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Unit 17


Unit Overview

As a continuation of Unit 16, this unit examines closely the tropical (A) and arid (B) climates of the Köppen climate classification system. The main sections are:

  • The major tropical (A) climates
  • The major arid (B) climates

The A climates are located at low latitudes, and typically straddle the equator. These climates are controlled by the inter-tropical convergence zone (ITCZ). The B climates are located in many places across the globe. These climates are defined by their relative aridity; they are associated with under subtropical high-pressure cells, extreme continentality, and the leeward side of mountains. The temperature and precipitation characteristics of the A and B as well as of the other climate types can be examined visually using a climograph. If a meteorological station has a minimum of 30 years of valid temperature and precipitation measurements, then the average monthly temperatures and precipitation totals can be plotted as a graph (temperature as a line and precipitation as a bar). Some major environmental problems associated with the A and B climates, respectively, are deforestation and desertification.

Unit Objectives

  • To expand the discussion of tropical (A) and arid (B) climates using climographs developed for actual weather stations
  • To highlight climate-related environmental problems within tropical and arid climate zones
  • To examine the causes and consequences of tropical deforestation and desertification

Glossary of Key Terms

Arid climate Dry climate where potential evaporation always exceeds the moisture supplied by precipitation; found in areas dominated by the subtropical high pressure cells, in the interiors of continents far from oceanic moisture sources, and in conjunction with rain-shadow zones downwind from certain mountain ranges.
Climograph A graph that simultaneously displays, for a given location, its key climatic variables of average temperature and precipitation, showing how they change monthly throughout the year.
Desertification The process of desert expansion into neighboring steppelands as a result of human degradation of fragile semiarid environments.
Tropical climate Climate dominated by warmth (due to low-latitude location) and moisture (from the rains of the Inter-Tropical Convergence Zone); contained within a continuous east-west belt astride the Equator, varying in latitutde from 30 to 50 degrees wide.
Tropical deforestation The clearing and destruction of tropical rainforests to make way for expanding settlement frontiers and the exploitation of new economic opportunities.

Unit Outline

  • The major tropical (A) climates
    • The tropical rainforest climate (At)
      • located close to Equator
      • high temperatures and large amounts of rainfall
      • convectional thunderstorms
      • dense canopy of tropical forest
    • The monsoon rainforest climate (Am)
      • located on tropical coasts, often near highlands
      • distinct short dry season in winter
      • evergreens and some grasslands
    • The savanna climate (Aw)
      • located in transitional area between subtropical highs and equatorial low pressure belts
      • extended dry season in winter
      • tall grasses, individual trees, thorny bushes
  • The major arid (B) climates
    • The desert (BW) climate
      • located near 30 degrees north and south at the subtropical highs
      • minimal precipitation
      • sparse vegetation
    • The steppe (BS) climate
      • located on the edges of the most arid (BW) deserts
      • semiarid, rainfall varying with latitude (more in lower latitudes)
      • short-grass prairie vegetation
    • Human activities in B climates
      • Virgin and Idle Lands Program (former Soviet Union)
      • Great Plains droughts
      • Nile Valley
      • desertification

Review Questions

  1. Compare and contrast the general types of plant life found in A and B climate zones.
  2. What is a climograph?