13
ATMOS 14 GEN 2
with coefficients defined by Buck (1981):
a
= 0.611 kPa
b
= 17.502,
c
= 240.97 °C, and
T
= temperature in degrees Celsius.
Then vapor pressure is calculated as the product of saturation vapor pressure and RH, with
RH expressed as a unitless ratio ranging from 0 to 1.
Equation 2
e RH
Vapor pressure
s
=
×
Vapor pressure is conservative across temperature differences and small spatial scales.
This means that the vapor pressure of the atmosphere near the ATMOS 14 is the same
as the vapor pressure at the ATMOS 14 sensor, even if the ATMOS 14 is not at the same
temperature as the atmosphere. Additionally, it is the vapor pressure of the atmosphere (not
RH) that controls the rate of vapor phase water transport (e.g., evaporation, transpiration,
and distribution of water vapor). As discussed, RH measurements below a temperature
of 0 °C introduce errors due to the use of liquid water as the reference. However, because
the Buck (1981) formulation for liquid water is used to calculate vapor pressure over the
full temperature range, ATMOS 14 vapor pressure output values are correct over the full
temperature range.
The METER ZENTRA system calculates and outputs vapor pressure deficit (VPD) in the
standard data stream. VPD is simply
e
s
(
T
air
) –
e
a
and gives a good indication of evaporative
demand.
3.3.2 RELATIVE HUMIDITY
The ATMOS 14 sensor provides an RH measurement that is referenced to saturation vapor
pressure over liquid water, even at temperatures below freezing, where ice is likely to be
present (WMO, 2008). Although this is the standard way to define RH, it has the disadvantage
of providing incorrect RH values below freezing when referenced to ice.
shows the maximum RH the ATMOS 14 measures at saturation over ice. RH values
over ice can be corrected by dividing reported vapor pressure values by saturation vapor
pressure calculated with the Magnus-Tetens equation (
) using the ice phase
coefficients of
b
= 21.87 and
c
= 265.5 °C. Note that supercooled liquid water is often still
present at temperatures well below 0 °C, and the liquid water coefficients should be used in
those cases.
