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Health & Fitness

Fun Weather Facts: Where do Forecasts Come From? (Part One)

When I started out forecasting in the 1960s there were no useable computer models of the atmosphere, no useable satellite pictures, and no available weather radars.

Finally-----finally--- I get to have some fun and get away from the “hot” topic of Global Climate Change, at least for now.

When I started out forecasting in the 1960s there were no useable computer models of the atmosphere, no useable satellite pictures, and no available weather radars.  All we did is plot data on maps that could be gathered from the ground (cloud cover, temperature, dew point, wind speed and direction, and air pressure) and similar data that could be gathered from instrumented balloons above the surface (radiosondes).

Then we analyzed those maps to tell us what the present atmosphere was like. Those maps really didn’t give us a very clear picture of what was coming the next day, let alone five days in advance.  Plenty of days I was surprised (pleasantly) by a snowstorm that wasn’t forecasted. 

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As a child, nothing could beat the feeling of having my mother waking me up by flinging open the blinds in my bedroom and announcing school was closed for the day. With modern forecasting techniques, I haven’t been surprised by a snowstorm in years.

Since I mentioned “dew point” I should probably explain what it is.  Water vapor is a small part of air, but an important one.  Without it we would not have snow and rain.  Water vapor is colorless and tasteless (lucky for us).  If you are reading this blog you are probably in a room filled with air (also lucky for you). There is most certainly water vapor in the air you are breathing, but most indoor rooms are warm enough to keep the water vapor as a gas (good planning on our parts). 

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If you start cooling the room you are in, the temperature when clouds form in the room is the dew point of the air in the room.  You would see the clouds as fog (the same as when we drive up into the mountains and come into a cloud layer extending into the mountains).  At night, fog forms when the air temperature drops below the dew point.  The bigger the difference between the dew point temperature and the room temperature, the drier the air in the room.

This is why in the winter when we heat up air to warm our rooms we tend to dry out everything in the room, including the furniture and us.  We are making the air drier by increasing the difference between the air temperature and the dew point.

To set the stage for an explanation of modern forecasting I probably should explain how the atmosphere works.  Think of the atmosphere as a man who is out for a walk, but has no path or destination in mind,  He walks a few steps to smell the flowers, then smelling the flowers makes him think about perfume.  He walks to a store to buy perfume which makes him think about wine.  He walks to look at wine, but the store is closed so he walks to a grocery store, and so on. Each new destination was determined by the previous one. 

In the atmosphere, air temperature produces pressure patterns that cause wind to blow.  The blowing wind can push together (converge) or pull apart (diverge) causing air to rise and sink.  Air blowing over mountains and even hills also rises and sinks.  Valleys force air to blow along them (Puget Sound is a big valley). Rising air forms clouds (the rising air temperature falls below the dew point) and snow and rain fall out. The sun rises and sets, warming and cooling the air.  All this changes the temperature, which changes the pressure patterns, which changes the wind, and so on.  The weather in the next instant is determined by the weather happening right now.

Well---I took so much space telling about dew point that I will have to continue explaining where modern forecasts come from in my next blog “Where Do Forecasts Come From? (Part Two).”

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