STRAIGHT FROM THE HORSES MOUTH

Duane A. Lienemann
UNL Extension Educator

     Is anyone else besides me tired of this incessant wind?  We finally get a little moisture (and I stress “little”) and then it seems it gets sucked right up by it.  I decided that it would be kind of fun to revisit my old science background and simply ask: What causes wind?  Some people will tell you that I have a lot of it, and may be the source, but let’s take a scientific look at what this four letter word really is this week. Just what is “wind”; what causes it; and do we need it? 
     It really was not until recent memory that what causes wind was fully understood.  I remember from the basics of Science 101 that wind is fundamentally caused by air flowing from high pressure to low pressure, in large part by the unequal heating of the earth's surface by the sun. However, to understand what makes the wind blow, we first need to understand what atmospheric pressure is. Pressure at the earth's surface is a measure of the 'weight' of air pressing down on it. The greater the mass of air above us, the higher the pressure we feel, and vice-versa. The importance of this is that air at the surface will want to move from high to low pressure to (drumbeat please) -- equalize the difference, which is what we know as wind.
     So wind is caused by differences in atmospheric pressure - but why do we get these differences? It's down to the rising and sinking of air in the atmosphere. Where air is rising we see lower pressure at the earth's surface, and where it's sinking we see higher pressure. In fact if it weren't for this rising and sinking motion in the atmosphere then not only would we have no wind, but we'd also have no weather. That can’t be good….or maybe some would argue that point with me.
     Of course, that isn’t the whole story, as we know it shifts directions and even forces.  The answer to why it shifts isn’t quite as simple. It seems that the earth’s rotation prevents that flow from being direct, but deflects it side to side (right in the Northern Hemisphere and left in the Southern), so wind actually flows around the high and low pressure areas. This movement around is important for very large and long-lived pressure systems. What I found interesting is that in small, short-lived systems; for instance the outflow of a thunderstorm, the wind will flow directly from high pressure to low pressure. The closer the high and low pressure areas are together, the stronger the pressure gradient, so the winds are stronger. So what that really means is that the further two systems are away from each other, the less wind, (or at least strength of wind) we experience and on the other end; the closer the systems are together the stronger winds we will have.
     Local winds, those that affect a comparatively small area, are often caused by heat transferred by convection. Direct radiation from the sun does little to heat the air. It is warmed chiefly by heat radiated from the earth. Intense local heating of the land causes air directly above to become greatly heated and to expand. As a result, some of the air aloft flows away, lowering the pressure over the heated area and increasing the pressure around it. The cooler, heavier air near the earth then flows to the heated area. This rising and sinking of air in the atmosphere takes place both on a global scale and a local scale. One of the simplest examples of a local wind is the lake breeze, think of Harlan County Dam for instance.
     On sunny days during the summer the sun's rays heat the ground up quickly. By contrast, the water surface has a greater capacity to absorb the sun's rays and is more difficult to warm up - this leads to a temperature contrast between the warm land and the cooler lake water. As the land heats up, it warms the air above it. The warmer air becomes less dense than surrounding cooler air and begins to rise, like bubbles in a pan of boiling water. The rising air leads to lower pressure over the land. The air over the lake remains cooler and denser, so pressure is higher than in the countryside. So we now have a pressure difference set up, and air moves inland from the lake to try and equalize this difference - this is our lake breeze. It explains why those sandy beaches are often much cooler than out in the country areas on a hot, sunny day.
     To be honest, I really hadn’t noticed before, but in watching the local weather on television this evening I did notice some lines. So what are they? On those weather maps, those are actually lines of constant pressure are drawn (isobars). These isobars are usually labeled with their pressure value in millibars (mb). The closer these lines are together, the stronger the wind. The curvature of the isobars is also important to the wind speed. Given the same pressure gradient (isobar spacing), if the isobars are curved anti-cyclonically (around the high pressure) the wind will be stronger. If the isobars are curved cyclonically (around the low pressure) the wind will be weaker.  Oh, let’s not stop there, we can find out even more.
     We know from experience that it makes a difference with barriers (windbreaks, stubble, etc.) and even the lay of the land on the type of winds we get. It should not be a surprise that friction from the ground slows the wind down. Well that makes sense. However, during the day convective mixing minimizes this effect, but at night (when convective mixing has stopped) the surface wind can slow considerably, or even stop altogether.  Therefore the “still of the night…”
     What is good about wind? There are times I really wonder about that.  We probably should admit that wind is one way that the atmosphere moves excess heat around. I can tell you from experience that on those hot, muggy days a wind or “breeze” is actually appreciated. We also must consider the fact that it helps remove odors, dust, and for our big city partners – smog. It is good for drying clothes on the line, hoisting kites and carrying smoke away from fires. It also helps in the harvesting process, whether it be drying hay down for baling or helping the dry down during the pre-harvest stage of most all crops. Since it is planting season, I guess I had better admit, it even dries the fields so that we can go out and get those seeds planted. Now, you have the answer to what causes wind and its primary function on our planet! Knowing all of this, I still don’t like these incessant winds, and I don’t really care what causes them, but now I know. But it still just blows!!

The preceding information comes from the research and personal observations of the writer which may or may not reflect the views of UNL or UNL Extension. For more further information on these or other topics contact D. A. Lienemann, UNL Extension Educator for Webster County in Red Cloud, (402) 746-3417 or email to: dlienemann2@unl.edu or go to the website at: http://www.webster.unl.edu/home