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21.7: Present Climate

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    10973
  • So far, we have focused on processes that create and modify global climate. But before we leave this chapter, let’s briefly summarize current climate.

    21.7.1. Definition

    Climate describes the slowly varying characteristics of the atmosphere, as part of the atmosphere/ hydrosphere/cryosphere/biosphere/land-surface system. Climate is found by averaging or filtering the hourly or daily weather data over 30 years, with the last year ending in zero (e.g., 1981-2010, or 1991- 2020). Mean values of weather, temporal and spatial variability, and extremes are the statistics that are recorded as components of the climate signal. 

    21.7.2. Köppen Climate Classification

    In 1884, Wladimir Köppen used temperature and precipitation to define different types of climate. He drew maps, classifying every point on land according to these climates. The climate at each location controls the dominant type of plants that naturally grow there. The Köppen climate map was modified by Rudolf Geiger in the early 1900s, and was significantly updated in 2007 by Peel, Finlayson and McMahon (Hydrol. Earth Syst. Sci., 11, 1633-1644) using modern weather data.

    A two- or three-letter code is used to identify each climate type (also see Table 21-8 in the Homework Exercises section). The criteria used to classify the climate types are detailed in Table 21-6. Fig. 21.25 shows a small portion of the global climate map.

    Figure 21.25 Köppen climate map for western North America. See Tables 21-6 & 21-8 for explanations of the climate code letters. [Color maps of Köppen climate classification for the whole world are available on the internet.]

    Screen Shot 2020-04-12 at 4.32.37 PM.png

    Table 21-6. Köppen climate codes and defining criteria. (Peel et al, 2007). Also see Table 21-8 near end of chapter.
    1st 2nd 3rd Description Criteria* (see footnotes below)
    A     Tropical Tcold ≥ 18
     

    f

    m

    w

     

    Rainforest

    Monsoon

    Savannah

    Pdry ≥ 60

    [not Af] and [Pdry ≥ (100 – MAP/25)]

    [not Af] and [Pdry < (100 – MAP/25)]

    B     Arid MAP < [10·Pthreshold]
     

    W

    S

     

    Desert

    Steppe

    MAP < [5·Pthreshold]

    MAP ≥ [5·Pthreshold]

       

    h

    k

    Hot

    Cold

    MAT ≥ 18

    MAT < 18

    C     Temperate [Thot ≥ 10] and [0 < Tcold < 18]
     

    s

    w

    f

     

    Dry summer

    Dry winter

    Without dry season

    [Psdry < 40] and [Psdry < (Pwwet/3)]

    Pwdry < [Pswet/10]

    [not Cs] and [not Cw]

       

    a

    b

    c

    Hot summer

    Warm summer

    Cold summer

    Thot > 22

    [not a] and [NmonT10 ≥ 4]

    [not (a or b)] and [1 ≤ NmonT10 < 4]

    D     Cold [Tcold ≤ 0] and [Thot > 10]
     

    s

    w

    f

     

    Dry summer

    Dry winter

    Without dry season

    [Psdry < 40] and [Psdry < (Pwwet/3)]

    Pwdry < [Pswet/10]

    [not Ds] and [not Dw]

       

    a

    b

    c

    d

    Hot summer

    Warm summer

    Cold summer

    Very cold winter

    Thot ≥ 22

    [not a] and [Tmon10 ≥ 4]

    [not (a or b or d)]

    [not (a or b)] and Tcold < –38

    E     Polar Thot < 10
     

    T

    F

     

    Tundra

    Frost

    Thot > 0

    Thot ≤ 0

    Table 21-6 (continuation). *Footnotes

    MAP = mean annual precipitation (mm·yr–1)

    MAT = mean annual temperature (°C)

    NmonT10 = number of months where the temperature is above 10 (°C)

    Pdry = precipitation of the driest month (mm·month–1)

    Psdry = precipitation of the driest month in summer (mm·month–1)

    Pswet = precipitation of the wettest month in summer (mm·month–1)

    Pwdry = precipitation of the driest month in winter (mm·month–1)   

    Pwwet = precipitation of the wettest month in winter (mm/month) 

    Thot = average temperature of the hottest month (°C)

    Tcold = average temperature of the coldest month (°C)

    Pthreshold (mm) = is given by the following rules: 

    If (≥70% of MAP occurs in winter) then Pthreshold = (2·MAT)

    Else if (≥70% of MAP occurs in summer) then Pthreshold = (2·MAT) + 28

    Else Pthreshold = (2·MAT) + 14 

    Summer (winter) is defined as the warmer (cooler) six month period of [1 Oct. - 31 Mar.] and [1 Apr. - 30 Sep.].