CLIMATE AND WORLD VEGETATION |
| Spherical habit of the Earth | Solar Insolation and Latitude Zones |
| Temperature and elevation | Precipitation |
| Global Pressure Systems and Winds | Ocean Currents |
| World Climate and Vegetation |
Reading: Text. Chapter 3: The Physical Setting [pp. 39-47]
Other sources: Strahler and Strahler. 1989. Elements of Physical Geography. Chapters 1-6. [On reserve in Biology Library]
[For additional practice with some of these topics, we recommend the tutorials on the "Understanding Weather and Climate" Software written by Dr. Burt in the Geography Dept. We are currently trying to get a copy for you to to download (PC format only so far). If/when we do, the link to it will be here.]
I. Introduction:solar energy (insolation) as key to driving biosphere
Plants are a general feature of the natural landscape and grow in all but the most extreme environments. However, no species occurs everywhere in the world, each being distributed according to its own unique tolerance of the multitude of factors that comprise its environment.
Species with similar ecological tolerances develop into recognizable plant formations with distinctive floristic and structural characteristics. At the broadest scale these represent the major biomes of the world. The regional extent of each biome is primarily determined by climatic conditions and this is the basis of most schemes of vegetation classification.
Of historical importance is the work of de Candolle who, in 1874, proposed that the major plant formations were distributed according to the heat requirements and drought tolerance of the plants. Although this early explanation has been superseded, his general concepts have been subsequently incorporated into most systems of vegetation and climate classification (for example, the Köppen, Holdridge, and Walter systems that we will examine later).
Climatic conditions are controlled by the amount of solar energy intercepted by the earth and its atmosphere. Some energy used in photosynthesis and stored in biosphere but most absorbed and converted to heat. In tropics or low latitudes, net surplus of energy occurs; but high latitudes experience a net negative radiation balance (i.e., more energy lost through re-radiation than received).This sets up an energy or heat circulation from low to high latitudes by movement of atmosphere (winds) and hydrosphere (ocean currents). These circulation patterns ultimately determine global temperature and precipitation
As we will see, these broad patterns are responsible for the specific climate and vegetation in specific areas:
wet, hot at equator give rise to wet tropical forest
dry, warm at tropics of capricorn and cancer (23°) on west side of large continents give rise to deserts
II. Spherical habit of the Earth
A. Rotation
Earth's rotation causes daily, diurnal, rhythm that plants and animals respond to and also causes the mysterious Coriolis effect we will look at later.
B. Revolution
Earth's revolution around the sun occurs in a tropical year = 365 1/2 days. It is important in setting the timing for climatic seasons that influence life on earth.
C. Tilt of Earth's axis
Plane of the ecliptic is the plane containing the earth's orbit around the sun. Imagine the earth's axis perpendicular to the plane of the ecliptic. Then the equator lies exactly in plane of ecliptic; the sun strikes the equator most directly (subsolar point); the sun's rays just graze N & S poles; and every day would have same climate - no seasons!
In reality, earth's axis inclined by 23 1/2% to the plane of the ecliptic. Must couple this fact of the earth's tilt with a second fact, the earth's axis maintains a fixed orientation to the stars, namely Polaris. What are consequences of these two facts (tilt & star orientation) ?
Solstice
summer (June 21,22) : North polar end of axis leans at max 23 1/2% to sun; subsolar point at tropic of cancerwinter (Dec. 21,22): South polar end of axis leans at max 23 1/2% to sun; subsolar point at tropic of capricorn
Equinox
vernal (Mar. 20,21): neither N or S pole has any inclination to sunautumnal (Sep. 22,23): neither N or S pole has any inclination to sun
subsolar point at the equator; circle of illumination passes through the two poles; and all points on earth get exactly 12 hrs light and 12 hrs darkness
The tilt of the earth's axis determines the following geographical lines of latitude
arctic circle 66 1/2% N 24 hr light in summer solstice
antarctic circle 66 1/2% S 24 hr dark in summer solstice
tropic of cancer 23 1/2% N subsolar point in summer solstice
tropic of capricorn 23 1/2% S subsolar point in summer solstice
III. Solar Insolation and World LatitudeZones
A. Solar insolation
Despite seasonality and daily changes in amount of day & night as earth revolves around the sun, all points on the earth receive the same hours of light averaged over a year!
