Six-In-One Weather Station
(630-0868A) Introduction To Weather Faxback Doc. # 46975
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" in Faxback Doc.
# 46979, or use the standard conversion formula:
C degrees = 5/9 (F degrees, minus 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 degrees 5 mph 10 15 20 25 30
Fahrenheit
35 33 22 16 12 8 6
30 27 16 9 4 1 -2
20 16 3 -5 -10 -15 -18
10 7 -9 -18 -24 -29 -33
0 -5 -22 -31 -39 -44 -49
-10 -15 -34 -45 -53 -59 -64
-15 -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 counterclockwise
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 counterclockwise
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 inches, 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 free 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
below.
The Beaufort Scale
Wind Weather
Beaufort Speed Forecast Wind's Effects on the Land
Number (MPH) Terms Environment
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
litter 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 winds; Whole trees in motion, resistance
very windy felt when walking against the wind
8 39-46 High winds; Twigs and small branches broken off
high wind trees
warning
9 47-54 Slight structural damage occurs,
slate 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
force debris 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 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.
The following table shows what each rainfall rate means.
Rainfall Amount (per hour) Rainfall Description
Trace to 0.10 inches Light rain or drizzle
0.11 to 0.30 inches Moderate Rain
Over .30 inches 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 snow fall) 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
following Relative Humidity Scale chart 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.
Relative Humidity Scale: Percent (See Faxback Doc. # 47255).
To determine the dew point temperature, subtract the wet bulb reading from
the dry bulb reading to determine the "wet bulb depression." Then look up
the wet bulb depression and dry bulb temperature readings on the Dew Point
Temperature Scale chart on Faxback Doc. # 47256. 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.
(BR/tc 4/20/98)
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