EXPLANATION OF WEATHER ELEMENTS AND
VARIABLES FOR THE DAVIS VANTAGE PRO 2
MIDSTREAM WEATHER STATION
The Weather Envoy consists of two parts: the Davis Vantage Pro 2 Integrated Sensor Suite (ISS) and
the Weather Console for data calculation and display. The data is further transferred from the
weather console via a data logger to a PC for computing with the Davis WeatherLink software for
additional reports and graphic display on the Internet.
Not all reported results are measured. Some of the variables do not have any sensors or circuitry.
They are calculated from measured parameters. Any conditions that affect the functions of the
measurements that are used to calculate these variables will affect the readings of these variables.
This includes the Weather Station Vantage Pro 2™ setup screen settings. In each case, unless
otherwise noted, the WeatherLink software uses international accepted scientific formulas and the
Weather Console uses a lookup table that closely approximates the formulas.
TEMPERATURE
Temperature is a measurement of the average kinetic energy of the molecules in an object or system
and can be measured with a thermometer or a calorimeter. It is a means of determining the internal
energy contained within the system.
The Weather Envoy uses the ISS temperature sensor to measure the outside air temperature. A
second temperature sensor in the Weather Console measures the inside air temperature. You may
use the probe to measure any other temperatures that are within the sensor’s range.
DEGREE-DAY CALCULATION METHODS
High / Low Method
If the high / low method is selected, the
WeatherLink software uses the highest
temperature and the lowest temperature for a
given day to calculate the average temperature
for that day. The difference between the
average temperature and the base threshold are
assumed to be the number of degree-days
accumulated on that day. For example, if the
average of the highest and lowest temperatures
is 24°C above the base threshold, the software
assumes 24°-days for the entire day.
Integration Method
If the integration method is selected, the software calculates degree-days using the average
temperature for an interval and the interval time. For example, if the average temperature during a
15-minute interval was 24°C above the base threshold, the software would calculate 0.25 degreedays during that interval (24 x 15 minutes in interval / 14440 minutes per day). The number of
degree-days during each interval are added together to arrive at a degree-day total. This method
calculates degree-day totals more accurately than the high / low method.
HIGH AND LOW TEMPERATURES
WeatherLink is constantly checking for new high and low temperatures. The highest and lowest
temperature values recorded during an archive interval are written into archive memory.
HEAT INDEX
The Heat Index uses the 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 saturated with water vapour) the apparent temperature “feels” higher than the
actual air temperature, because perspiration evaporates more slowly.
The WeatherLink software uses the Steadman (1979 and 1998) formula to calculate the Heat Index,
which is more accurate than the method used by the Weather Console and is calculated for all
temperatures. The Heat Index is not stored in archive memory or in the database, it is rather
calculated as necessary (for example, when plotting or displaying database information). When Heat
Index data is needed, the software calculates an average for each archive interval based on the
temperature and humidity readings for the archive period.
The formula by the WeatherLink software is also used by the U.S. National Weather Service. The
Heat Index is an indicator to determine indoor and outdoor comfort levels.
HEAT VS TEMPERATURE
Note that temperature is different from heat, though the two concepts are closely linked.
Temperature is a measure of the internal energy of the system, while heat is a measure of how
energy is transferred from one system (or body) to another. The greater the heat absorbed by a
material, the more rapidly the atoms within the material begins to move, and thus the greater the
rise in temperature.
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 warm air surrounding
the body away. The faster the wind blows, the faster heat is carried away and the colder you feel.
Wind has a warming effect at higher temperatures.
HUMIDITY
Humidity itself simply refers to the amount of water vapour in the air. However, the total amount of
water vapour 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
vapour 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 vapour in the air, but a ratio of the air’s
water vapour 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 realise that relative humidity changes with temperature, pressure, and water
vapour content. For a parcel of air with a capacity for 10g of water vapour, which contains 4g of
water vapour, the relative humidity would be 40%. Adding 2g more water vapour (for a total of 6g)
would change the humidity to 60%. If that same parcel of air is then warmed so that it has the
capacity for 20g of water vapour, the relative humidity drops to 30% even though water vapour
content does not change.
Relative humidity is an important factor in determining the amount of evaporation from plans and
wet surfaces since warm air with low humidity has a large capacity to absorb extra water vapour.
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 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 to occur during the night. In addition a high dew
point indicates a better chance of rain, severe
thunderstorms and tornados.
Dew point is also a good indicator of the air’s actual water vapour content (as opposed to relative
humidity). High dew point indicates high vapour content and low dew point indicates low vapour
content.
One can even use dew point to predict the minimum overnight temperature. Provided no new
weather fronts are expected overnight, the afternoon’s dew point gives a good indication of what
minimum temperature can be expected at night, since the air is not likely to get colder than the dew
point anytime during the night. Since condensation occurs when the air temperature reaches the
dew point, and condensation releases heat into the air, reaching the dew point halts the cooling
process.
AVERAGE WIND
The anemometer measures wind speed and wind direction and is part of the Integrated Sensor Suite
(ISS). WeatherLink samples the wind speed reading from the station a number of times during the
interval (the actual number depends on your archive interval). Those readings are averaged to
determine the average wind speed for the interval.
The WeatherLink software accumulates the following wind speed statistics for each minute (as
determined by the computer clock). The number of data points received, the highest wind speed
value, and the sum of all of the wind speed values received. These statistics are kept for the last full
10 minutes, plus the last minute that is currently being updated.
HIGH WIND SPEED
The WeatherLink is constantly checking for a new high wind speed. The highest wind speed value
recorded during the interval gets written into archive memory. The average and high wind values
are calculated by aggregating the last X minute statistics, plus the statistics for the current minute.
