Six-In-One Weather Station (630-0868) Introduction Faxback Doc. # 18073 WHAT IS WEATHER? Weather is the condition of the earth's atmosphere (air). Weather is all around us, all of the time. Rain, snow, clouds, hot or cold air, humidity, and wind are all forms of weather. WHAT CAUSES WEATHER? Weather is the result of the sun heating the earth. Even on cloudy days, the sun continues to shine on the earth - although we can't see it! The sun always heats the earth more at the equator and less at the north and south poles. This uneven heating of the earth causes warm air masses to form at the equator and cold air masses to form at the poles. The warm air masses tend to move towards the poles, while the cold air masses tend to move toward the equator. When these air masses meet and move against each other, clouds and precipitation (rain or snow) result. WHAT MAKES UP WEATHER? Air Temperature - how hot or cold the air feels Air Pressure - the weight of the air on the earth Wind Speed/Direction - how fast the wind is blowing and from what direction (North, South, East, or West) Rainfall/Snowfall - how much rain/snow falls on the earth Relative Humidity - a relative measure of the humidity in the earth's air Clouds - the type and amount of clouds in the sky, and the types of precipitation produced by the clouds Visibility - how far we can see and what types of clouds and precipitation block our view Because there are several factors that make up weather, there are also several different ways to measure it. The following sections describe each of these weather factors and tell you how you can measure them with your weather station. AIR TEMPERATURE The earth's air temperature is a very important weather condition because it often determines other weather conditions. As you might already know, the temperature is measured using a dry bulb thermometer, which is a piece of thin glass attached to a glass bulb that is mounted against a numbered scale. A liquid (mercury or alcohol) is sealed inside the glass. As the air around a thermometer gets hotter or colder, the liquid rises or falls inside the glass. The temperature is measured in either the Fahrenheit (F) or Celsius (C) scale. The Celsius scale is used worldwide, except in the United States where the Fahrenheit scale is commonly used. Measuring the Air Temperature Your weather station's dry bulb thermometer measures the air temperature on the Fahrenheit scale (F). To tell what the temperature is, simply compare the level of the thermometer's mercury against the numbered scale on the thermometer. NOTE: Even though your weather station uses the Fahrenheit scale, you can easily calculate the equivalent Celsius temperature reading. Simply read the Fahrenheit number on the thermometer, then look up that reading on the "Fahrenheit/Celsius Conversion Table" on Page 30 of the manual, or use the standard conversion formula: Degrees C = (F-32 Degrees) WIND CHILL As the wind blows, it takes heat away from your body, making the temperature feel much colder than the thermometer says it is. This effect is called wind chill. The wind chill factor is figured by comparing the current wind speed and temperature readings on a special wind chill chart. The lower the wind chill factor, the colder you feel. Measuring the Wind Chill To determine the wind chill factor using your weather station, first read the wind speed and dry-bulb (air) temperature. Then look up those readings on the WIND CHILL CHART printed on the back of your weather station. The wind speed is listed horizontally (across the top) and the dry bulb temperature is listed vertically (down the left side). The intersection of those readings is the wind chill factor. WIND CHILL CHART WIND 5 MPH 10 15 20 25 30 --------------------------------------------------------------------------- 35 DEGREES F 33 22 16 12 8 6 --------------------------------------------------------------------------- 30 DEGREES F 27 16 9 4 1 -2 --------------------------------------------------------------------------- 20 DEGREES F 16 3 -5 -10 -15 -18 --------------------------------------------------------------------------- 10 DEGREES F 7 -9 -18 -24 -29 -33 --------------------------------------------------------------------------- 0 DEGREES F -5 -22 -31 -39 -44 -49 --------------------------------------------------------------------------- -10 DEGREES F -15 -34 -45 -53 -59 -64 --------------------------------------------------------------------------- -15 DEGREES F -21 -40 -51 -60 -66 -71 --------------------------------------------------------------------------- For example, if the wind is blowing at 20 MPH and the temperature is 10 Degrees F, the wind chill is approximately -24 degrees F. The higher the wind speed and lower the temperature, the colder the wind chill factor. AIR PRESSURE The air that surrounds the earth is constantly placing pressure on the earth's surface. The pressure in the earth's atmosphere is measured using an instrument called a barometer. Like a thermometer, a barometer usually contains mercury that rises and falls to indicate rising (high) or falling (low) barometric pressure. Barometric pressure is most often recorded in inches of mercury, millibars, or kilopascals. Changes in air pressure are critical in forecasting the weather. When the air pressure is low, warm, moist air is rising and moving counter-clockwise toward the center, creating less pressure. A barometer usually shows falling pressure as a storm system approaches. Low Pressure System An area of lower pressure at the surface where winds blow counter-clockwise