SME
Physical Geography · Problem type
Dry climates: desert or steppe
Use this for “A place averages 18 °C with 300 mm of rain spread through the year. Is it a desert, a steppe or not dry?”
Chart
300 mm
This is an example. Type your own values to start a new problem.
T
Mean annual temperature
example · °C
P
Yearly rain
example · mm
Pₛ
Season term
example · mm
Pₜₕ
Dry threshold
calculated · mm
Pd
Desert line
worked out (not typed) · mm
r
Rain ÷ threshold
calculated
B
Dry group
worked out (not typed)
h/k
Heat letter
worked out (not typed)
Formulas
Pₜₕ
=
20
×
T
+
Pₛ
500 = 20 × 18 + 140
Pd
=
Pₜₕ
2
250
=
500
2
r
=
P
Pₜₕ
0.6
=
300
500
B
=
BW
if
P
<
Pd,
BS
if
P
<
Pₜₕ,
else
not
B
300 < 500, so BS (steppe)
h/k
=
h
if
T
≥
18,
else
k
18 ≥ 18, so h (hot)
Assumptions
•
Köppen’s dry threshold is Pₜₕ = 20T + Pₛ mm, with T in °C and the season term Pₛ = 280 (summer rain), 140 (even) or 0 (winter rain).
•
Summer rain means 70 % or more of the year’s rain falls in the warmer six months; winter rain, 70 % or more in the cooler six.
•
Below Pₜₕ the climate is dry (B): a desert (BW) below half of it (the desert line), a steppe (BS) between.
•
The last letter is h (hot) when T is 18 °C or more, k (cold) below.
Step-by-step
WE KNOW
P = 300 mm, T = 18 °C, Pₛ = 140 mm
FIND
dry threshold (Pₜₕ), desert line (Pd), rain ÷ threshold (r), dry group (B), heat letter (h/k)
Step 1 · Find dry threshold (Pₜₕ)
Use Pₜₕ = 20 × T + Pₛ
Warmer air evaporates more, so a hot place needs more rain not to be dry; summer rain evaporates most.
Pₜₕ = 20 × 18 + 140
Pₜₕ = 360 + 140
Pₜₕ = 500 mm
Step 2 · Find desert line (Pd)
Use Pd = Pₜₕ ÷ 2
A desert gets less than half the threshold.
Pd
=
500
2
Pd = 250 mm
Step 3 · Find rain ÷ threshold (r)
Use r = P ÷ Pₜₕ
The rain as a share of the threshold: below 1 is dry, below 0.5 a desert.
r
=
300
500
r
=
0.6
Step 4 · Find dry group (B)
Use B = BW if P < Pd, BS if P < Pₜₕ, else not B
The rain is under the threshold but at least half of it, so it is a steppe.
300 < 500: BS (steppe)
B
=
BW
if
300
<
250,
BS
if
300
<
500,
else
not
B
B
=
BS
(steppe)
Step 5 · Find heat letter (h/k)
Use h/k = h if T ≥ 18, else k
A
dry
climate
takes
h
when
its
mean
annual
temperature
is
18
°C
or
more,
k
below;
the
letter
means
nothing
outside
B
.
18 ≥ 18: h (hot)
h/k = h if 18 ≥ 18, else k
h/k = h (hot)
Check
Put the numbers back in. Both sides should match.
500 = 20 × 18 + 140 ✓
250
=
500
2
✓
0.6
=
300
500
✓
300 < 500, so BS (steppe) ✓
18 ≥ 18, so h (hot) ✓
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