Letter

Criteria

A

Avg. temp. of coolest month 18°C or higher

f

precip. in driest month at least 6 cm

m

precip. in driest month < 6 cm but >= 10 - r/25

w

precip. in driest month < 10 - r/25

B

Neither half of year with more than 70% of annual precip. and r < 2t + 14

W

r < 1/2 upper limit of applicable requirement for B

S

r < upper limit for B but more than 1/2 that amount

h

t > 18°C

k

t < 18°C

C

Avg. temp. of warmest month > 10°C and of coldest month between 18° and 0°C

s

precip. in driest month of summer half of year less tan 4 cm and < 1/3 the amount in wettest winter month

w

precip. in driest month of winter half of year < 1/10 of amount in wettest summer month

f

precip. not meeting conditions of either s or w

a

Avg. temp. of warmest month 22°C or above

b

Avg. temp. of each of four warmest months 10°C or above; temp. of warmest month below 22°C

c

Avg. temp. of from one to three months 10°C or above; temp. of warmest month below 22°C

D

Avg. temp. of warmest month > 10°C and of coldest month 0°C or below

s

Same as under C

w

Same as under C

f

Same as under C

a

Same as under C

b

Same as under C

c

Same as under C

d

Avg. temp. of coldest month below - 38°C

E

Avg. temp. of warmest month equal to or below 10°C

T

Avg. temp. of warmest month between 10° and 0°C

F

Avg. temp. of warmest month 0°C or below

H

Temp. requirements same as E, but due to altitude (generally above 1500 m)

* t = average annual temperature (°C); r = average annual precipitation (cm).