DAVIS Vantage Pro Console - Manual (Page 50)

Appendix A: Weather Data
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).
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.
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 tempera-
ture 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 humid-
ity, 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 humid-
ity.
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.
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.