Calculate Dew Point

Function to calculate dew point (°C) from temperature (°C) and relative humidity (%).

The dew point is the temperature at which air becomes saturated with moisture and water vapour begins to condense.

Usage

calcDP(Temp, RH, method = c("Magnus", "Buck"))

Arguments

Temp: Temperature (°Celsius)
RH: Relative Humidity (0-100%)
method: Character; formula to use, either "Magnus" or "Buck". Defaults to "Magnus".

Value

Td (DP), Dew Point (°Celsius)

Details

This function supports two methods for dew point calculation:

"Magnus" (default): Uses the August-Roche-Magnus approximation, valid for 0°C < Temp < 60°C and 1% < RH < 100%.
"Buck": Uses the Arden Buck equation with Bögel modification, valid for -30°C < Temp < 60°C and 1% < RH < 100%.

Both methods compute saturation vapour pressure and convert relative humidity to dew point temperature. The Magnus method is chosen as the default because it is more stable when used with the calcTemp and calcRH_DP functions.

Note

More details of the equations are also available in the source R code.

References

Alduchov, O. A., and R. E. Eskridge, 1996: Improved Magnus' form approximation of saturation vapor pressure. J. Appl. Meteor., 35, 601–609

Buck, A. L., 1981: New Equations for Computing Vapor Pressure and Enhancement Factor. J. Appl. Meteor. Climatol., 20, 1527–1532, https://doi.org/10.1175/1520-0450(1981)020<1527:NEFCVP>2.0.CO;2.

Buck (1996), Buck (1996), Buck Research CR-1A User's Manual, Appendix 1.

https://bmcnoldy.earth.miami.edu/Humidity.html

Examples

# Dew point at 20°C and 50% relative humidity (RH)
# Default Magnus method
calcDP(20, 50)
#> [1] 9.261107

# Using Buck method
calcDP(20, 50, method = "Buck")
#> [1] 9.250632

# Validation check
calcDP(20, calcRH_DP(20, calcDP(20, 50)))
#> [1] 9.261107
calcDP(20, calcRH_DP(20, calcDP(20, 50, method = "Buck"), method = "Buck"), method = "Buck")
#> [1] 9.250632


# mydata file
filepath <- data_file_path("mydata.xlsx")
mydata <- readxl::read_excel(filepath, sheet = "mydata", n_max = 5)

mydata |>
  dplyr::mutate(
    DewPoint = calcDP(Temp, RH),
    DewPoint_Buck = calcDP(Temp, RH, method = "Buck"))
#> # A tibble: 5 × 7
#>   Site   Sensor Date                 Temp    RH DewPoint DewPoint_Buck
#>   <chr>  <chr>  <dttm>              <dbl> <dbl>    <dbl>         <dbl>
#> 1 London Room 1 2024-01-01 00:00:00  21.8  36.8     6.38          6.39
#> 2 London Room 1 2024-01-01 00:15:00  21.8  36.7     6.34          6.35
#> 3 London Room 1 2024-01-01 00:29:59  21.8  36.6     6.30          6.31
#> 4 London Room 1 2024-01-01 00:44:59  21.7  36.6     6.22          6.22
#> 5 London Room 1 2024-01-01 00:59:59  21.7  36.5     6.18          6.18