The weather variables we measure and store

• Atmospheric Pressure

  Atmospheric pressure is the force per unit area exerted by the weight of the air. In general it is the pressure differences in space and time that we are interested in. The spatial differences that we measure along the surface of the Earth give useful information about the local wind. The day to day changes at one location tell us what kind of weather to expect. Usually, rising pressure brings fair weather and falling pressure brings poor weather. We show the pressure corrected to sea level. The units of pressure are hectopascals (hPa). Example.

• Humidity

  Humidity is a measure of the amount of moisture (water vapour) in the air. We are measuring the relative humidity. Relative humidity tells you how water vapour is present as a fraction of the maximum amount of water vapour that could be present in a parcel of air at the current temperature and pressure. If the temperature goes down and the amount of water vapour does not change then the humidity will go up. Relative humidity is expressed in percent (%). Example.

• Rain

  Rainfall is measured using a tipping bucket rain gauge that doesn't have to be emptied. An event is recorded when about one quarter millimetre of rain falls. Total daily rainfall is stored as a running total that is reset to zero each day at midnight. Our rain gauges are not heated so the occasional winter ice and snow events that we see here in Victoria will not be recorded properly. Any snow or ice that accumulates in the bucket will be recorded when it melts. Example.

• Incoming Solar Radiation (Insolation)

  The solar radiation sensor on the weather station measures the intensity of sunlight reaching the Earth's surface. This radiation is coming from the entire sky, not just the sun. It is interesting to note the effect of closely approaching clouds. Reflection of energy from clouds often causes a noticeable peak in the strength of the intensity. The intensity of the incoming radiation is measured in Watts per square metre (Wm^-2). Example.

• Temperature

  The weather station contains a temperature sensor in a special chamber that is shielded from direct exposure to the sun. The shield is ventilated to allow air to circulate over the sensor. Temperature is measured in degrees Celsius (°C). Example.

• Ultraviolet (UV) Radiation

  Some of the energy that comes from the sun reaches us in the form of ultraviolet light. This kind of light is responsible for causing the skin to burn. The UV sensor on the weather station allows us to report the UV index during the course of the day. Here is a table of UV index values and your risk of exposure to the sun. Human skin comes in a wide range of sun sensitivity. Some skin types burn more quickly at a particular UV Index than others. Example.

  UV Index	Exposure Risk
  0 - 2	Low
  3 - 5	Moderate
  6 - 7	High
  8 - 10	Very High
  11+	Extreme

• Wind Speed

  Wind speed is measured with a cup anemometer. This device spins in the wind in such a way that the rate of spin is proportional to the wind speed. We store the wind speed in metres per second (ms^-1) and display it in kilometres per hour (kmhr^-1). Wind speed is variable at different timescales. We sample wind speed for a period of one minute and store the average. With a one minute sampling interval a lot of variation caused by eddies is observed. This makes the wind speed observations appear "noisy". The plots show two sets of data. The orange points show the one minute time average and the blue ones show the maximum gust in the one minute period. Example.

  The wind speed is also available in the Beaufort wind rose. A wind rose is a histogram of wind directions. This plot combines the observed directions and speeds (as Beaufort numbers). The speeds are drawn as coloured segments of each bar in the rose.

• Wind Direction

  To measure the wind we need to determine both its speed and direction. The direction is measured using a weather vane. In our case it is a simple device that rotates to point into the wind, to indicate in what direction the wind is coming from. We store the direction as degrees from north and report it both in degrees and using compass headings. We plot the wind direction two ways. In the first, we show wind direction as a function of time through the day, week or month (Example). In the second, we plot a wind rose which shows the relative amount of time the wind blows from a particular direction (Example).

• Dew Point

  The dew point temperature is the temperature to which the air must cool for it to reach saturation, for a given pressure and humidity. If the air temperature cools to the dew point, dew will form. If the dew point temperature is less than zero, frost will form.

• Weather Symbol

  The weather symbol shows the current values of Temperature, Pressure, UV Index, Solar Radiation, Dew Point, Precipitation, Wind Speed and Direction. Pressure is given in tenths of a hectopascal with the leading 10 or 9 omitted. Since the sea level pressure usually ranges between 980 and 1040 hPa, this convention offten works well. In this example, the pressure is 1019.0 hPa. Wind speed and direction are represented by the black arrow with the arrow pointing to the direction that the wind is coming from. The wind speed can be determined by the feathers on the arrow. Half a barb is 5 km/h, a full barb is 10 km/h and a pendant is 50 km/h. In this example the wind is coming from the NW at 65 km/h.

• Weather Data Sensor Specifications

  Here we list the specifications of the Davis Vantage Pro2 weather station.

  Weather Variable	Resolution	Range	Accuracy (+/-)
  Barometric Pressure	0.1 hPa	880 to 1080 hPa	1.0 hPa
  Outside Humidity	1 %	0 to 100%	3% 4% above 90%
  Daily Rain	0.25 mm	0 to 999.9 mm	greater of 4% or 1 tip
  Solar Radiation	1 Wm-2	0 to 1800 Wm-2	5% of full scale
  Outside Temperature	0.1°C	-40° to 65°C	0.5°C
  Time and Date	1 minute	24 hours month/day	8 seconds/month
  UV Index	0.1 Index	0 to 16	5% of full scale
  Wind Direction	1°	0 to 360°	7°
  Wind Speed	0.5 ms-1	1 to 68 ms-1	greater of 1 ms-1 or 5%

  Links to the Vantage Pro2 manuals may be found at Davisnet.com.