Introduction to Climate
 
 Rev  3-13-2002

 
 
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Idaho State Climatologist Additional weblinks for Climate provided by Prentice Hall, Publishers of your text.  Click to "Chapter 6," then to "Destinations."  (You may need to first establish yourself as a student user of the text.)

Note:  The "lecture" material presented here here on this website should be considered  "generic."  References to your textbook are in this color.

We will NOT do these items in the textbook:

• Fig 6-1
• Fig 6-3

You should read and study your text as well as these notes.  If your time is so limited that you cannot do both, then know these notes, first!

This lecture web-page-set is the first of three web parts on Global Climates, plus some additional information provided by your professor.

What is Climate?

• "Climate is what you expect; weather is what you get!"
• Climate is the long-term average of the weather

Climate classification is based on two environmental variables

• Annual cycles of mean monthly temp and precipitation
• Not because they are best; because they are available
• "Temp" and "precip" relate to natural vegetation distribution
• Temperature and precipitation influence human activities

Global Temperatures

Controls of temperature

• Latitude
  • Warmer; less annual temp range, near equator
  • Cooler, to colder, with increasing annual range of temp, farther from equator
• Coastal vs continental locations
  • Coastal locations have milder annual temperature ranges
    • Cooler summers than inland
    • Warmer winters than inland
  • Coastal locations have smaller diurnal temperature ranges
    • Nights less cold than inland
    • Day less warm than inland
  • Coastal locations have their annual temperature cycles delayed
    • The warmest month is 2 months or more later in the year than for inland locations
    • The coldest month is delayed 2 months or so

Temperature affects precipitation, too

• Warm air holds more moisture ...
• Hence, warmer latitudes have potential to have more moisture than cooler latitudes

Global Precipitation

Controls of precipitation, or, lack thereof

• Continental vs. marine location
  • Ex: Moist U.S. NW coast vs. interior east of Rockies
• Orographic precipitation and rain shadows vs. non-orographically affected geographic regions
  • Ex: Moist west and top of Cascades and mountain passes such as Snoqualmie
  • Dry east side of Cascades such as cities of Yakima, WA and Sunnyside
• Pressure cells and latitude
  • Intertropical convergence zone
    • Wet all year
  • Subtropical high pressure cells (STH)
    • Hawaiian High affecting (usually) dry California
  • Midlatitude cyclones (low pressure cells)
    • Cyclonic/frontal precipitation in cool season
  • Extremely high latitudes -- too cold, so very dry
    • Arctic and Subarctic geographic regions
• Warm-season convectional precipitation
  • U.S. Middlewest, Gulf (of Mexico) Coast and Southeast Atlantic Coast

Above are brief  U.S. examples -- many others, world-wide

Global Precipitation Pattern

World Precipitation Map     
      (Text reference is Fig. 6-2)

• Isohyets -- lines of equal precipitation show geographic regions of high and low precipitation.  (These are not shown on your text map, Fig. 6-2.)
• High Precipitation (mostly are rainy, no snowy) Regions
  • Rainfall high in belt around equator -- convectional
  • Orographic precipitation on mountainous monsoon coasts
    • Monsoon coast = winds blowing towards shore-side mountains
  • High precipitation on midlatitude west coasts -- cyclones with fronts, plus orographic effects from mountains
    • Coast Ranges of Oregon, Washington, British Columbia
    • Olympic Mountains of Washington
• Dry Regions
  • Some subtropical west coasts -- east side of Subtropical High
    • Southern California
  • Zones under outflow of Subtropical Highs
    • Huge dry region in N Africa, Middle East   -- Beneath STH
  • Interior of Asia -- continentality -- too far from oceans
  • Polar regions -- very cold, so very dry ...

