Earth Science Today
Russ Colson
Minnesota State University Moorhead

Weather and Climate:

Topic 1: Weather, Winds, and Rain

     In October of 1732, an eclipse of the Moon, which Benjamin Franklin had hoped to observe from Philadelphia, was obscured by a storm.  The storm was a "Nor'Easter", a storm with strong winds from the Northeast.  Yet, further to the Northeast, the storm began much later and the eclipse was visible.  Benjamin Franklin writes "This puzzles me, because the storm began with us so soon as to prevent any observation; and being a northeast storm, I imagined it must have begun rather sooner in places farther to the north-eastward than it did in Philadelphia."
     But it didn't.  The storm began first downwind.  Thus Benjamin Franklin documents one of the first clues to the nature of storms and where they come from.
     This story also illustrates that the real discoveries in science are the things that are confusing, that don't fit our existing mental model of nature.  It is interesting that from the disappointment of missing the eclipse was born a more fundamental discovery about weather.

• Where does the wind come from?

     All weather, including winds, results from the attempt to balance out the uneven distribution of solar energy on Earth.  In general, for example, there is more energy at the equator and less at the poles.  Sometimes local heating imbalances arise due to differences in how much sunlight is absorbed by the earth.
     These heating imbalances result in differences in pressure from one place to another on both global and local scales.  Differences in pressure result in winds, as the air attempts to move from areas of high pressure to areas of low pressure (illustration of how pressure differences and winds result from differential heating in coastal areas).
     The key to what makes winds go is not simply pressure, but pressure differences from one place to another.  Pressure differences are usually shown on a map with isobars, lines that pass through points with equal pressure.  Isobars allow us to quickly see where the pressure at Earth's surface is high and low (exercise in drawing isobars, est4a1.html).  The difference in pressure can be expressed as pressure gradient.  Pressure gradient indicates how steeply pressure changes from one location to another.  It is analogous to the steepness (gradient) of a ramp.  Just like a ball rolls faster down a steeper ramp, wind blows more strongly in a steeper pressure gradient.  (illustration of pressure gradient).
     One might expect that wind will blow directly from high pressure to low pressure areas, but, because the Earth is spinning, this does not occur.  The Earth's spin causes the wind to bend due to a Coriolis Effect.  You feel this effect on a spinning merry-go-round when you try to move inwards or outwards on it.  You feel a force pulling you to one side.  Winds on a spinning Earth also respond to this effect.  You can also illustrate this effect by rolling a marble on a spinning carousel (such as might be used for cakes) (illustration).
     The coriolis effect causes winds to be bent to the right in Earth's northern hemisphere and bent to the left in the southern hemisphere.  If we ignore the effect of friction between the wind and Earth's surface, the winds will be bent until the pressure force and the coriolis "force" balance each other.  This occurs when the winds are parallel to the isobars (illustration).
    The result of this effect is that winds circulate counterclockwise around low-pressure centers in the northern hemisphere of Earth (you can understand this by imagining the winds trying to blow toward the low, but getting bent to the right, causing them to circulate counterclockwise).  Winds circulate clockwise around high pressure centers.  In the southern hemisphere the circulation is the opposite, clockwise around lows and counterclockwise around highs.  This circulation means that given the wind direction you can estimate where a low pressure storm is relative to your own location.  Stand with your back to the wind, stretch out your left hand sideways and it points approximately in the direction the low is from you (You always wondered what those guys were doing standing around with their arms out!).  (wind direction and velocity puzzle).
     Because strong storms often move out of the southwest, an easterly or southeasterly wind can presage a storm.  Also, the strongest part of a winter storm is usually in the northwest quadrant of the low pressure center (go look at the isobar exercise above, and figure out what direction the wind in Fargo-Moorhead was blowing from during the storm of Nov 28, 2000, and what quadrant of the low we were in - answer).  So winter storms often occur when a low passes to our south.
     The way that the wind changes direction as a storm approaches reveals whether the low is passing north or south of us.  A backing wind is one in which the wind shifts in a counterclockwise direction (from south to southeast to east to northeast).  A veering wind is one in which the wind shifts clockwise (from south to southwest to west to northwest).  You can figure out whether a backing or veering wind is a harbinger of a major winter storm (backing and veering wind puzzle).