Insolation: amount of solar energy (shortwave) intercepted by an exposed surface is not constant over time or place.
solar constant = 2 langley (ly) / min
= 2 g calories / cm2 / min
[langley or gram calorie / cm2 is the amount of heat to raise 1g water by 1%C]Only at subsolar point will solar energy be intercepted at this full value of the solar constant. Insolation received at any particular place on earth depends on 2 factors
1. angle at which sun's rays strike earth
2. length of time of exposure to the sun's raysThis interplay results in three important facts about insolation on earth:
1. equator gets a little less than "no tilt" model; only at equinoxes is it at subsolar point; therefore two humps or seasonality in energy
2. insolation amount on longest days increases poleward in respective summer, but so does contrast between 2 solstices!
3. polar regions actually receive over 40% of equatorial value, "added insolation". This had major impact on plants, adaptations, distributions; and permits plant vegetation to flourish further poleward than expected.
B. World latitude zones
The angle of attack of sun's rays and length of day determines the flow of solar energy reaching a given unit of earth's environment and therefore governs the thermal environment of life in the biosphere. This concept provides a basis for dividing the globe into latitude zones:
1. Equatorial zone: 10%N - 10%S
intense insolation
days and nights roughly equal in duration
aseasonal more or less2. Tropical zone: 10% - 25% N & S [up to the Tropics of C. & C.]
large total annual insolation
marked seasonal cycle [sun at zenith at one solstice and low at the other
3. Subtropical zone: 25% - 35% N & S
transitional4. Mid-latititude zone: 35% - 55% N & S
strong seasonal contrasts in insolation
strong seasonal contrasts in day/night lengths5. Subarctic / Subantarctic zones: 55% - 60% N & S
enormous seasonality in insolation / day length6. Arctic / Antarctic zones: 60% - 90% N & S
sometimes split into two
i. arctic / antarctic zones: 60% - 75% N & S
ii. polar zones: 75% - 90% N & S
6 month day and 6 month night
ultimate in seasonal contrasts of insolation & day length
IV. Temperature and Precipitation
The processes just described account for seasonal and latitudinal variations in temperature. The energy input of the sun is also instrumental in setting the global pressure system and the very important patterns of precipitation and wind/ocean currents.
These patterns are critical factors on where vegetation biomes exist and set the parameters for why species can tolerate the local environment.
A. Temperature and elevation
We first need to look at the perhaps counter intuitive fact that air gets colder as you ascend to higher elevations.
Why are there arctic-like conditions (paramo, puna, etc.) as you near the tops of high mountains in the tropics and in fact snow covered on Mt. Kenya in tropical East Africa?
Answer lies in the thermal properties of air. Density and air pressure decrease with increasing elevation. Less energy stored in gas molecules at lower densities!
Environmental Temperature Lapse Rate (=Normal lapse rate)
= decrease of temperature with altitude in still airAttach thermometer to balloon and measure temperature as it rises
6.4°C decrease per 1000 m (1 km) or 3.5°F per 1000 ft
e.g. Mt. Kenya (with perpetual snow at summit)
32°C (90°F) at sea level
0°C (32°F) at 5000 m (5 km) or 16,250 ft = snow line
- Mt. Kenya is 5,895m or 19,160ft-60°C (-76°F) at 14 km [46,000 ft or ca. 9 miles]
Top of troposphere it gets no colder but remains constant
B. Hopkin's Bioclimatic Law
This elevational decrease in temperature is mirrored in latitudinal decrease in temperature as you move away from the equator - or Hopkin's Bioclimatic Law
1000 feet of altitude = 100 miles of latitude = 3.5°F temperature drop
C. Adiabatic cooling
Now, instead of still air, consider what happens when a body of air (a bubble) rises: the body of air will drop in temperature as a result of the decrease in air pressure at higher elevations - even though no heat energy is lost to the outside
This process is reversible; air warms as it descends
= adiabatic process
Adiabatic cooling not be confused with the environmental lapse rate: applies to nonrising air only
Dry adiabatic rate = with no liquid water in the air:
10°C per 1000 m vertical rise
(5.5°F per 1000 ft)At altitude of about 1000m (3300ft) air temperature meets dew point (saturation) and condensation of water vapor into liquid water (i.e., clouds)
Going from high energy water vapor to low energy liquid water (condensation) releases 600 calories / gram of H2O
This latent heat liberated by condensation causes adiabatic rate to be reduced in further rising air
Wet adiabatic rate = with liquid water in the air:
3°C per 1000 m
(2°F per 1000 ft)Further rising causes convectional precipitation; like a bonfire - the latent heat pushes the air even higher, causing more condensation, etc., etc.
