Mediterranean climate (Cs)

Topic: World Climate Classification: Koeppen's Scheme · NCERT: Class 11, Ch 11 "World Climate and Climate Change"

Meaning

The Mediterranean climate (Cs) is a warm temperate (Group C) climate with a dry summer. Its wettest winter month gets at least three times as much rain as its driest summer month [2]. It is found on west coasts between 30° and 40° latitude [3]: summers are hot and dry, winters are mild and rainy, and the year's rain is 35–90 cm.

It is the only major climate where the rain comes in winter and summer is the dry season. That is why it is a favourite exam topic. It also explains a whole farming system (citrus, olives, grapes and wine) and a special kind of vegetation (evergreen scrub).

Explanation

Koeppen letters: what "Cs" means

• C (1st letter): the coldest month averages between −3°C and 18°C.
• If the coldest month is above 18°C, the climate is tropical (Group A).

• If it is below −3°C, the climate is a cold, snowy-forest one (Group D).

• s (2nd letter): the dry season is in summer.

• The Cs rain rule: the wettest winter month gets at least 3 times the rain of the driest summer month [2].
• Third letter (subtypes) [2]:
• Csa (hot-summer Mediterranean): the warmest month is above 22°C. Most lands around the Mediterranean Sea are Csa.
• Csb (cool-summer Mediterranean): no month is above 22°C. This happens on coasts cooled by cold ocean currents.

How it forms: the pressure belts shift with the Sun

• Summer: the subtropical high takes over
• The pressure belts move towards the pole with the overhead Sun.
• The subtropical high (a belt of sinking, dry air at about 30° latitude) moves over the west coast.

• Sinking air warms up and does not form clouds. So summer is hot and dry, at about 25°C.

• Winter: the westerlies arrive

• The pressure belts move back towards the equator.
• The westerlies (winds that blow from west to east in the mid-latitudes) and their storms reach the coast.

• So winter is mild and rainy, below 10°C.

• Result: rain falls mostly in winter, 35–90 cm a year.

Why only west coasts

• At the same latitude, east coasts have warm ocean currents and onshore summer winds. These bring rain all year (Cfa), or summer monsoon rain (Cwa).
• Only the west coast comes under the dry subtropical high in summer and the rain-bearing westerlies in winter.
• Further poleward, at about 40°–60°, the westerlies blow onshore all year. Cs then gives way to Cfb (marine west coast).

Vegetation and human use

• Evergreen scrub: dense shrubs, bushes and small trees with broad evergreen leaves. They are usually under 2.5 m tall [4].
• Their leaves are thick and waxy, so they lose little water in the dry summer.

• Local names: maquis (Mediterranean), chaparral (California), fynbos (South Africa).

• Mediterranean agriculture: growing citrus fruits, olives, grapes and wine.

Where it occurs

All Cs regions lie on west coasts between 30° and 40° [3]:

Region	Continent / coast
Lands around the Mediterranean Sea (southern Europe, North Africa)	Europe, Africa
Central California	North America, Pacific coast
Central Chile	South America, Pacific coast
South-western and southern Australia (around Perth and Adelaide)	Australia, Indian Ocean coast
Cape region of South Africa	Africa, southern tip

• India: no part of India has a Cs climate. The Ganga plain is Cwg, which has the opposite pattern: a wet monsoon summer and a dry winter.

Don't confuse with

• Cwa / Cwg (humid subtropical with dry winter): the opposite rain season. Cwa has rainy summers from the monsoon and dry winters. Cs has dry summers and rainy winters.
• Cfb (marine west coast): also on west coasts, but poleward of Cs. The westerlies bring rain in every month, so there is no dry season.
• Cfa (humid subtropical): at similar latitudes, but on east coasts. Thunderstorms in summer and fronts in winter give rain all year, 75–150 cm.
• Csa vs Csb: both have dry summers. In Csa the warmest month is above 22°C. In Csb no month is above 22°C, because of cold currents [2].

Prelims Hooks

• "s" in Koeppen = dry season in summer. Cs is the only C climate with a dry summer.
• Cs regions: Mediterranean basin, central California, central Chile, south-west and southern Australia, Cape region. All are west coasts at 30°–40° [3].
• Scrub and place: maquis = Mediterranean; chaparral = California; fynbos = South Africa (Cape).
• Cs rain rule: wettest winter month ≥ 3 × driest summer month [2]. Annual rain is 35–90 cm.
• Rainfall ranges, widest to narrowest: Cfb (50–250 cm) > Cfa (75–150 cm) > Cs (35–90 cm).
• Trap: a dry summer comes from the subtropical high, not from offshore monsoon winds. Winter rain comes from the westerlies.

Mains Points

• Why west and east coasts differ (GS-I):
• On the west coast, the subtropical high in summer gives Cs, with rain from the westerlies in winter.
• On the east coast, warm currents and onshore winds give Cfa, or Cwa where a monsoon controls the winds.

• Use this to explain why Lisbon and Shanghai, at similar latitudes, have opposite rain seasons.

• Human use and problems (GS-I/GS-III):

• Benefit: Mediterranean agriculture produces citrus, olives, grapes and wine for export.
• Problems: rain comes in the cool season, when crops need it least. Dry summers cause water stress, so farmers depend on irrigation.

• The dry summer scrub burns easily, which leads to scrub fires.

• Plant adaptation (GS-I, biogeography): the thick, waxy, evergreen leaves of maquis, chaparral and fynbos are a good example of how climate shapes vegetation. The plants survive drought in summer and grow during the wet winter.

Related concepts

• Warm temperate climates (C)
• Humid subtropical climate with dry winter (Cwa)
• Humid subtropical climate (Cfa)
• Marine west coast climate (Cfb)

Read more

• Group C: warm temperate climates (Cwa, Cs, Cfa, Cfb)