• Where does rain come from?

     Why, from clouds, of course!  But where do the clouds come from?  A friend told me of a teacher in Fargo who asked his 8th grade class what clouds are made of.  When they said "water" he ran out of the classroom, got a big bucket full of water, ran back in, and threw the whole bucket full of water up into the air.  When it came splashing down, he asked them with a tone of dismay "What happened?  "Where is my cloud?"!!  (someday I'm going to work up the courage to do that!)
     Clouds are formed from water that was dissolved in the air and condenses out in small droplets.  We can watch water dissolve into the air when we boil water.  The water becomes part of the air.  When we set our cold Coke can out on a hot, muggy day, we can see the water undissolve, it condenses back out of the air.  From these two observations, we can infer how the solubility of water in air (that is, how much water dissolves in the air) depends on temperature, which is the key to understanding how we get clouds (thought puzzle, est4a5.html).  
Dewpoint is the temperature at which water will start to condense from air as it is cooled.  Dewpoint is therefore a measure of how much water is in the air.
Relative Humidity is the amount of water in the air/amount of water the air can hold at that temperature x 100%.  Relative humidity is therefore a measure of how much water is in the air compared to how much water the could possibly hold at that temperature.
    Since cold air can hold less water than warm air, we can squeeze water out of the air simply by changing the temperature of the air (to make dew, or fog, or snow or clouds, ~ 3 minutes to load at 14.4K baud)!  If we cool the air, it will hold less dissolved water, and some of that water will condense out as liquid or solid water.
     So how can we cool air?
     One way is simply to radiate energy away from the Earth at night (black body radiation).  This cools the ground, which cools the air next to the ground.  Under these conditions we may get dew, or frost, or fog as water condenses out of the air near the ground.
     But the big way to cool air for making clouds and rain is by adiabatic decompression (isn't that the coolest thing to say?).  When air is compressed, its temperature rises.  When air is decompressed, its temperature falls.  You can observe this with a manual tire pump.  The pump gets hot as you compress the air into your tire.  Cans of compressed air can be purchased (for cleaning items you don't want to get wet).  The nozzle of these get very cold as the compressed air decompresses on its way out.  But maybe the easiest way to observe this effect is with a balloon.  Blow up a balloon, then hold it against your cheek and let the air out quickly.  You can feel the balloon cool as the air in it decompresses.
     Because pressure is lower higher up in the atmosphere, rising air will decompress and cool adiabatically.  Air may rise because it is warm and buoyant (warm air is less dense and rises), or air may be forced up over mountains by winds, or air may be forced up by a cold or warm front (illustration).  Regardless of how the air is made to rise, the rising results in decompression and cooling.  The cooling can squeeze out the water dissolved in the air, and we get clouds and precipitation.
      The evaporation and condensation of water is one of the main ways that Earth moves energy (remember that weather is the effort to even-out Earth's energy distribution).  To evaporate water, you have to add energy to it (think about adding heat to boiling water, or how water evaporating from your skin takes energy from you and makes you cooler).  That energy doesn't simply vanish from existence.  When the water is recondensed as rain or snow, we get that energy back.  This is a major source of energy driving storms such as hurricanes, thunderstorms, and even winter storms.  When water evaporates from the Gulf of Mexico, travels on winds up to Fargo-Moorhead and falls as snow in a big winter storm, the effect is to move a huge amount of heat from down closer to the equator to up here closer to the north pole!

 Illustrative activity:  What are condensation/humidity/dewpoint?.

Isobar and wind direction activity-map reading (in MSword)

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