Transcript Weather in a Nutshell - City University of New York

Introduction to the Atmosphere
Weather is always in the news. It is the extreme weather events such as storms, floods,
droughts, heat waves and cold snaps that rivet our attention. But even in its quietest
moments, the atmosphere affects us more strongly than we might think. In recent years
we have also become aware of the changing climate. Climate has always changed, but
we are beginning to realize that human activity has
reached the point where we are changing climate
more rapidly than at almost any time in Earth’s 4.6
billion year history.
This course should heighten your awareness of the awesome power and breathtaking
beauty of the Earth and atmosphere so that you may become guardians of the fragile
planet we are now altering.
And now, permit me to introduce our planet and its atmosphere.
Apollo 17 View of Earth 08 Dec 1972
This famous photo, shown
in almost every textbook on
Astronomy, Meteorology,
Oceanography, Geology,
and Climatology contains
an extraordinary amount of
information about Earth and
its atmosphere.
Before you look at the key
on the next slide and the
descriptions on the slide
after that, see how many
features you can identify.
Atmospheric Features in the Photo
1. Africa
2. Kalihari Desert
3. Sahara Desert
4. Arabian Peninsula
5. Congo Rainforest
6. Antarctic Ice Sheet
7. Extratropical Cyclone
8. Tropical Rain Belt
9. Sahel
10. Sun Glint
11. Orographic Cloud
12. Tropical Cyclone
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Major Features of Earth and Atmosphere
Our perspective of the Earth and atmosphere has broadened from a narrow, local view of incomprehensible
mystery to a comprehensive global picture. On December 8, 1972, the Apollo 17 astronauts photographed
the Earth from space on their way to the Moon (Next slide). The photo contains a wealth of information
about our planet and its atmosphere listed in the Key (Slide after next). The continent of Africa (1), largely
surrounded by deep blue ocean, holds center stage. The golden regions are Africa’s great deserts, the
Kalihari (2) and Namib in the southwest, and the Sahara (3) stretching across the width of northern Africa,
and continuing in the Arabian peninsula (4). These deserts are largely confined between 20° and 30° latitude,
in subtropical belts that contain most of the world’s great deserts. The broad, green band in the center of
Africa houses the Congo Rainforest (5), which straddles the equator like all tropical rainforests. Much
further south, bathed in the December sun, lies the white, ice-covered continent of Antarctica (6), outlined
by a giant seam that distinguishes it from the surrounding sea ice and from clouds. The North Pole, out of
sight at the top and cast into December darkness, is also covered with ice and snow.
White clouds cover the globe in a variety of intricate patterns. Several large, comma-shaped cloud swirls
occupy the ocean between Antarctica and Africa, and one covers Africa’s southern tip (7). They mark the
low pressure areas called extratropical cyclones, that bring the middle and higher latitudes most of their
stormy weather and supply Antarctica with its snow. The band of cloud spots and blotches over Africa’s
green central band constitute the tropical rain belt or ITCZ (8) and include many towering thunderstorms.
Few of these clouds venture over Africa’s deserts. Even the northern fringe of green land just south of the
Sahara (9) is clear, for December is the dry season there since the sun and the tropical rain belt is centered
south of the Equator where sun glint (10) brightens the water between Madagascar and Africa. The cloud
line on the east Coast of Madagascar (11) shows that the trade winds strike the coast there and produce
clouds and rain as they ascend the mountains.
A small pinwheel cloud system resembling a spiral galaxy appears at the upper right (12), near the
southern tip of India. This system is a tropical cyclone or hurricane and the dots are thunderstorms that are
analogs to stars in galaxies. Its majesty from space contrasts with its destructiveness on Earth.
Some Important Terms
Trade Winds: Tropical winds that blow toward the Equator from the east.
Inter-Tropical Convergence Zone: (ITCZ) The zone (roughly parallel to the
Equator) along which the Trade Winds converge and rise. The ITCZ is marked
by abundant convective clouds and showery rain. It is sometimes called the
Tropical Rain Belt.
Cyclone: A storm marked by low atmospheric pressure and characterized by
counterclockwise (in the North Hemisphere), inward spiraling winds. Cyclones
are generally regions of cloudiness and precipitation.
Tropical Cyclone: A cyclone that forms over warm tropical waters (but not on
the equator) marked by a central clear eye and spiral cloud bands. It has the
appearance of a spiral galaxy with spiral bands containing thunderstorms.
(Also called Hurricanes, Typhoons, Cyclones and Baguios)
Extratropical Cyclone: A cyclone that forms outside the tropics when polar
and tropical air masses collide. Marked by fronts. (Example - Blizzard)
Front: An often stormy boundary line separating tropical and polar air masses
The Cause of Wind and Rain
The Sun: The Source
Sunshine and Temperatures on Earth
Solar energy available for heating is proportional to
width of sunbeams striking a given area of ground
90 N Latitude
60
30
0
DAY
Sunlight variations over Earth produce
variations of air temperature, density and
pressure. The higher the Sun in the sky,
the stronger its heating. The Tropics are
warmer than the Poles because the Sun
gets high in the sky every day in the
Tropics but never does near the Poles.