around its center. Unsettled weather is often associated with low pressure because air rises, cools, and eventually becomes saturated. When the air pressure is high, cold, dry air is dropping (moving toward the earth) and moving clockwise away from the center, creating more pressure. A barometer usually shows rising pressure as nice, sunny weather approaches. High Pressure System An area of higher pressure at the surface where winds blow clockwise around its center. Clear weather is often associated with high pressure because descending air becomes warmer and drier. Measuring the Air Pressure Your weather station does not include a barometer, but you can usually find the daily barometer reading in the weather section of your local newspaper. The weather section might look like this. YESTERDAY'S STATISTICS Humidity Sunday at noon........56% Sunday's average......62% Monday's forecast.....40-80% Barometer Sunday at noon........30.10 degrees, falling Winds Sunday at 6 p.m.......N at 8 mph Monday's forecast.....NNW at 6-12 mph WIND SPEED/DIRECTION Because the sun heats the earth more at the equator than at the poles, warm air masses at the equator move north, while cold air masses at the poles move south. Since the earth rotates, the force of the earth's rotation deflects the air masses to the right. That change in the wind's direction creates whirling masses of air called high-pressure cells (highs) and low pressure cells (lows). The highs generally bring fair weather, like breezy, sunny days. The lows usually bring unsettled weather, like storms. Measuring Wind Speed/Direction Both the speed and direction of the wind are important when observing the weather. An instrument called a wind vane tells the direction of the wind, and an instrument called an anemometer measures the speed of the wind. These instruments can come in different shapes. A wind vane is simply a piece of specially shaped metal or plastic (like the body of your weather station) that is mounted on an upright rod or axle so it can spin freely when the wind blows against it. Because of its special shape, one end of the wind vane always points in the same direction that the wind is blowing from. A compass mounted on the rod or axle tells you what direction (north, south, east, or west) the wind is blowing from. To determine from what direction the wind is blowing, be sure that the compass on the weather station's swivel axle is pointing north (N). Then compare the position of the blue-tipped end of the weather station with the compass marking beneath it. Your weather station's anemometer measures wind speed using a hollow tube that is open on both ends, with one end of the tube lower than the other. The outside of the tube has marks measuring miles per hour (MPH). As wind blows through the tube, a lightweight ball trapped inside blows from the low end of the tube toward the high end. To measure the speed of the wind with your weather station, simply compare the height of the round ball inside the wind tube against the MPH scale marked on the outside of the tube. NOTE: Be sure that the opening is clear at both ends of the wind tube. Otherwise, you might get a false reading. Estimating Winds on the Beaufort Scale In 1805 Admiral Francis Beaufort developed a scale based on wind speed to estimate the effect of wind on the environment and things around us, such as water, trees, buildings, etc. The first scale estimated the effects of wind on the ocean. Later, a separate scale was designed for wind effects on land. After you take a wind speed reading, look it up on the chart on the following page. THE BEAUFORT SCALE Beaufort Wind Weather Wind's Effects on the Land Environment Number Speed Forecast (MPH) Terms --------------------------------------------------------------------------- 0 Under 1 Calm Calm smoke rises vertically; no leaf motion. --------------------------------------------------------------------------- 1 1-3 Light Smoke drift shows wind direction; wind vanes don't respond --------------------------------------------------------------------------- 2 4-7 Light Wind felt on face; leaves start rustling; wind vanes respond --------------------------------------------------------------------------- 3 8-12 Breezy Leaves, small twigs in constant motion; light flags extended --------------------------------------------------------------------------- 4 13-18 Breezy Dust, leaves, loose paper and other liter is raised; small branches move --------------------------------------------------------------------------- 5 19-24 Breezy Small trees in leaf begin to sway --------------------------------------------------------------------------- 6 25-31 Windy Large tree branches in motion; whistling heard in overhead wires --------------------------------------------------------------------------- 7 32-38 Strong Whole trees in motion, resistance felt winds; when walking against the wind very windy --------------------------------------------------------------------------- 8 39-46 Twigs and small branches broken off trees -------------- High winds; ------------------------------------------- 9 47-54 High winds Slight structural damage occurs, slate warning blown off roofs; wires may snap --------------------------------------------------------------------------- 10 55-63 Severe Structural damage; trees broken -------------- thunderstorm ----------------------------------------- 11 64-73 wind Widespread damage --------------------------------------------------------------------------- 12 >73 Hurricane Violence and destruction; flying debris force from