometric Chart.

Measuring Temperature

The dry-bulb temperature of air is measured by a thermometer which is freely exposed to the air but is shielded from radiation and moisture. A thermometer can be made from any substance with a property that changes predictably with temperature. The most commonly seen thermometer uses mercury, a substance whose volume increases with increasing temperature.

A quantity is placed in a reservoir connected to a very thin glass tube. A small change in volume is therefore made quite evident as a noticeable movement within the thin tube. Other liquids can also be used in this way. Other forms of volume-based thermometers use the differential expansion of metals in the form of a wound spring.

More recently, materials have been found whose electrical resistance change with temperature. These are usually manufactured from the oxides of transition metals such as manganese, cobalt, copper or nickel. They are called thermistors and are usually found in data-loggers and as computer-based sensors. When the temperature increases, the resistance of the thermistor decreases. Conversely, if the temperature decreases, the thermistor's resistance increases. Thermistors have this property because at high temperatures, their electrons gain more kinetic energy and vibrate more vigorously.

Such vigorous vibrations break the bond and free some electrons for the conduction of electricity. The higher the temperature, the more kinetic energy the electrons gain - more bonds are broken and thus more electrons are freed. Thus the resistance of the thermistor decreases.

Figure 3 - Different types of dry bulb thermometer.

In a weather station, the thermometer is usually housed in a Stevenson Screen. A Stevenson Screen is a white boxed shelter that contains temperature and relative humidity equipment. It shields the instruments from sunshine and precipitations and has louvered sides to permit the free movement of air. Ideally the shelter is placed over grass, mounted at 1 meter above the ground and as far from any buildings as circumstances permit.

Figure 4 - A Stevenson Screen used to protect dry bulb thermometers from radiant energy and moisture whilst still providing a relatively free flow of outside air.

Related Links

INNOVA Thermal Comfort booklet

http://www.innova.dk/books/thermal/thermal.htm

About Temperature

http://www.unidata.ucar.edu/staff/blynds/tmp.html

How Thermometers Work

http://www.howstuffworks.com/therm.htm