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Weather Watch - Wind and Sun Print E-mail
Written by William Finney   
Saturday, 25 August 2007

Snowy Scene




In this part we talk about three more weather observations that we can make, wind speed, direction and the duration of sunlight.  Making these measurements are a little more involved and good for someone looking for a project as commercial instrumentation is likely to be out of the price range of most people.  These measurements round out the observations that we have previously discussed and help with making a complete record of the daily weather.


Wind Speed and Direction

I discuss both of these together because talking about the speed of the wind doesn’t make much sense without talking about the direction that the wind is coming from.  Generally that is how winds are described, not in the direction that they are traveling but in the direction from which they are originating.  The direction of the wind is influenced by several factors that work on several scales.  The first and largest factor is the pressure gradient.  In Part 2 we discussed barometric pressure and how it changes from day to day.  When we are between areas of higher and lower atmospheric pressure there is a gradient of pressures.  The air in the atmosphere moves from the regions of higher pressure to the regions of lower pressure.  Imagine you have a long thin balloon, like the from which animal shapes are made.  If you squeeze, or increase the pressure, on one end of the balloon the other end expands and gets larger because the air has moved from the end you were squeezing to the other side.  Other factors include the Coriolis effect (rotation of the Earth on its axis), localized weather phenomenon and physical geography.  By observing the direction from which our winds come we can sometimes tell what kind of weather to expect. 


The device used to measure the direction of the wind is the weather vane or wind vane.  Some suggestions on the materials you might need to make one can be found here .  Make sure your wind vane is placed high enough so that trees and buildings don’t interfere with the observed direction of wind travel. 

Wind speed can be measured with a device called an anemometer.  This is a device that has cups that catch the wind, spinning it around.  As the speed of the wind increases, so does the rate of the rotation of the cups.   Anemometers are often sold as parts of weather stations that can get to be a little expensive, with a little work, however, you can make your own. Simple anemometers useful for demonstrating the concept can be made with cups or with plastic egg pieces .

These are read by counting the number of rotations that the cups make in a given amount of time.  These are not too likely, however, to stand up to being left outside, high winds, or heavy use.  To build an anemometer that is a little more robust and easer to read you might want to construct one using the sensor from a bicycle speedometer to give you a digital readout.  Instructions can be found here and here.

In all cases you will need to calibrate your anemometer if you want to record wind speed in miles per hour or kilometers per hour.  The simplest way is to very carefully mount your anemometer on a car and to have someone drive the car around at different speeds, counting the number of rotations in a given amount of time or recording the value given by the bicycle speedometer at that speed.  If this is done at several speeds a graph can be constructed that relates the number of rotations per second or the bicycle speedometer reading to the speed of the car.  You will probably have to drive in both directions, to account for the effect of the wind speed on your measurements unless you have a perfectly still day.

Wind speed is important because it can make the air temperature feel colder than it actually is.  This effect is called wind chill.  Think about it, in the summer we use a fan to move air over our bodies to make us feel cooler, this works when the movement of the air is from the wind outside too!  This can result in dangerous conditions in the winter where even short exposures can result in severe injury.

Cloud Cover / Duration of Sunlight

The length of a day is controlled by your latitude, the rotation of the Earth on its axis and the position of the Earth in its orbit of the sun.  Whether the Earth receives the maximum amount of direct sunlight that is possible each day is effected by cloud cover.  Direct sunlight is important in agriculture and in our health. 

One device that you can use to measure the duration of sunlight is a Campbell-Stokes recorder or a sunshine recorder.  Here a glass sphere is used to focus the sunlight onto a piece of paper.  When the sun is bright, it burns a hole in the paper.  A cloudy sky will burn no hole or just a faint mark depending on the reduction in the sunlight.  As the sun “moves” across the sky the focus moves across the paper, so a record from sunrise to sunset can be obtained on one paper strip.  They can be purchased but are quite expensive.  You could probably build your own with a crystal ball and some practice to get the distance between the paper and the crystal ball just right.

If you are looking for an electronics project you could also build your own pyrometer .  I seem to remember an electronics book by Forest Mims that I purchased from RadioShack some time ago that had quite a good section on these measurements, but my understanding is that these books are now out of print.  If I can find it I will post the title and ISBN.  If  you want a day long record, like with the Campbell-Stokes recorder you will have to interface the pyrometer with a computer. 


What to do with it all?

 I know that we have talked a lot about what to measure but what can we do with these measurements?  In our final part we will talk about what you can do with all these weather phenomena that you have learned to record.  We will also have some questions for further thought?


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