Climate Classification

Vladimir Köppen (1918) devised a numerical climate classification

• Advanced his work with his student, Geiger.  Major publication, 1928.
• Köppen noticed zones of similar natural vegetation
  • He wondered why the regularity in nature?
  • He devised climate zones which somewhat matched vegetation distribution
  • He used available temperature and precipitation data
• Köppen's climate classification system is widely used to teach climate
• We will briefly cover the Köppen system
• Know who Köppen was and what he did
• Before looking at details of Köppen's system, we will evaluate other aspects of climate

Classification of climates by temperature and precipitation is accurate

• Requires a long "record" of properly collected temperature and precipitation data
  • "Long" means at least 30 years; a century is good
  • "Properly" means measurement of temperature  with instruments which are properly sheltered as shown earlier in the course
  • Precipitation collected via acceptable instruments and methods
• Climatologists set numerical/statistical "rules" and divide the world into climate zones based on the temperature and precipitation "numbers" for each weather "station"
• This section of the course uses 14 Köppen climate zones

Climographs

Climate types are depicted on Climographs

• Annual cycle of temperature -- red line
  • Horizontal (left-right) equal months of year -- Jan - Dec
  • Vertical height of red line = average monthly temperatures
  • Look for how high or how low temperature is on the graph
    • Does it swing a little, or a lot, with the seasons?
  • Look for annual range -- the difference between the warmest and coldest month on the red line graph
    • Is it flat and even, or is there much variation?
• Annual cycle of precipitation
  • Blue vertical bars = average monthly precipitation
  • The taller the blue bar, the greater the precipitation
  • Look for the annual total amount
  • Look for seasonal maximums and minimums
  • Look for the pattern through the year
    • About same amount of precip throughout year?
    • Are there wet and dry seasons?
    • Very wet and very dry, or, only moderately so?
• "Extras" which are not always shown on a climograph
  • Seasonal positions of the Sun
  • Look for dominant pressure systems
  • Wind "roses" for some months of year

Different climates can be compared via their distinctive climographs

• Differences in annual precipitation are very different -- both the amount and the seasonal patterns
• Differences in annual temperature patterns are very evident
• Remember, "annual" means month by month, for a year

How should you study the climographs and 13 climate types?

• Köppen's system is based on numeric values
  • You will just have to follow the lecture notes to determine when these values should be remembered
  • There will be some other rules of thumb, as well.
• Mostly, we talk about the annual pattern of the amounts of temperature and precipitation
• Which air masses go with which climate types?  What does that mean?
• If a climograph is attached to the question sheet, you should be able to identify it!
• If a world map is attached to the test with sample locations marked, you should be able to match world locations with climate types
• Each climate type has something special which characterizes it
  • These are the "somethings" to know
  • These will be identified in lecture

Köppen's climate classification system

Köppen uses a system of code letters to go from very generalized climates to very specific

• There are a mixture of capital and small letters
• There can be as many as four code letters to designate a single climate type, in great climatic, statistical detail
• We will mostly use only two letters; sometimes three

Capital letters are used for the first level of climate classification

• A plus C, D and E correspond to increasing distance from the Equator, and thereby signify temperature characteristics
  • A climates are Tropical Rainy climates
    • Average temperature every month is above 64.4 F
    • No winter season
    • Annual precipitation is large and exceeds annual evaporation
  • C climates are Mild, Humid (mesothermal) climates
    • The coldest month has an average temperature under 64.4 F, but above 26.6 F
    • At least one month averages above 50 F
    • C climates have both a summer and a winter
  • D climates are Snowy-forest (microthermal) climates
    • The coldest month has an average temperature under 26.6 F
    • The average temperature of the warmest month is above 50 F (The 50 F line generally marks the limits to forests)
  • E climates are Polar climates
    • The average temperature of the warmest month is below 50 F
    • There is no true summer
  • H climates stand for Mountains and Highlands where climates change in short distances and cannot be depicted on world maps
• B climates are Dry climates
  • Evaporation exceeds precipitation on the average, all year
  • No water surplus; hence no streams begin here!
• Note that Köppen climate groups A, C and D have sufficient heat and precipitation to support growth of forests and grasslands
• Here are boundaries of the major Köppen climate classes using the first, capital letter
  (Note the broad groupings, without a letter designator, in Fig 6-4 of your textbook.)

Additional capital or small letters add details to the Köppen climate classification system

• S means semiarid (semi-dry)
• W (waste) means arid (semi-dry)
• f means moist, adequate precipitation in all months, no dry season
  • Used for the A, C, and D climates
• w means a dry season is in the winter
• s means the dry season is in the summer
• m means a rainforest type climate, despite a brief dry season in a monsoon cycle

The World Map of Köppen Climates looks like this   

(See the map of Fig 6-5 in your textbook.)

In the web pages which follow, we will pursue second levels of detail as we peruse a little more than a dozen climate types

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