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Dewpoint temperature

Fire » Generic composite input variables » Dewpoint temperature

Definition

"The dewpoint temperature is the temperature to which the air needs to be cooled to make the air saturated. The actual vapour pressure of the air is the saturation vapour pressure at the dewpoint temperature, The drier the air, the larger the difference between the air temperature and dewpoint temperature" (Allen et al. 1998).

Formula

Dewpoint temperature \(T_{dew}\) [°C] can be calculated based on actual vapor pressure \(e_a\) [kPa] as follows (Allen et al. 1998):

\[T_{dew}=\dfrac{116.91+237.3\cdot\ln(e_a)}{16.78-\ln(e_a)}\]

References

Allen et al. (1998)

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Symbols

Variable	Description	Unit
\(T\)	air temperature	°C
\(T_{dew}\)	dew point temperature	°C
\(H\)	air humidity	%
\(P\)	rainfall	mm
\(U\)	windspeed	m/s
\(w\)	days since last rain (or rain above threshold)	d
\(rr\)	days with consecutive rain	d
\(\Delta t\)	time increment	d
\(\Delta{e}\)	vapor pressure deficit	kPa
\(e_s\)	saturation vapor pressure	kPa
\(e_a\)	actual vapor pressure	kPa
\(p_{atm}\)	atmospheric pressure	kPa
\( PET\)	potential evapotranspiration	mm/d
\(r\)	soil water reserve	mm
\(r_s\)	surface water reserve	mm
\(EMC\)	equilibrium moisture content	%
\(DF\)	drought factor	-
\(N\)	daylight hours	hr
\(D\)	weighted 24-hr average moisture condition	hr
\(\omega\)	sunset hour angle	rad
\(\delta\)	solar declination	rad
\(\varphi\)	latitude	rad
\(Cc\)	cloud cover	Okta
\(J\)	day of the year (1..365/366)	-
\(I\)	heat index	-
\(R_n\)	net radiation	MJ⋅m^-2⋅d^-1
\(R_a\)	daily extraterrestrial radiation	MJ⋅m^-2⋅d^-1
\(R_s\)	solar radiation	MJ⋅m^-2⋅d^-1
\(R_{so}\)	clear-sky solar radiation	MJ⋅m^-2⋅d^-1
\(R_{ns}\)	net shortwave radiation	MJ⋅m^-2⋅d^-1
\(R_{nl}\)	net longwave radiation	MJ⋅m^-2⋅d^-1
\(\lambda\)	latent heat of vaporization	MJ/kg
\(z\)	elevation	m a.s.l.
\(d_r\)	inverse relative distance Earth-Sun	-
\(\alpha\)	albedo or canopy reflection coefficient	-
\(\Delta\)	slope of the saturation vapor pressure curve	kPa/°C
\(Cc\)	cloud cover	eights
\(ROS\)	rate of spread	m/h
\(RSF\)	rate of spread factor	-
\(WF\)	wind factor	-
\(WRF\)	water reserve factor	-
\(FH\)	false relative humidity	-
\(FAF\)	fuel availability factor	-
\(PC\)	phenological coefficient	-

Suffix	Description
\(-\)	mean / daily value
\(_{max}\)	maximum value
\(_{min}\)	minimum value
\(_{12}\)	value at 12:00
\(_{13}\)	value at 13:00
\(_{15}\)	value at 15:00
\(_{m}\)	montly value
\(_{y}\)	yearly value
\(_{f/a}\)	value at fuel-atmosphere interface
\(_{dur}\)	duration
\(_{soil}\)	value at soil level

Constant	Description
\(e\)	Euler's number
\(\gamma\)	psychrometric constant
\(G_{SC}\)	solar constant
\(\sigma\)	Stefan-Bolzmann constant