objects destroyed earlier damages additional structures --------------------------------------------------------------------------- RAINFALL/SNOWFALL Water from the earth's oceans and lakes is constantly evaporating into the atmosphere. The water vapor evaporated from the oceans and lakes creates clouds. Under the right conditions, such as a low pressure cell/front, the moisture in the clouds condenses (comes together) causing either drops of water (rain!) or ice crystals (snow!) to fall from the clouds to the earth. NOTE: The temperature in the atmosphere must be 32 degrees F or lower for snow to form and fall. Measuring Rainfall The easiest way to measure rainfall is to use a clear, round or cone-shaped tube called a rain gauge, which is placed outside in an open area. When rain falls, it collects in the rain gauge's tube. The outside of the tube has marks measuring tenths of hundredths of inches. To measure rainfall, compare the rain that accumulates in the gauge's tube to the marks on the scale. To measure the rainfall with your weather station, remove the rain gauge tube from the weather station, then compare the rainwater level in the tube against the scale on the outside of the tube. To record your reading, slide the TOTAL RAIN button to the appropriate mark. NOTE: The scale on both the rain gauge and TOTAL RAIN is measured in inches (" to the right of a number means inches). Empty and dry out the rain gauge tube after each reading, then put it back in the weather station to prepare for the next reading. NOTE: Meteorologists measure and report rainfall on an hourly basis to determine such things as the possibility of flooding and the need for watering lawns or crops. To keep track of rainfall like a meteorologist, measure the rainfall in your weather station once every hour, then compare it against the last reading you took. This table shows what each rainfall rate means. Rainfall Amount Rainfall Description Amount (per hour) -------------------------------------------------------------------------- Trace to 0.10" Light rain or drizzle -------------------------------------------------------------------------- 0.11 to 0.30" Moderate Rain -------------------------------------------------------------------------- Over .30" Heavy Rain -------------------------------------------------------------------------- Measuring Snowfall You can measure snowfall by simply sticking an ordinary ruler down into snow that has fallen on the ground. Because the wind can cause snow to blow and accumulate differently in different places, it is best to take snowfall measurements in several different places, and then take the average of all measurements. For example, if you took snowfall measurements of 4, 3, 3 1/2, and 4 1/2 inches at four different locations, the average would be 3 3/4 inches ((4 +3 +3 1/2 + 4 1/2)/4 = 3 3/4). NOTE: Be sure you don't stick the ruler down into prior snow accumulations (if you are measuring new snowfall) or underlying soil (such as gravel). RELATIVE HUMIDITY AND DEW POINT TEMPERATURE Moisture in the atmosphere comes from the evaporation of large bodies of water. The amount of moisture in the air (in the form of water vapor) is measured as the relative humidity. When no more water can evaporate into the atmosphere, the air is saturated (full of water molecules) and is said to be at 100% relative humidity. Another way meteorologists measure the amount of water vapor in the atmosphere is by using the dew point temperature. The dew point temperature is the point at which the air will become saturated. Cooler air can hold less water vapor than warmer air. Therefore, as the air cools, the saturation level increases. Fog or low clouds are a good indicator of saturated atmosphere. Measuring the Relative Humidity and Dew Point Temperature 1. Wet the supplied cotton ball with water, then place it around the outside of the bulb at the bottom of the wet bulb thermometer. NOTE: Be sure to stretch the cotton ball so it covers the bulb completely. 2. After a few minutes, read the temperature on both the wet and dry bulb thermometers. NOTE: The air around the wet bulb thermometer must be moving (like wind) in order to get the most accurate reading. If there is no wind, either fan the air around the weather station (if the station is permanently mounted) using a flat, thin piece of paper or cardboard, or move the weather station through the air (if you are holding it in your hand). To determine the relative humidity, look up those readings on the Relative Humidity Scale chart on page 15 of the manual or the chart (RELATIVE HUMIDITY) printed on the front of your weather station. The dry bulb temperature is listed horizontally (across the top) and the wet bulb temperature is listed vertically (down the left side). The intersection of those readings (where they meet) is the relative percent of humidity. To determine the dew point temperature, subtract the dry bulb reading from the wet bulb reading to determine the "wet bulb depression." Then look up the wet bulb depression and dry bulb temperature readings on the following chart on page 16 in the manual. The wet bulb depression is listed horizontally (across the top) and the dry bulb temperature is listed vertically (down the left side). The intersection of those readings is the dew point temperature. NOTES: The water in the cotton ball could evaporate. To get accurate readings at a later time, be sure to keep the cotton wet. To avoid false readings, always remove the cotton ball from around the thermometer bulb before adding water to the cotton. (PH 11/21/95)