Did You Know....(That There are 3 Types of Temperatures)?

Meteorologists commonly use three!   Dry-Bulb, Wet-Bulb, and Dew Point Temperature.

When most hear the term temperature, they often think of the measurements taken by a common thermometer that many people have in their backyards and that many media outlets use on a daily basis to let us know how warm or cool it will be in the next few hours or days. However, this only scratches the surface of how many different temperatures meteorologists use to calculate how much moisture is in the air and to determine the state of the atmosphere at a given moment in time.

 


 1.  DRY BULB TEMPERATURE

The cornerstone of these temperatures is the one most people observe and use on a daily basis called the dry-bulb temperature.  The dry-bulb temperature basically refers to the ambient air temperature, or in other words, the surrounding air temperature.  A dry-bulb temperature is measured by a basic thermometer (the one on th left below) in which the instrument is exposed to the surrounding air but cannot be in contact with moisture source, direct radiation from the sun, or something that radiates a lot of heat such as a black-top surface.

Dry Bulb vs. Wet Bulb Temperature

 


2.  WET BULB TEMPERATURE

If a thermometer would also be exposed to moisture while being in contact with the air (the gray, wet, piece of cotton wrapped around the right thermomenter above), the reading would be called the wet-bulb temperature.  The technical definition of the wet-bulb is the temperature an air parcel would have if it was cooled adiabatically, in which no heat is transferred to or from the parcel, to saturation at constant pressure by evaporating water into it.  So essentially, since no heat is added or taken away, the evaporation of water into the parcel cools it.

Now this may seem counter-intuitive because many may think that evaporation is a heating process since it occurs when a pot of water is boiled.  However, think of when you just come out of the pool and you feel chilled as you run for a towel.  This chill comes from the evaporation of water off your body, leaving you with water droplets on your skin that are at a lower energy, and thus a lower temperature.  As water evaporates on your skin, heat is removed from your skin and you feel cooler.  As a result, evaporation is indeed a cooling process!  You can make your own wet-bulb thermometer by wrapping a damp sock-like cloth over the bulb of an ordinary thermometer and then reading the thermometer.


 3.  DEW POINT TEMPERATURE

Knowing the wet-bulb and dry-bulb temperatures can help to determine the dew point temperature through a complex calculation.  This term may be more common knowledge as it is used in many cases to talk about how humid or sticky the air may seem.  In a more scientific definition, the dew point is the temperature in which a parcel of air (the "block" of air in image below), cooled at constant pressure and water vapor, becomes saturated.  In order to understand how the dew point is calculated, imagine a parcel of air at the surface, where the dry bulb temperature at 5 pm, is 80 degrees and the dew point is measured to be 60 degrees.  Let's assum that around this parcel of air is an invisible boundary that keeps the pressure and the amount of water vapor inside constant.

Now throughout the night, the dew point remains constant inside our air parcel, but the outside temperature decreases such that the dry bulb temperature at 5 am is now 60 degrees outside and inside the parcel, the same as the dew point. It is under these circumstances that we see dew form.  The dew forms because once the temperature cools down to the dew point, the air is saturated, meaning it cannot hold any more water vapor.  Consequently, the excess water vapor condenses into very small water droplets - called dew.  The result is similar to trying to put more water into a full glass of water, it overflows.   In our example, the dew that is formed is like a glass of water overflowing due to the fact that the air cannot hold any more water vapor.

 Dew Formation


 RELATIVE HUMIDITY

When dew forms, or when a cloud forms (see image below), it is at this point when the relative humidity, the ratio of how much water vapor is in the air over how much it can actually hold, is 100%.  By the way, the dew point temperature can never be greater than the dry-bulb temperature.  A simple experiment to calculate the dew point can be done at home with a can full of water and a thermometer.  Keep adding ice to the water and stir it with the thermometer.  When water begins to condense on the outside of the can, check the temperature of the water. It should be the same as the dew point!

Cloud Formation

Now our example from before can also be used to understand why clouds form and how high they form in the sky (see image above).  Imagine again that the temperature on the ground is the same as before, 80 degrees, and the dew point temperature is 60 degrees.  The air parcel is forced to rise and due to the imaginary boundary, the pressure and water vapor content remains the same inside the parcel.  Now, for a parcel like this the temperature cools at roughly 5 degrees per 1000 feet.  So after the parcel rises roughly 4000 to 4500 feet, the temperature of the parcel should be 60 degrees, the same as the dew point.  Just like the above example, the water vapor condenses at this point and produces a cloud, which is essentially just condensed water vapor! In this circumstance the relative humidity is, again, 100%.


HEAT INDEX (APPARENT TEMPERATURE)

All of the concepts from above relate in some way to how we calculate the heat index or wind chill. Both of which are "feels like" temperatures.  The heat index is a perceived temperature based upon the actual temperature and the dew point or relative humidity.  It was mentioned earlier that evaporation of sweat or water off of the skin was a cooling process.  So when the dew point or relative humidity is high, that means there is more water vapor in the air and and your sweat will not evaporate off your skin as quickly as opposed to when the dew point or relative humidity are low.  When you can't evaporate your sweat as rapidly, the body won't cool as quickly and your body will perceive a warmer temperature than the actual temperature.  This is how we measure the heat index and what we mean by "the feels like" temperature.

Below are a couple conversion charts for heat index that take into account both the relative humidity and the dew point.

 


 WIND CHILL

Calculating the wind chill, a perceived temperature, also depends on the rate water is evaporated off of the skin.  However, wind chill is more dependent on the speed of the wind than the heat index.  The wind simply removes heat from your skin surface.  Additionally, a higher wind speed corresponds to a more severe wind chill because more wind will evaporate greater amounts of water off of the skin, making you feel alot colder.  This is opposite to  the summer, where more moisture makes you feel warmer.  Essentially, more evaporation, the cooler you feel, less evaporation, the warmer you feel.  Below is a chart that calculates the wind chill based upon wind speed as well as the actual dry-bulb temperature.


Overall there is a great more deal of science behind these concepts and how they are actually calculated.  A lot of these calculations are done under the mathematics of psychrometrics, which can be easily researched and investigated on the internet and on various other NWS office websites.  Also it is extremely important to play close attention to the wind chill and heat indices because these values tell you how much danger your body is in during extreme weather.  When you see these values approach dangerous levels, it is important to take precautions that can limit your exposure to the elements.  Overall, now when you see a cloud up in the sky, see dew forming on your lawn outside, or experience extreme temperatures,  you have a basic understanding of the science behind them.

Andrew Winters – Student Volunteer NWS Milwaukee/Sullivan
Portions of information courtesy of NWS Office- El Paso, Texas.


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