Apparent
Temperatures Vantage Pro calculates three apparent temperature
readings: wind chill, Heat Index, and the Temperature/Humidity/Sun/Wind
(THSW) Index. Apparent temperatures use additional weather data to
calculate what a human body perceives the temperature to be in those
conditions.
Wind chill Wind chill takes into
account how the speed of the wind affects our perception of the air
temperature. Our bodies warm the surrounding air molecules by transferring
heat from the skin. If there’s no air movement, this insulating layer of
warm air molecules stays next to the body and offers some protection from
cooler air molecules. However, wind sweeps that comfy warm air surrounding
the body away. The faster the wind blows, the faster heat is carried away
and the colder you feel.
Heat Index The Heat Index uses temperature
and the relative humidity to determine how hot the air actually “feels.”
When humidity is low, the apparent temperature will be lower than the air
temperature, since perspiration evaporates rapidly to cool the body.
However, when humidity is high (i.e., the air is more saturated with water
vapor) the apparent temperature “feels” higher than the actual air
temperature, because perspiration evaporates more slowly.
Note: Vantage Pro measures Heat Index only when
the air temperature is above 57° F (14° C), because it’s insignificant at
lower temperatures. (Below 57°, Heat Index = the air temperature.) The
Heat Index is not calculated above 135° F (52° C). |
Rain Vantage Pro incorporates a
tipping-bucket rain collector in the ISS that measures 0.01” for each tip
of the bucket. Your station also logs rain data in inch units. If you
select millimeters for the rain measurement unit, your station still logs
rain data in inches but will convert the logged totals from inches to
millimeters at the time it is displayed. Converting the logged rain totals
reduces possible conversion losses to a minimum. Four separate variables
track rain totals: “rain storm”, “daily rain”, “monthly rain”, and “yearly
rain”. Rain rate calculations are based on the interval of time between
each bucket tip, which is each 0.01” rainfall increment. |
Humidity Humidity itself simply refers to
the amount of water vapor in the air. However, the amount of water vapor
that the air can contain varies with air temperature and pressure.
Relative humidity takes into account these factors and offers a humidity
reading which reflects the amount of water vapor in the air as a
percentage of the amount the air is capable of holding. Relative humidity,
therefore, is not actually a measure of the amount of water vapor in the
air, but a ratio of the air’s water vapor content to its capacity. When we
use the term humidity in the manual and on the screen, we mean relative
humidity. It is important to realize that relative humidity changes
with temperature, pressure, and water vapor content. A parcel of air with
a capacity for 10 g of water vapor which contains 4 g of water vapor, the
relative humidity would be 40%. Adding 2 g more water vapor (for a total
of 6 g) would change the humidity to 60%. If that same parcel of air is
then warmed so that it has a capacity for 20 g of water vapor, the
relative humidity drops to 30% even though water vapor content does not
change. Relative humidity is an important factor in determining the
amount of evaporation from plants and wet surfaces since warm air with
low humidity has a large capacity to absorb extra water
vapor. |
Temperature/Humidity/Sun/Wind (THSW)
Index The THSW Index uses humidity and temperature like the Head
Index, but also includes the heating effects of sunshine and the cooling
effects of wind (like wind chill) to calculate an apparent temperature of
what it “feels” like out in the sun. The THSW Index requires a solar
radiation sensor. |
Solar
Radiation What we call “current solar radiation” is technically
known as Global Solar Radiation, a measure of the intensity of the
sun’s radiation reaching a horizontal surface. This irradiance includes
both the direct component from the sun and the reflected component from
the rest of the sky. The solar radiation reading gives a measure of the
amount of solar radiation hitting the solar radiation sensor at any given
time, expressed in Watts /sq. meter (W/m2). Solar radiation requires the
solar radiation sensor. |
Barometric
Pressure The weight of the air that makes up our atmosphere exerts
a pressure on the surface of the earth. This pressure is known as
atmospheric pressure. Generally, the more air above an area, the higher
the atmospheric pressure, this, in turn, means that atmospheric pressure
changes with altitude. For example, atmospheric pressure is greater at
sea-level than on a mountaintop. To compensate for this difference and
facilitate comparison between locations with different altitudes,
atmospheric pressure is generally adjusted to the equivalent sea-level
pressure. This adjusted pressure is known as barometric pressure. In
reality, the Vantage Pro measures atmospheric pressure. When you enter
your location’s altitude in Setup Mode, the Vantage Pro stores the
necessary offset value to consistently translate atmospheric pressure into
barometric pressure. Barometric pressure also changes with local weather
conditions, making barometric pressure an extremely important and useful
weather forecasting tool. High pressure zones are generally associated
with fair weather while low pressure zones are generally associated with
poor weather. For forecasting purposes, however, the absolute barometric
pressure value is generally less important than the change in barometric
pressure. In general, rising pressure indicates improving weather
conditions while falling pressure indicates deteriorating weather
conditions. |
Dew
Point Dew point is the temperature to which air must be cooled for
saturation (100% relative humidity) to occur, providing there is no change
in water vapor content. The dew point is an important measurement used to
predict the formation of dew, frost, and fog. If dew point and temperature
are close together in the late afternoon when the air begins to turn
colder, fog is likely during the night. Dew point is also a good indicator
of the air’s actual water vapor content, unlike relative humidity, which
takes the air’s temperature into account. High dew point indicates high
water vapor content; low dew point indicates low water vapor content. In
addition a high dew point indicates a better chance of rain and severe
thunderstorms.