For example, the two-minute high wind speed is the highest value recorded in the last two full
minutes plus the current minute divided by the number of samples over the same period.
WIND DIRECTION
The WeatherLink samples the wind direction reading a number of times during the interval (the
actual number depends on one’s archive interval). If wind speed is greater than zero (0) when the
WeatherLink samples wind direction, it places a “marker” into one of sixteen “bins” that correspond
to the sixteen compass points. At the time of the archive, the WeatherLink determines which bin
contains the most markers and writes the corresponding wind direction to the archive memory as
the dominant wind direction.
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 Weather
Station Vantage Pro2 measures atmospheric pressure.
When entering the location’s altitude in “setup” mode,
the Vantage Pro and Vantage Pro2 calculates the
necessary correction factor 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.
RAINFALL
In calculating both daily and yearly rainfall totals for the software, the WeatherLink checks the total
rain register on the station. The WeatherLink compares the current total rain value to the previous
total rain entry in the archive memory to determine the amount of rainfall that occurred during the
interval.
Vantage Pro2 incorporates a tipping-bucket rain collector in the ISS that measures 0.01” for each tip
of the bucket. The station logs rain data in the same units it is measured in and converts the logged
totals into the selected display units (inches or millimetres) at the time it is displayed. Converting at
display time reduces possible compounded rounding errors over time.
HIGH RAIN RATE
Rate calculations are based on the interval of time between each bucket tip, which is each at a 0.01”
rainfall increment. When there is rainfall within the archive period, the highest measured value is
reported. When no rainfall occurs, the rain rate will slowly decay based on the elapse time since the
last measured rainfall. Parameters used include Rain Total (actually, rain rate is a measured variable
in the sense that it is measured by the ISS and transmitted to the display console, whereas all other
calculated variables are determined by the Weather Console from data received from the ISS).
Under normal conditions, rain rate data is sent with a nominal interval of 10 to 12 seconds. Every
time a rain tip or click occurs, a new rain rate value is computed (from the timer values) and the rate
timers are reset to zero. Rain rate is calculated based on the time between successive tips of the
rain collector. The rain rate value is the highest rate since the last transmitted rain rate data packet.
(Under most conditions, however, a rain tip will not occur every 10 to 12 seconds). If there have
been no rain tips since the last rain rate data transmission, then the rain rate based on the time
since that last tip is indicated. This results in slowly decaying rate values as a rain storm ends,
instead of showing a rain rate which abruptly drops to zero. This results in a more realistic
representation of the actual rain event.
STORM RAINFALL
It takes two clicks (equivalent of 0.5mm rain) to begin a storm event and 24 hours without rain to
end a storm event.
SOLAR RADIATION
Solar radiation is 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 per square meter (W/m²). The value
logged by WeatherLink is the average solar radiation measured over the archive interval.
HI SOLAR RADIATION
This is the peak solar radiation measured during the archive interval.
TEMPERATURE HUMIDITY SUN WIND (THSW) INDEX
Like Heat Index, the THSW Index uses humidity and temperature to calculate an apparent
temperature. In addition, THSW incorporates the heating effects of solar radiation and the cooling
effects of wind (like wind chill) on our perception of temperature. Thus, this index indicates the level
of thermal comfort including the effects of all these values.
TEMPERATURE HUMIDITY WIND (THW) INDEX
The THW Index uses humidity, temperature and wind to calculate an apparent temperature that
incorporates the cooling effects of wind on our perception of temperature. This index represents
only humidity, temperature and wind to determine a level of thermal comfort.
U.V. INDEX
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 sun burn, skin cancer, skin aging, and cataracts;
and can suppress the immune system. The Vantage Pro 2 Integrated Sensor Suite (ISS) can help
analyse the changing levels of UV radiation and can advise of situations where exposure is
particularly unacceptable.
The UV readings do not take into account UV reflected off snow or water, which can significantly
increase the amount of UV to which one is exposed. Nor does the Vantage Pro 2 take into account
the dangers of prolonged exposure to UV radiation. The readings do not suggest that any amount of
exposure is safe or healthful.
Do not use the Vantage Pro 2 to determine the amount of UV radiation to which one is exposed.
Scientific evidence suggests that UV exposure should be avoided and that even low UV doses can be
harmful.
The UV Index measures the intensity of UV. It was first defined by Environment Canada and since
has been adopted by the World Meteorological Organisation. UV Index uses a scale of 0 to 16 to
rate the current intensity of UV. The UV value logged by WeatherLink is the average UV measured
during the archive interval.
HI U.V. INDEX
The Hi UV Index is the peak UV measured during the archive interval.
EVAPOTRANSPIRATION (ET)
Evapotranspiration (ET) is the amount of water that moves from the ground (and plants on the
ground) to the atmosphere through both evaporation and transpiration. It is primarily important to
people who are monitoring plant growth and associated water usage.
Measuring actual ET for a given location requires the measurement of weather variables at different
heights at the same location and is beyond the capabilities of our current Davis weather station.
Instead, a single set of weather data measurements are used to calculate a reference ET (ETo). ETo
is the amount of ET that is expected at a location with specified reference conditions under the
actual weather conditions.
The two most common reference conditions used for agricultural purposes are the grass reference –
well watered grass that completely shades the ground, is uniformly clipped to a few inches in height
– and the alfalfa reference – similar to the grass reference with alfalfa instead of grass, and a
different height. The Davis ET calculations all calculate ETo for a grass reference.
Effectively, ET is the opposite of rainfall, and it is expressed in the same units of measure
(millimetres). The Weather Envoy uses air temperature, relative humidity, average wind speed, and
solar radiation data to estimate ET, which is calculated once per hour on the hour.
Source: Compiled from Davis Weather Information Reports