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Weather Watch - Pressure and Humidity Print E-mail
Written by William Finney   
Friday, 24 August 2007

Picture of Happy Clouds



We left off our discussion of the weather talking about recording rainfall and the temperature.  We continue our discussion talking about barometric pressure and relative humidity.  These two parameters can tell us much about the direction the weather is heading, whether we will have fair skies or rainy nights. 





Barometric Pressure


Barometers are devices that can measure the pressure of gases.  Historically they were glass tubes that were filled with mercury and the pressure was described as the height of the column of mercury supported by the atmosphere.  Earth’s atmosphere extends hundreds of miles above Earth’s surface.  Even though air appears very light when you have hundreds of miles of it, the weight can add up.  That weight, under the effect of gravity, spread out over the surface of objects is what we call atmospheric pressure.  Atmospheric pressure changes slightly with temperature and the amount of water in the atmosphere.  It is one of the most predictive weather phenomenon that we can observe.

We now know that mercury can be a very hazardous substance so instead of using mercury filled barometers we now use aneroid barometers almost exclusively.  A typical aneroid barometer has a needle that you can adjust to the current measurement so that when you make your next measurement you can easily tell whether the pressure went up or down.  As a side note, barometric pressure usually changes quite slowly over the course of a day or several days, so if you happen to see a sharp drop in barometric pressure over the course of an hour or two, be advised that hazardous weather is in your area. 

Relative Humidity

Water is important to the energy transfer processes in our atmosphere because it absorbs a lot of energy when it goes from the liquid state to the gas state and it releases that energy when it goes from vapor to liquid.  We have all heard the weather forecast talk about whether it is going to be a “humid” day or that the air is “dry” but what does that mean?  Relative humidity is not the fraction of the air that is water vapor as people sometimes think it is, but it is the fraction of water that the air can possibly hold at that temperature that is present in the air.  Confused?  Let me try to explain.

The air in our atmosphere always has some water vapor in it, but the amount can vary.  The warmer the air is, the more water vapor the same amount of air can hold compared to when that air is cold.  The temperature at which a certain amount of air is saturated with water vapor, its relative humidity is 100%, and cannot hold any more is called the dew point.  If air cools below its dew point temperature water must leave that volume of air going from the vapor state to the liquid state, we see this as precipitation. 

Imagine this, you have a box of air at 70 degrees F.  The air in this box is saturated with water vapor, it cannot hold any more.  The relative humidity of the air in this box is 100%.  Now imagine you could take a pair of tweezers and sort out the water molecules from all the other molecules in the box, and you removed half of the water molecules that were in the air in that box.  You would now have air with a relative humidity of 50%, the air is holding half of the water molecules that it can possibly hold.  


Measuring the relative humidity is probably the hardest measurement that anyone recording weather data can try to measure.  If you are looking for a device that is similar to the barometer or thermometer in that it has a scale you can directly read there are hair hygrometers.  Hair hygrometers have several drawbacks in that they tend to be very delicate, have very large errors when it is very humid or very dry and need to be recalibrated often.  A more accurate and practical way to measure relative humidity is to do so indirectly with a sling psychrometer.  A psychrometer has two thermometers, one with a small piece of cotton to cover the end of the bulb.  You wet the cotton with water and make sure that it covers the bulb of one of the thermometers completely and either fan air over its surface or spin it around above your head until the “wet bulb” temperature stops changing.  You then use a dew point temperature chart  to compute the relative humidity using the wet and dry bulb temperatures. 


Weather or Knot!  More is on the way!

In our next installment we will discuss two other weather phenomenon that we can observe and record, wind speed and duration of sunlight.


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