Air Conditioning Guides & Resources

What is Dry Bulb Temperature?

Measuring dry air temperature allows for direct measurements of ambient air temperature. The dry bulb temperature is just what it sounds like. The term 'dry' denotes that measurements are being taken off the moist air movement, as opposed to measuring ambient air humidity or ambient temperature levels . As a result, this temperature differs from wet bulb temperature. The dry-bulb temperature (opposite of wet bulb temperature) is measured by allowing the sensor tip of a thermometer to rest directly in the humid air pressure. At sea level, this will provide an accurate reading of the humid air temperature unlike wet bulb temperature. The dry bulb temperature can be measured by a normal thermometer freely exposed to the actual air but it must be shielded from moisture, while this process isn't applicable to measure wet bulb temperature. The temperature is measured in degrees Fahrenheit or degrees Celsius or Kelvin. Zero Kelvin is equal to -273 deg C. ## How is it different from a wet bulb temperature? The wet-bulb temperature is measured by allowing moisture content to evaporate off of an moistened thermometer bulb exposed to the moist air molecules, cooling it in the process. By comparison, measuring dry air temperature allows for direct measurements of ambient air temperature. Since both methods are used to measure the same general phenomenon, they are related by what is known as a psychrometric chart. To properly use this tool one must be familiar with the various properties and environmental factors that will affect readings of an air parcel. The wet bulb temperature is also called as adiabatic saturation temperature or some experts use "isobaric wet bulb temperature" to define thermodynamic wet bulb temperature. Wet bulb temperatures and dew point temperature are usually correlated. Dew point temperature can also be measured through dew point temperature thermometer. The dew point temperature around a wet bulb will be different from the dew point temperatures in the surrounding. The temperature difference is corrected through relative humidity. The relative humidity also plays a role in measuring wet bulb temperature. The dew point will have a saturation line because after all, dew is the completely saturated water vapour that was created when the heat gained. This saturation line stays constant. The partial pressure of the water vapour around the wet bulb will be different from the partial pressure near the steam. The wet bulb temperature also denotes adiabatic evaporation. ## What affects dry bulb temperatures? The amount of solar radiation received at ground level affects dry-bulb temperature, as do wind speeds and cloud cover. Other elements, such as the height of the station above the ground and the amount of water vapour present in the saturated air parcel also help to change the same temperature readings. Dry-bulb temperature measurements are only useful in evaluating the air temperature. This is because humid air starts to vary considerably from visible moisture to gaseous water vapour concentrations. In most cases, the height at which a thermometer is placed will affect how much water vapour present in the air sample. When one is measuring the temperature of a cloud layer, for example, it's important to make sure that the thermometer is high enough above the ground. If it isn't, the dry bulb temperature reading will not be true and a lower wet bulb reading will show up in its place. ## How does a dry bulb thermometer work? A typical dry bulb thermometer is made up of a small aluminium or copper-bimetallic strip which turns red when the air temperature has risen enough. This strip is attached to the metal cap covering the bulb of an ordinary thermometer. As the sun warms things up, this thin strip expands, pulling on the metal cap and turning it until eventually, all of the mercury in the column is at the top end. At this point, some of it starts to flow back down into its original place again while most will have gone over into a reservoir at the top end. The amount that does this depends on how hot it was outside originally. More mercury comes out if things are really hot than if they are just warm or cool. Once the temperature starts to drop again, the strip contracts and pulls on the cap which turns it back once more until all of the mercury is at the lower end again. In this way, a continuous record of temperature variations is created. The relative humidity helps find dew point temperatures, which is figured out by the water vapour and latent heat around the bulb. The water vapour will evaporate near the bulb, which is measured through a thermometer. A bulb wrapped with any coating will have less latent heat and increased water vapour near it. For more information on calculating relative humidity, take a look at this explanation from the [Bureau of Meteorology](http://www.bom.gov.au/climate/averages/climatology/relhum/calc-rh.pdf). To understand how a bulb thermometer works, you should be aware of these terms- constant pressure, heat content, wet bandage, latent energy, vapour pressure, mass transfer coefficient, etc. The difference between a dry bulb thermometer and a wet-bulb thermometer is that in a wet-bulb thermometer version, water vapour from water at the bottom end of the thermometer evaporates up into its place creating a cooling effect which is measured by the lowering of the mercury in both columns. ## How do I use a dry bulb thermometer? A constant dry bulb temperature thermometer is typically used in conjunction with a constant wet-bulb temperatures thermometer to determine the dew point and relative humidity levels. The two readings, when plotted on a psychrometric chart, can also help meteorologists and climatologists estimate future weather conditions including rainfall totals. In order to use this tool properly, it is necessary to find out what the prevailing wind direction is from reliable sources such as weather stations or even wind direction maps published online at various websites. This information will be needed to determine which end of the strip should face into the wind. One must also make sure that there is no obstruction preventing accurate measurements from being taken. ## Where are dry bulb thermometers used? Dry bulb thermometers can be used in a wide variety of applications which include testing the accuracy of other types of weather instruments and recording temperature range variations at various points in different environments such as near airports or shipping ports. Because the information is transmitted via radio waves, it is possible to monitor such things without having to place equipment directly on the ground. They're different from thermometers used to measure wet bulb temperature. These tools are also commonly used in meteorological applications such as weather stations and by companies looking for ways to keep track of outdoor conditions in order to optimise product storage and transport. ## How is atmospheric pressure measured? A dry bulb thermometer works by measuring how much heat has been removed from the air property in order to form water droplets (droplets are formed when the moisture condenses, and the technical term is evaporative cooling). The wet bulb version does this by producing an uninsulated canvas sleeve that is kept wet with distilled water. As this evaporates, it cools down the surrounding air, just like when perspiration evaporates off your skin. The difference between these two temperatures can then be used to determine relative humidity levels and dew point details. Atmospheric pressure can also be measured using a barometer. If you bring ice cubes or a cold bottle near the bulb, you'll notice the heat lost and reduced water vapour in the case of a wet bulb as opposed to a dry bulb. Latent heat will be less and less water vapour will be produced (also called wet bulb depression). The convective heat transfer in a wet bulb is low, hence the atmospheric pressure will be low (also called thermodynamic wet bulb temperature). Sensible and latent heat (otherwise called sensible and latent energy) are different from each other. ## How do I choose the right thermometer? When looking to purchase a dry bulb thermometer, it is important to consider what it will be used for and whether you will need any additional accessories such as adapters or stands. Some models also come with an integrated hygrometer and wind gauge as well as a [complementary thermometer](/guides/air-conditioning/what-is-a-dry-bulb-thermometer). If you know the dew point and humidity levels, it will be easier to make sure that the device being considered provides accurate readings. ## What are the alternatives to a dry bulb thermometer? Today there are relatively low-cost devices that can provide information on atmospheric temperature variations via mobile phone apps or consumer websites. These may not always be suitable for more demanding professional use but they can help to determine current weather conditions with a degree of accuracy so it is possible to plan accordingly. A more sophisticated alternative that can be used to measure dew point and relative humidity levels is a data logger which can collect information automatically for longer periods of time while providing high-resolution details. ## How can I get the most out of my dry bulb thermometer? When taking measurements, you need to ensure that the device is stable during operation so it remains accurate. There should also be no obstructions blocking the dry airflow between it and the object being measured. Since these devices are not overly expensive, it makes sense to purchase one with an integrated hygrometer in order to collect additional data on airflow speed and direction if they are needed. It may help you run your home [air conditioning more efficiency](/guides/air-conditioning/energy-saving-air-conditioner).