You can also use dew
point to predict the minimum overnight temperature. Provided no new fronts
are expected overnight and the afternoon Relative Humidity ≥ 50%, the
afternoon’s dew point gives you an idea of what minimum temperature to
expect overnight, since the air cannot get colder than the dew point
anytime.
Soil
Moisture Soil moisture, as the name suggests, is a measure of the
moisture content of the soil. Soil moisture is measured on a scale of 0 to
200 centibars, and can help choose times to water crops. The soil moisture
sensor measures the vacuum created in the soil by the lack of moisture. A
high soil moisture reading indicates dryer soil; a lower soil moisture
reading means wetter soil.
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UV (Ultra Violet)
Radiation Energy from the sun reaches the earth as visible,
infrared, and ultraviolet (UV) rays. Exposure to UV rays can cause
numerous health problems, such as sunburn, skin cancer, skin aging, and
cataracts, and can suppress the immune system. The Vantage Pro can help
analyze the changing levels of UV radiation and can advise of situations
where exposure is particularly unacceptable. UV radiation requires the UV
radiation sensor. The Vantage Pro displays UV readings in two scales: MEDs
and UV Index. CAUTION: Your station’s UV readings do not take into account
UV reflected off snow, sand, or water, which can significantly increase
your exposure. Nor do your UV readings take into account the dangers of
prolonged UV exposure. The readings do not suggest that any amount
of exposure is safe or healthful. Do not use the Vantage Pro to
determine the amount of UV radiation to which you expose yourself.
Scientific evidence suggests that UV exposure should be avoided and that
even low UV doses can be harmful. UV MEDs MED stands for
Minimum Erythemal Dose, defined as the amount of sunlight exposure
necessary to induce a barely perceptible redness of the skin within 24
hours after sun exposure. In other words, exposure to 1 MED will result in
a reddening of the skin. Because different skin types burn at different
rates, 1 MED for persons with very dark skin is different from 1 MED for
persons with very light skin. Both the U.S. Environmental Protection
Agency (EPA) and Environment Canada have developed skin type categories
correlating characteristics of skin with rates of sunburn. See “EPA SKIN
PHOTOTYPES” and “ENVIRONMENT CANADA SKIN TYPES AND REACTION TO THE SUN”
for a description of skin types. |
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UV
Index Vantage Pro can also display UV Index, an intensity
measurement first defined by Environment Canada and since been adopted by
the World Meteorological Organization. UV Index assigns a number between 0
and 16 to the current UV intensity. The US EPA categorizes the Index
values as shown in 2. The lower the number, the lower the danger of
sunburn. The Index value published by the U.S. National Weather Service is
a forecast of the next day’s noontime UV intensity. The Index values
displayed by the Vantage Pro are real-time measurements. |
Evapotranspiration (ET) Evapotranspiration
(ET) is a measurement of the amount of water vapor returned to the air in
a given area. It combines the amount of water vapor returned through
evaporation (from wet vegetation surfaces and the stoma of leaves) with
the amount of water vapor returned through transpiration (exhaling of
moisture through plant skin) to arrive at a total. Effectively, ET is the
opposite of rainfall, and it is expressed in the same units of measure
(Inches, millimeters). The Vantage Pro uses air temperature, relative
humidity, average wind speed, and solar radiation data to estimate ET,
which is calculated once an hour on the hour. ET requires the optional
solar radiation sensor. |
Air
Density Measuring oxygen concentration in air is difficult; other
measures are commonly used to estimate oxygen content. For example,
determining changes in the density of the air (i.e., how much a given
volume of air weighs) can produce a reasonable estimate of changes in
oxygen concentration. The formula used to calculate air density in the
Weatherlink Software requires measures of absolute pressure (barometric
pressure uncorrected for altitude), relative humidity, and
temperature.
One must develop “rules of thumb” to use air density
effectively in tuning carburetors for optimal performance. One such rule
of thumb is that a five percent (5%) change in air density may require
readjustment of the carburetor jetting. High air density readings indicate
more oxygen in the air, thus more fuel is needed (richer tune), while low
readings indicate less oxygen, requiring less fuel (leaner
tune). |
Comparison of Outside Temperature, Attic Temperature ( Temp
2nd ), Inside Temperature and Wind Speed. This shows the
comparison on March 10th 2004 with an outside temperature maximum of 93.2
degrees at 3:21pm. The maximum attic temperature was 103 degrees at
3:12 pm. A delta temp of 10 degrees and 9 minutes. The wind
during the day was consistently between 1 and 4mph. |
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