D. Orographic precipitation
"Orogeny" refers to mountain building; orographic precipitation generated by the forced ascent of moist air over a mountain barrier
After precipitation - air descends over the back of mountain and thus warms up (adiabatic increase)
But this warming on the backside occurs at the faster dry adiabatic rate since no more water in air; therefore, air actually warms up more than when it started!
Produces hot, dry rain shadow on leeward side
Examples:
"Chinook" winds in California are these westerly winds coming down the leeward sides of mountains [not "Santa Ana" which is actually easterly with dust from desert to Pacific]
Winter temperature maps often show leeward side of Rockies with higher temperatures than windward sides
Rain shadow deserts on leeward sides of North American Rockies & Sierra Nevada
Kona Coast in Hawaii (leeward and dry) is where people go to vacation, not Hilo side (windward and wet) - we will see that this sets up an incredible array of local climatic variations in Hawaii
V. Global PressureSystems and Winds
Now lets tie in insolation of sun, latitude, and precipitation by examining global atmospheric pressure
A. Equatorial trough
High insolation over the equatorial zone causes warming of air and rising of air bubbles in convective cells
This rising air causes lower than normal air pressure
As condensation and precipitation occur due to convectional precipitation, enormous amounts of latent heat are literated; updrafts continue to increase in intensity ("firebox of the globe") as times goes on releasing alot of rain
The equatorial region is therefore cloudy quite often, especially from midday on
B. Subtropical high (Horse latitudes)
At higher altitudes, when water finally depleted, air finally cools enough and becomes denser and stops rising
The now cooled and drier (denser) air cells sinks at around. 30°N & S - called Horse Latitudes (sinking air)
Causes high pressure (air is forced down) at these latitudes, and usually dry and warm (it has been warming at the dry adiabatic warming rate!)
Circular flow of air set up or Hadley Cell; air moves up from equatorial region, sinks at the subtropical high belts, and then rushes equatorward to replace the rising air
Water evaporates from the ocean surface over the subtropical highs (dry air) and is carried towards equator in form of water vapor to further feed the equatorial convection and Hadley cell
The winds from the subtropical highs towards the equator are called trade winds
Now lets consider the very important Coriolis effect before looking at the other global pressure belts
These winds do not blow exactly in N-S direction but appear to be deflected by rotation of the earth
equator's rotation velocity = 40,000 km / 24 hr or 1,700 km / hr
other latitudes will have progressively slower rotational velocitiesTherefore, northern hemisphere winds approaching the equator (faster rotation) from the subtropical highs (slower rotation) will "not keep up with points on the equator" and appear to be deflected to the right
These winds will come from the NE going SE = Northeast trades (or "easterlies")
Likewise, southern hemisphere winds approaching the equator will be deflected to the left = Southeast trades
"Trades" get their name from the trading ships coming from Europe to the New World that travelled south towards the subtropics to catch the NE trades before heading west
The trades converge at the narrow intertropical convergence zone ("doldrums" at times when not convergent)
C. Subarctic / subantarctic highs
In the same way, winds and moisture are carried poleward from the subtropical high belt to the subarctic and subantarctic low pressure belts at ca. 60° N & S
Coriolis effect causes these to be westerlies in both hemispheres; taken by trading ships back to Europe
In Northern Hemisphere, land masses disrupt the westerlies somewhat, but in Southern Hemisphere there is almost unbroken belt of ocean (great Southern Ocean) and the great winds at different latitudes are there referred to as the:
"roaring 40s"
"furious 50s"
"screaming 60s"
D. North Temperate continentalpressure systems
1. winterdry monsoon
2. summerwet monsoon
A. Windand gyres
B. Equatorialcurrent: westward
C. Westwinddrift: eastward
D. Oceancurrents and coastal climates
1. warmcurrents: east coast humid climates
2. coldcurrents: west coast arid climates
VII. World Climate and Vegetation
A. Climatetypes
1. Interplay of insolation, wind, and ocean currents
2. Precipitationpatterns (some examples)
a. equatorial wet belt
b. trade wind coasts
c. humid temperate east coasts
d. subtropical deserts
e. temperate steppe / grasslands
f. temperate wet westerly belts
3. Climate types and vegetation classification systems
a. Vegetation biomes/types vs. floristic kingdoms/regions
b. Köppen climate classification
c. L. Holdridge's world plant formations
d. H. Walter's zonobiomes and climate diagrams
