Lecture 9, 2016
MT LEMMON –
Meet outside at 8 AM; we’ll be back by 5 PM.
Bring:
Handouts – oaks/maple
Day pack
Hat, umbrella or rain coat. Could get cold, with hail. Shoes, not sandals. Long Pants.
Lunch.
Water: two quarts.
Plastic bags for collecting plants; newspaper bag, or plant press.
Notebook, pencil.
GPS if you’ve got one. Add aside on locations: can get off of Google earth. Lat, long,
elevation.
Hand lens – can borrow one from class.
TODAY: why and where of deserts. POWERPOINT
Start with: Mercator grid. Explain latitude (distance from equator) and longitude
(distance from the PRIME MERIDEAN in Greenwich, England). (And what is
distance? Degrees or UTMs.)
Equator – real. Greenwich – arbitrary.
From Wikipedia:
A prime meridian, based at the Royal Observatory, Greenwich, in London,[1] was
established by Sir George Airy in 1851. By 1884, over two-thirds of all ships and tonnage
used it as the reference meridian on their charts and maps. In October of that year, at the
behest of U.S. President Chester A. Arthur, 41 delegates from 25 nations met in
Washington, D.C., USA, for the International Meridian Conference. This conference
selected the meridian passing through Greenwich as the official prime meridian due to its
popularity. However, France abstained from the vote and French maps continued to use
the Paris meridian for several decades.
What makes a desert?
Image: precip map of US, and world.
What's the pattern?
Why is there a pattern? Shape of the earth, to start with. If the Earth truly was flat, there
would be no difference in heat gain from equator to pole. (as well as no day or night).
But the earth is round.
Hotter where the sunlight hits it straight on. Image.
So some parts are hotter than others. Like the equator. Air rises when warmed. What
happens when air sinks or rises?
Adiabatic cooling and heating is due to changes in pressure of a gas. Decrease in
pressure = cooler. Feel it when you blow up a tire, or deflate.
1 degree C / 100 meters
about 5 degrees F/1000 feet
Figures are for DRY air. It’s easy to change the temperature of air; hard to change the
temp of water. It’s dense.
Back to the equator. As it rises it cools and rain falls. Cool air holds less moisture than
warm air.
What happens to the now-drier air way up high? Gravity keeps it in the atmosphere, but –
but where does it go?
Towards the poles. Descends at around 30 degrees. SLIDE.
Another SLIDE
This is the Hadley Cell: Air circulation pattern of rising air at equator, and sinking
at 30 degrees latitude.
Similar pattern at 60 degrees, but for different reason: colliding air masses. We won't
focus on this.
Back to 30 degrees. Sinking air, high pressure, it warms. Whatever moisture it has, it
retained. Many of the great deserts are located at these latitudes. Image: back to earlier
global precip.
But something else is going on.
Image: Global circulation. Note the Hadley cell – tropical circulation pattern between
0 and 30 degrees N and S.
Something else is going on: tradewinds, or easterlies. And westerlies.
What’s going on? Coriolis effect is an apparent deflection of moving objects (apparent
because of the frame of reference; it’s actually going straight from a different perspective,
from that of the object itself, which is simply maintaining its trajectory)
Give merry go round example, toss candy.
Coriolis effect is important, because it explains the distribution of Rain Shadow Deserts.
(diagram on board, of the Andes in South America: Patagonia and Atacama)
Adiabatic cooling – lower rate, because it’s wet.
Condensation because colder air holds less water. Rain falls out.
Adiabatic heating – dry rate is higher, so air ends up hotter. This also increases water
use, so the conditions are tougher still.
So: rainshadow deserts are in the lee of mountains.
BUT …why is the Atacama dry when it’s on the ocean? Because it’s a cold ocean, and
it’s cold because of upwelling and circulation.
Image: Ocean temperature
When air moves off the cold water onto warm land, it’s doing the exact opposite of when
it gets pushed over a mountain. The water does not condense. The fog vanishes.
So: recap. Key concepts:
Sinking air at the Horse latitudes, around 20-40 degrees, N and S.
Lee of Mountains – Rain Shadow
Cold water, warm land
Examples follow…