NIGHT
Temperature Differences
Drive the Winds
When air is heated it expands. Warm
air tends to rise because it is light.
When air is cooled it contracts and
becomes denser. Cold air tends to
sink because it is dense.
The Thermoscope
Expansion and contraction of air
caused by temperature changes are
illustrated by the thermoscope, which
is essentially an air thermometer. As
the bulb is cooled the air inside
contracts and draws the colored water
up into the tube. When air is heated it
expands, forcing water out of the tube
(and some air if it is heated enough).
THERMOSCOPE
Driving the Winds
Differences
of
temperature,
density and pressure from one
place to another drive the winds.
If the Earth did not rotate and its
surface were uniform, each
hemisphere would have a single
circulation cell with warm, light,
rising air at the equator and
cold, dense, sinking air at the
poles.
But the Earth Does Rotate
Winds on the Rotating Earth
Earth’s rotation breaks winds
in each hemisphere into three
cells or zones.
1. Tropical EW Trade Winds
2. Mid Latitude W  E Winds
3. Polar E  W Winds
Cloudy, rising air prevails near
Equator and 60° latitude
Dry, sinking air prevails near 30°
and 90° lat.
Coriolis Force
Earth’s rotation deflects
moving air to its right (left)
in the North (South)
Hemisphere.
Important Result:
Tropical circulation (Hadley)
Cells are created with rising
air at the Equator, sinking air
in the subtropics, and Trade
Winds along the ground that
are deflected to the West as
they approach the Equator.
CORIOLIS
http://www.windows.ucar.edu/tour/link=/earth/Atmosphere/hadley_cell.html
http://www.nodvin.net/snhu/SCI219/demos/Chapter_7/Chapter_07/Present/animations/50_1_2_1.html
Clouds, Rain, and Rising Air
Rising air produces clouds and rain. But why?
Condensing Vapor – Producing Precipitation
Vapor
Capacity
All clouds and rain are formed by the process of cooling air.
As air cools its water vapor capacity decreases. When
vapor capacity falls below the original vapor content the
excess vapor condenses to form liquid water or ice.
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Vapor
Content
HOT
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Cooling….Produces Condensed Water
COLD
The fastest way to cool air in the atmosphere is to make it rise. Thus...
Most clouds and almost all precipitation are produced by rising air.
To make most clouds and almost all
precipitation
1. Air must rise
2. As air rises pressure, p decreases ()
3. As p  air expands and T .
4. As T  vapor capacity 
5. With enough cooling, vapor capacity
decreases below original vapor content
so excess vapor condenses to form
drops or crystals
Air pressure decreases () with height
because there is less air weighing
down from above. In a pile of football
players, the player on the bottom has
the greatest pressure while the player
on the top has the least pressure. The
bottom player is squeezed most, and if
he were made of air he would be
compressed to the smallest volume.
Compress
Overview of Africa’s Wind and Rain
Rain Forest is located where air rises and deserts where air sinks.
Composite satellite map of Africa’s
Vegetation with all clouds removed
In Slides #4 and #15, we noted
30
that Tropical rainforests straddle
the Equator (0 latitude) while the
Sahara and many of the world’s
great deserts are located in the
Subtropics between about 20 and
30 lat. We now explain these
major climate features. Around the
Equator, the air is hot and rises,
making it rainy. The air that rises
over the Equator moves towards the Poles. But long
before the air reaches the Poles, it cools and sinks in
the Subtropics, producing deserts.
The picture is complicated by the seasons. The Sun
does not remain directly over the Equator all year but
moves North and South. It stands directly overhead at
23.5 North Latitude on June 21, and directly overhead
at 23.5 South Latitude on Dec 21. This causes clouds
of the Tropical Rain Belt to follow the Sun and migrate
about the Equator (see next slide), producing distinct
rainy and dry seasons in the Tropics, widening the
Tropical Rainforest, but narrowing the Subtropical
deserts (although they remain quite wide).
0
-30
24 January 2006
14 August 2011
Comparing two (METEOSAT) satellite images several months apart shows how the seasons affect
climate. The Tropical Rain Belt (green ellipse) is South of the Equator in January and North of the
Equator in August. Most of its clouds are spotty thunderstorms, with anvils typically less than 100
miles wide. The Mid Latitude Storm Belt is also further south in January and further north in August.
NW Africa is covered by a winter storm with a comma-shaped front (red circle) in January but is
clear in August, when storms are located further North over Europe.
People throughout the tropics (and in North Africa) wait for the rainy season to come. But near the
edges of their range, the rains are sporadic. If they fail to fall, drought, famine, and starvation follow.