What Is the Wet Bulb Temperature?
If you are a skier and you are interested in snow level or snowmaking, you should get to know something called the wet-bulb temperature.
Modern thermometers work differently but the concept is the same. The dry-bulb temperature is the ambient temperature measured with shielding from radiation and moisture and the wet-bulb temperature is the lowest temperature that can be achieved through evaporation under ambient conditions.
Most of the time when we refer to temperature we are talking about the ambient air temperature or what is known to meteorologists at the dry-bulb temperature. This is the temperature measured by a thermometer that is exposed to the air but protected from radiation and moisture. Bulb refers to the rounded end of an old school liquid thermometer and dry bulb indicates that the air temperature recorded is not affected by moisture.
The wet-bulb temperature is the lowest temperature that can be achieved through evaporation under current ambient conditions. In the past, it was measured by wrapping the bulb of a thermometer in a wet muslin (hence "wet bulb").
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| Old school dry and wet-bulb thermometer setup. Source: https://collection.sciencemuseumgroup.org.uk/objects/co518421/wet-and-dry-bulb-psychrometer-for-humidity-measurement |
Modern thermometers work differently but the concept is the same. The dry-bulb temperature is the ambient temperature measured with shielding from radiation and moisture and the wet-bulb temperature is the lowest temperature that can be achieved through evaporation under ambient conditions.
The dry-bulb and wet-bulb temperatures are equivalent at saturation when the relative humidity with respect to water is 100%. Under such conditions, there is no way to achieve net evaporation into the air. However, if the relative humidity is below 100%, then the wet-bulb temperature is lower than the dry-bulb temperature. This is because evaporation can occur, which results in cooling of the air. That cooling is sometimes referred to as evaporative cooling or latent cooling. Latent means "hidden" cooling arising from changing the state of a substance. In this case water is changing from liquid to vapor.
Because of our dry climate, most Utahn's are pretty familiar with evaporative cooling. Evaporative coolers, sometimes referred to as "swamp" coolers, use this physical process to cool air. During a hot summer day with low humidity, this process is quite effective. During a hot summer day with high humidity, such as during a monsoon surge, this process is less effective. The reason for this is that the wet bulb temperature is higher on a humid day, so you can't cool the air as much through evaporation.
It turns out for skiers, the wet-bulb temperature is often the more important temperature. Snowmaking, for example, can occur at dry-bulb temperatures above 32°F if the wet-bulb temperature is below 28°F. This is because evaporation of the sprayed water droplets cools the air and cools the water droplets, allowing them to freeze even if the ambient dry bulb temperature is above 32°F.
The wet-bulb temperature is also useful for determining snow level. The traditional freezing level is based on the altitude where the dry-bulb temperature is 0°C. If the air is saturated, the falling snow will begin to melt as soon as it falls below this level. However, if the air is dry, the falling snow will experience sublimational cooling (sublimation is the conversion of ice to water vapor) and fall below the freezing level before it begins to melt, typically to a level where the wet-bulb temperature is 0°C. Additionally, if precipitation persists, the airmass itself will cool to the wet-bulb temperature, lowering the traditional freezing level.
This is why we use the wet-bulb zero (i.e., 0°C) or wet-bulb 0.5°C levels in products produced on weather.utah.edu. They are more closely related to snow level than freezing level. The former represents the wet-bulb freezing level and the latter represents the level at which we assume snow is transitioning to rain. We typically use our machine learning estimated snow-to-liquid ratio (SLR) at and above the wet-bulb 0.5°C level and assume there is a transition to all rain as one descends 200 m below the wet-bulb 0.5°C level.
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| Schematic transition zone in the Utah Snow Ensemble |
This is a simple approach. The reality is that snow can sometimes fall well below the wet-bulb zero (i.e., 0°C) or wet-bulb 0.5°C level if temperatures are relatively constant with height (i.e., near 0°C), precipitation rates are high (which increases not only evaporative and sublimational cooling rates but also cooling from melting), or the falling snow is dense and has a high fall speed. Graupel for example can penetrate a lot farther below the wet-bulb freezing level than small dendrites.
If you are interested in more on this topic, see the following posts:
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