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Weather Conditions Measured/Calculated
GroWeather Manual Page 11
Relative Humidity
Humidity itself simply refers to the amount of water vapor in the air. However,
the amount of water vapor which 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 per-
centage of the amount the air is capable of holding. Relative humidity, there-
fore, 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.
It is important to realize that relative humidity changes with temperature, pres-
sure, 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 evapo-
ration from plants and wet surfaces since warm air with low humidity has a
large capacity for extra water vapor.
• Relative Humidity of Outside Air (Alarm: High/Low)
• “Daily” High and Low Relative Humidity reading with time of occurrence (Auto-
Clear)
Dew-Point & Leaf Wetness
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 after-
noon 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 (as
opposed to relative humidity). High dew-point indicates high vapor content;
low dew-point indicates low vapor content. You can even use dew-point to
predict the minimum overnight temperature. Provided no new fronts are
expected overnight, the afternoon’s dew-point gives you an idea of what mini-
mum temperature to expect that night, since the air is not likely to get colder
than the dew-point anytime during the night.
Leaf wetness provides an indication of whether the surface of foliage in the
area is wet or dry by indicating how wet the surface of the sensor is. The leaf
wetness reading ranges from 0 (dry) to 15.
• Current Dew-Point (Alarm: when temperature is within 2˚F of dew-point)
• Leaf Wetness