Major hot deserts in northern hemisphere are located between 20–30 deg N latitudes and on the western side of the continents. Why?

Reasoned account of distribution

1. Subtropical High Pressure Belt

• around 30°N and S
• descending air - desiccating effect as no instability
• (Eg) Sahara desert ; Rub-Al khali

1. Offshore wind system - Contributing to low moisture conditions

• (Eg) Sonoran desert

1. Cold ocean currents - prevent evaporation, cloud formation, precipitation in coastal areas

• (Eg) Atacama desert - South America

1. Rain-shadow regions - (Eg) Patagonian desert in Argentina

• Leeward side of Andes
• adiabatic warming after loss of moisture on windward slopes

1. Continentality - Contributes to cold desert formation in interior Asia

• (Eg) Gobi desert, Takla Makan etc
• too far away from oceans, moist air masses and no physical barrier to prompt instability in wind

1. lack of physical barriers can in general lead to desert-formation

• Eg: Thar desert in India - monsoon winds simply move through Gujarat, Rajasthan
• minimal rainfall near Aravallis

Impact

Hence desert formation is the result of many different factors and in the era of climate change, desertification and land degradation are increasing - (Eg) Sahel in Africa.

Local winds are those that arise and due to local, regional conditions and impact limited areas for limited duration in the year. That makes them different from planetary winds (Trade winds etc) and seasonal winds (large-scale like monsoon).

Impact of local winds on associated regions

• North America

  • Chinook is a major example: It is the descending hot air on the leeward side of the Rockies; also called 'snow eater'; leads to the ripening of grapes; allows earlier cultivation - harvests.

• Africa

  • Harmattan - also called the 'doctor's wind' - hot and dry offshore winds that blow from the Sahara, Sahel to the Gulf of Guinea; they carry dust.
  • Sirocco - blow from the Sahara - are hot but pick up moisture while moving north over the Mediterranean; cause rainfall in southern Italy, France and known as 'blood rain' due to dust, soil particles; destructive towards grape, citrus orchards, olive.

• Europe

  • Mistral is like Chinook and is hot, down-slope moving air - adiabatic heating takes place to melt snow in Alps valleys and encourages cultivation.
  • Fohn and Bora - gusts of cold wind moving through Rhone valley, Balkan mountains - cause snowstorms, cut off communication.

• Asia

  • Blizzards - seen in northern Siberia - cold - snowstorms, violent winds.
  • Loo in north-West India, Pakistan - hot, dry - can cause heatstroke; carry dust.