Calculating rainfall

[Shaheen hassan]

Originally Posted by Mhc1985

I don’t know what you are exactly looking for, as you have already answered the question…

The threshold for calculating a Köppen B climate is defined by a simple linear function f(x) = ax + b, in which x is the annual average temperature, a = 20, and b can get three different values depending on the precipitation pattern (0, 140, 280).

A place where the annual average precipitation is below the threshold is a B climate.
A place where the annual average precipitation is above the threshold is not a B climate.

If b = 0 (winter precipitation max), no place with an average temperature below zero can be a B climate, as a place can’t have a negative amount of precipitation.

For instance, for a place with winter precipitation maximums that average -3.0°C, the threshold would be 20 x (-3) + 0 = -60. As having negative precipitation is impossible, a place like this can’t be a B climate. Simple.

Many places in Central and North Asia have negative mean annual temperatures but are classified as BWk or BSk. If this formula is impossible to use, how did they know that these places are BWk or BSk.

[Mhc1985]

Originally Posted by Shaheen hassan

Many places in Central and North Asia have negative mean annual temperatures but are classified as BWk or BSk. If this formula is impossible to use, how did they know that these places are BWk or BSk.

You answered it before:

Originally Posted by Shaheen hassan

The formula for distinguishing B group climates is:-
The mean annual temperature is multiplied by 20 and then
a. Add 280 if 70% or more of precipitation occurs in summer.
b. Add 140 if 30-70% of precipitation occurs in summer.
c. Don't add anything if less than 30% of precipitation occurs in summer.

The formula can always be used. It is the precipitation that can’t be negative. The formula doesn’t use precipitation anywhere. It just provides us a number that can be compared to the annual average precipitation.

Those places in Central Asia have dry winters and relatively wet summers, so we must add 280 after the multiplication.

So, for a site that averages -2°C and precipitation mostly occurs in summer we have:

ax + b
20 . (-2) + 280
-40 + 280 = 240

If such place averages below 240 mm, it's a B climate.