Weather and
the Atmosphere
RAP Short Course
Syllabus
1) Vertical structure and composition of the
atmosphere, basic weather variables
2) Warming Earth and its atmosphere – the
diurnal and seasonal cycles
3) Humidity, condensation, clouds, fog
4) Cloud development and precpitation
5) Large-scale processes – upper-air winds,
mid-latitude cyclones, fronts
Syllabus continued
6) Mesoscale processes – mountain-valley
breezes, sea breezes
7) Severe weather – hurricanes,
thunderstorms, tornadoes
8) Global climate and climate change
9) Atmospheric optics – mirages, rainbows,
etc.
10) The weather-forecast process
More about the course
• Selected topics and lectures from a CU course –
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ATOC 1050 (45, 50-min. classes).
The course is aimed at non-science as well as
science majors – so no math and physics
beyond high school are assumed.
No meteorologists allowed in class.
Text available from NCAR bookstore, but not
required – Essentials of Meteorology, by Ahrens
• Please interrupt with short questions when
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necessary.
If the answer is not also short, I may ask you to
see me after class.
No exams
No homework
Course web site with notes at
http://www.rap.ucar.edu/general/events.html
Class dates – all 1:30-2:20
1) 6 March, Thur
2) 13 March, Thur
3) 20 March, Thur
4) 27 March, Thur
5) 3 April, Thur
6) 10 April, Thur
7) 29 April, Tues
8) 1 May, Thur
9) 8 May, Thur
10) 15 May, Thur
Composition of the
Atmosphere
The atmosphere – A molecular cocktail
COMPOSITION OF THE ATMOSPHERE
NEAR EARTH’S SURFACE
“Permanent” gases (percent is the same
everywhere)
• Nitrogen – 78 %
• Oxygen – 21 %
• Hydrogen - .00006 %
• Others (less than 1 %) – ozone, methane, helium, etc.
Variable gases (amount depends on place and time)
• Water vapor – 0-4 %
• Carbon dioxide – about .037 %
CO2 varies with time
Upward trend from 1958-2000, plus seasonal
variational
Not
zero
Why the variation with time?
• Overall upward trend – carbon dioxide from
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combustion by industry, autos, etc.
Sawtooth pattern – seasonal variation.
Q: What would cause winter-summer
oscillation?
A: Vegetation is active in warm season,
consuming carbon dioxide
Q: But shouldn’t two hemispheres balance?
A: More land and vegetation in Northern
Hemisphere
Seasonal Variation in Vegetation
Equator
Distribution of forests
Equator
Water-vapor Content
Content of the atmosphere
in addition to gases
• Mineral particles – dust from ground,
man-made pollution
• Water droplets – clouds, fog
• Water drops – rain
• Ice – small crystals, snow flakes, hail
Structure of the atmosphere
• Varies depending on the meteorological
conditions
• But we can talk about average conditions
- horizontal structure (as shown on a
weather map)
- vertical structure
Vertical structure of the
atmosphere
• First – how deep is it?
White line is the thickness
of the atmosphere (99%
of molecules)
Earth
Within this thin atmosphere,
there are three ways of
defining layers
• How temperature changes with height
(increase versus decrease with height)
• Composition (mixture of gases)
• Electrical structure
Different Layers of the Atmosphere
A Glossary of Weather
Variables
• Air temperature
• Air pressure
• Humidity – amount of water vapor
• Clouds
• Precipitation – rain, snow, hail, etc.
• Wind – direction and speed
• Visibility
Air Temperature
Temperature
• Related to rate of motion of molecules: The
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warmer the air, the more rapidly the molecules
move.
Primary temperature scales
– Fahrenheit (F): freezing point of water = 32 F,
boiling point = 212 F
– Celsius (C): freezing = 0 C, boiling =100 C
– Absolute (A) or Kelvin (K): absolute zero = 0 A = 273 C (all molecular motion stops)
• Temperature observations near the surface
(2m) are reported in degrees F, and above that
are reported in degrees C.
The temperature scales
,A
Vertical Temperature Structure
of the Atmosphere
Commercial
Aircraft
All the
Weather
Denver temperature profile at 5:00 AM
28 Feb 03
Stratosphere
Tropopause
Te
m
p
Up
Troposphere
Experiences that confirm that
temperature decreases with height
• It is colder on the tops of mountains
• Some of the hottest places in the world
are below sea level – Death Valley, the
Dead Sea
• Commercial aircraft need to be heated
Air Pressure
Why do we care about
pressure?
Air Pressure - Definition
• Pressure = force/area (e.g., pounds per
square inch)
1) Hydrostatic pressure – weight of the
atmosphere above the surface
2) Dynamic pressure – force of the wind
(e.g., against a door)
Air Pressure - Units
• In this course, pressure is expressed in millibars
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(mb) – average sea-level pressure = 1013 mb
Sometimes (e.g., in weather forecasts) it is
given in inches of mercury – average = 29.92 in.
mercury
Surface pressure values are “adjusted” to sea
level, so that values show high and low pressure
patterns rather than the elevation of the
observation
How is sea-level pressure estimated
for plotting on weather maps?
Vertical Pressure Variation in the
Atmosphere
Experiences that confirm that
pressure decrease with height
• Ears pop in elevator or driving up mountain
• Commercial airplanes need to be pressurized •
pain in ears when landing
Analogy with water – ears sometimes hurt as
you dive deeply
Air Density
• General definition of density - Mass per
unit volume
• Air density – Mass (e.g., kilograms) of air
molecules per unit volume (cubic meter)
More dense
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Less Dense
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Density Variation With Height
Sea level
6-7 km, 450 mb, 4 mi ASL – 50 % reduction
20 km, 50 mb, 12 mi ASL – 90 % reduction
Personal experience showing
that air density decreases
with height
• It is more difficult to breath at higher
elevations – fewer oxygen molecules
Humidity
• A measure of the amount of water vapor
in the air
• There are many different measures of
humidity, and a few used in this class are
– Relative humidity
– Vapor pressure
– Dew point temperature
Clouds
• Percent of sky covered by clouds
• Types of clouds
– Cumulus
– Cirrus
– Stratus, etc.
Wind
• Speed – miles per hour or knots (nautical
miles per hour), meters per second
• Direction - Given in terms of the direction
from which the wind is blowing. A
northwesterly wind is blowing from the
northwest.
• Horizontal wind is measured, but vertical
wind is also very important
Visibility
• The maximum distance at which an
observer can distinguish an object against
the sky as a background
• May be limited by fog, air pollution, etc.
Precipitation
• Type of precipitation
- rain, snow, sleet, hail, etc.
• Rate at which it is falling
(inches/hour)
Scales of atmospheric motion Storms of all sizes
• Mid-latitude cyclones – 1000+ mile
• Hurricanes – 500+ miles
• Thunderstorms – 1-100 miles
• Tornadoes – few hundred yards
• Turbulence – centimeters-meters
Measuring the atmosphere
• Weather balloons, or radiosondes make
“upper-air” measurements of horizontal
wind, temperature and humidity.
Radiosonde –
helium
balloon with
measurement
instruments
Measuring the atmosphere
• Weather balloons, or radiosondes make
“upper-air” measurements of horizontal
wind, temperature and humidity.
• Surface weather stations (land and seas)
measure horizontal wind, temperature,
humidity, cloud cover
• Satellites can estimate winds, temperature
and humidity
Radiosonde launch locations
“Surface” observations - NWS
Zooming in to see even more
Northern Utah