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Often when we hear a weatherman speaking of temperature, dew point, humidity, barometric pressure and other things we rather easily accept it all and really do not know precisely what he is talking about. Let me try, as a former weatherman in the Navy, to explain.
Temperatures are fairly easy to understand. Converting to metric is also fairly simple if, like me, you are oriented towards Fahrenheit. Think of 32º, freezing to you and me, as zero degrees Celsius. 40º Fahrenheit is 5 Celsius. 50º Fahrenheit is 10 Celsius. 60º is 15 Celsius and 70º is 20 Celsius. 80º Fahrenheit is 26 and 90º is a shade over 32 Celsius. That will set you on the road to understanding Celsius, especially if you visit Canada. Humidity is a dab more complicated. They call it relative humidity. Related to what? Here it is: humidity is the measurement of the water content of the air as it relates to the air's capacity to hold water at the temperature also taken when humidity is measured. In other words; humidity is the water content as it relates to the temperature. In an instrument measuring temperature and humidity on a graph paper, you see in the morning the temperature rise and the relative humidity fall. In the tropics where it is humid most of the time, the water content will stay pretty much the same all the time but the humidity relative to the temperature falls as the temperature rises. OK? Barometric pressure. Strange as it seems the air around us has a weight due to gravity. Instead of the air remaining more or less static, it responds to temperatures and the rotation of the earth in a process called the coriolis effect. As a response the atmosphere goes from lower weight [pressures] to higher weight [pressures]. In the tropics the atmosphere is generally of lower pressures as the temperatures are warm and the air is not as dense as in higher latitudes. By the same token, the atmosphere in the arctic tends to be of higher weight. Higher barometric pressure tends much more to fair weather. Low pressure areas foster storms. In the temperate zones such as the latitude of Melrose, several times per year there will be upper level low pressure areas which are slow moving unless the jet stream flows through it. These areas create extended raw and cloudy days. If they are then replaced with high pressure, the days tend to become bright and sunny. Storms: almost like a parade, fronts of cooler air flow down from the north to displace its warmer counterparts here. As the flow rushes to the south the rotation of the planet helps to create a counterclockwise flow in the front. As the pressure falls due to the rotation, the temperature also falls and the water in that air cools and condenses into clouds and a storm is formed. In the rotation the clouds shed water as rain in the warmer [southern] part of the storm and as anything from sleet to snow in the northern areas as the clouds flow over the colder air rushing in from the north. Below the equator the rotation is clockwise for lows. Also, as the heavier, colder air slides south, it plows underneath its warmer counterpart and causes that air to rise. In rising the air cools, condenses its water and we get precipitation. If, on the other hand, warm air is thrust at cold air, it does not have the strength to displace it so that air rides up and over the cold air as a warm front and once again we get overcasts and eventual precipitation. Hurricanes: In our summer [winter below the equator] the southeast trade winds meet the northeast trade winds and that is referred to as the intertropical convergence zone. That zone during our summer rises north of the equator to between 10º and 15º latitude. Extremely hot air flows from the Sahara Desert into the Atlantic ocean forming large thunderstorm areas. Here again, the coriolis effect of the Earth's rotation occasionally sets up a rotation in these stormy areas as warm surface air rises up, often to the tropopause at 60,000 feet! This rotation is somewhat self sustaining. As the rotation increases, so the central pressures decrease and the storm intensifies. It then moves according to the direction of the air mass in which it formed. All of this allows Mother Earth to regulate her temperatures as the hurricane pumps huge amounts of hot air on the surface up to the high troposphere. Theoretically that air cannot rise above the border between the troposphere and the stratosphere [called the tropopause] but I have measured huge strato-cumulus build ups higher than 60,000 feet in the tropics and even photographed them with infrared film. Instruments: Having a small weather station is a lot of fun. First, you need an anemometer. That is a cupped device which rotates with the wind and sends you the wind speed at a metering device in your home or office. With any anemometer you mount a wind direction device so the result is wind speed and direction at the same time. Then you will need a temperature measuring device; a thermometer. In my office I have the receiving devices for these instruments, one is a large 12 inch square electronic window, which shows the wind speed and direction with the temperature outside at all times in 1 inch tall red numbers. It can be read at a distance. If you have a computer, all of your weather information can be recorded in hourly increments on charts so you can see it all as the days flow by. The better companies from which you can acquire a weather station for your home are PEET BROTHERS COMPANY and DAVIS INSTRUMENTS. Peet Brothers can be reached at 601 Woodland Road, West Allenhurst, New Jersey 07711. Davis instruments is at 3465 Diablo Avenue, Hayward, Ca. 94545. Upon mounting your anemometer on a mast: Radio Shack has mating 15 foot masts on which you can mount the anemometer and other instruments. A caution here: file the paint off the bottom end of the mast and clamp a 3/8 copper tube to the mast and run it to a grounding rod in the ground with another clamp. That way you won't tempt lightning! Thanks for visiting. Len
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