Rain or Snow

Knowledge of surface precipitation type can be critical during snow events at low altitudes or in regions not used to this phenomena. For this purpose, previous studies developed several methodologies to discriminate precipitation types using meteorological surface observations. Some of them are implemented in this package.

There are different approaches to address this issue:

  • Single threshold

  • Linear transition

  • Koistinen and Saltikoff

  • Dual threshold

Single threshold

A single temperature value is set as a threshold from which precipitation type is discriminated. If temperature is above the threshold, precipitation is classified as rain, otherwise as snow.

Air temperature (TA)

An air temperature (\(T_{a}\)) value is used to discriminate precipitation between rain and snow. If precipitation occurs above the air temperature value considered, rain is assumed. Otherwise, precipitation is classified as snow.

\[ \begin{align}\begin{aligned}T_{a} <= T_{a_{threshold}} \longrightarrow Snow\\T_{a} > T_{a_{threshold}} \longrightarrow Rain\end{aligned}\end{align} \]

The best air temperature single threshold may be different depending on the region. For more information on which is the most suitable threshold for your area, see https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5861046/.

Wet bulb temperature (TW)

A wet bulb temperature (\(T_{w}\)) value is used to discriminate precipitation between rain and snow. If precipitation occurs above the air temperature value considered, rain is assumed. Otherwise, precipitation is classified as snow.

\[ \begin{align}\begin{aligned}T_{w} <= T_{w_{threshold}} \longrightarrow Snow\\T_{w} > T_{w_{threshold}} \longrightarrow Rain\end{aligned}\end{align} \]

The best wet bulb temperature single threshold may be different depending on the region. Still, it is common to use a wet bulb temperature value of 1.5°C.

Linear transition

Two threshold values are set to discriminate precipitation type between rain (th_r) and snow (th_r). It can be either used with any meteorological field, but with thresholds properly defined. If a value of the meteorological field is above th_r, precipitation is classified as rain. On the other hand, if the value is below th_s, precipitation is classified as snow. A linear transition is assumed for values between th_s and th_r, then precipitation is classified as a mixed type.

If the meteorological field chosen to discriminate precipitation is air temperature:

\[ \begin{align}\begin{aligned}T_{a} <= T_{snow} \longrightarrow Snow\\T_{snow} < T_{a} < T_{rain} \longrightarrow Mixed\\T_{a} >= T_{rain} \longrightarrow Rain\end{aligned}\end{align} \]

Koistinen and Saltikoff (KS)

The methodology proposed by Koistinen and Saltikoff (1998) provides an empirical formula to calculate the probability of precipitation type using temperature and relative humidity observations. Formally, the formula calculates the probability of rain and two thresholds are set to discriminate between snow, sleet and rain. In our case, the equation is flipped, so probability of snow is determined by (1) which may be expressed as

\[p(snow) = 1 - \dfrac{1}{1 + e^{22 - 2.7\cdot T - 0.2\cdot RH}}\]

where T corresponds to temperature in Celsius and RH to relative humidity in %. If p(snow) obtained values are below 0.33 precipitation is in form of rain, if they are between 0.33 and 0.66 in form of sleet and classified as snow if they are above 0.66.

Dual thresholds

Two threshold values are set to discriminate precipitation type between rain (th_r) and snow (th_r). It can be either used with any meteorological field, but with thresholds properly defined. If a value of the meteorological field is above th_r, precipitation is classified as rain. On the other hand, if the value is below th_s, precipitation is classified as snow. Finally, if the values are between th_s and th_r, then precipitation is classified as a mixed type.

If the meteorological field chosen to discriminate precipitation is wet bulb temperature:

\[ \begin{align}\begin{aligned}T_{w} <= T_{snow} \longrightarrow Snow\\T_{snow} < T_{w} < T_{rain} \longrightarrow Mixed\\T_{w} >= T_{rain} \longrightarrow Rain\end{aligned}\end{align} \]