1. Earth’s Weather Chapter 17
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2. The atmosphere
A relatively thin shell of gases surrounding the solid Earth
Density decreases with increasing altitude
50% within 5.6 km of Earth’s surface
99% within 32 km of the surface
Earth’s radius = 6,373 km
32/6,373 = 1/200

3. Composition of the atmosphere
Mostly nitrogen (78%), oxygen (21%) and argon (1%)
Nitrogen and oxygen cycle in and out of atmosphere
Argon: inert; of radioactive origins
Trace components
Water, carbon dioxide, neon, helium, krypton, xenon, hydrogen, methane, nitrous oxide, …
Aerosols: dust, smoke, salt and other tiny solid or liquid particles

4. Measurement of atmospheric pressure
Barometer: pressure measuring device
Mercury barometer
Depends on atmospheric pressure supporting a column of mercury
Standard atmospheric pressure (1 atmosphere) corresponds to cm of Hg
Pressure variation with altitude
10.0 N/cm2 at sea level
5.0 N/cm2 at 5.6 km

5. Warming the atmosphere
Heated by incoming solar radiation
Direct heating
16% absorbed by air
3% absorbed by clouds
Indirect heating
51% absorbed by surface
Infrared reemitted by surface and absorbed by atmosphere
Involves water and CO2
Greenhouse effect
Losses
20% reflected by clouds
6% scattered by air
4% reflected by surface

6. Structure of the atmosphere
Temperature generally decreases with increasing altitude
Heating mechanisms more efficient at lower altitudes
Cooling mechanisms more pronounced at higher altitudes
Observed lapse rate is 6.5ºC per km
Inversion: colder temperatures near surface

7. Layered structure Troposphere Stratosphere Ionosphere
Surface to where temperature stops decreasing with height
Most weather here
Stratosphere
Temperature increases with height
Less turbulent layer
Ionosphere
Merges with vacuum of space
Free electrons and ions

8. The wind Three general motions
Upward movement over a region of greater heating
Sinking of air over a cooler region
Horizontal movement between warmer and cooler regions
Wind: this horizontal movement of air

9. Local wind patterns Two important relationships
Air temperature and air density
Density decreases with temperature
Hot air rises; cold air sinks
Air pressure and air movement
Upward movement leaves low pressure behind “lifting effect”
Downward movement builds higher pressures “piling up effect”
Air generally moves from higher pressure areas to lower pressure regions
Examples: sea breezes, mountain winds, Chinook (compression) winds

10. Wind Chill Chart

11. Global wind patterns
Earth receives more direct solar radiation in the equatorial region
Intertropical convergence zone
Hot air rises here
Rising air cools, causing precipitation
Cooler air descends at higher latitudes
High pressure belt 30º N and S of equator
Jet stream
Meandering loops of wind near the top of the troposphere
Speeds of 100 mi/h or more

12. Water and the atmosphere
Three phases of water
Liquid generally above 0ºC (98% of Earth’s water)
Solid below 0ºC in the form of ice, snow and hail
Water vapor
Smallest component of the three
Contributes to greenhouse effect
One of the principal weathering and erosion agents
Responsible for maintaining life on land

13. Evaporation and condensation
Liquid-to-gas and gas-to-liquid phase changes occur at any temperature
Liquid molecules with higher than average kinetic energy can evaporate
Gas molecules with lower than average kinetic energy can condense
Saturation
Equilibrium between evaporation and condensation
Influenced by temperature
Warm air can hold more water vapor than cold air

14. Humidity Absolute humidity Relative humidity
The amount of water vapor in the air at a particular time
Ranges from near zero to the temperature dependent saturation limit
Relative humidity
A relationship between the actual absolute humidity and the maximum absolute humidity
Capacity of air to hold water changes with temperature
Capacity increases, relative humidity decreases

15. The condensation process
Condensation factors
Relative humidity
Temperature of the air
Water molecules join together to:
produce a liquid
On a surface as dew
In the air in droplets
or to produce a solid
As frost on a surface
As snow in the air

16. Condensation, cont. Air must be saturated for condensation to occur
Saturation processes
Water vapor added to the air via evaporation
Seeing your breath on a cold day
Jet aircraft contrails
Cooling reduces the capacity of the air to hold water vapor
The dew point
A combination of 1 and 2.
Dew and frost tend to form on cool, clear and calm nights
Related to radiative cooling of surfaces
Grass blades have relatively high surface areas
Cooling occurs in low-lying areas near the surface

17. Fog and clouds Fog - near the ground; clouds - higher up
Comprised of small, suspended water droplets
Form around condensation nuclei
Particles about which fog or cloud droplets form
Cloud classification schemes
Cirrus (curly), cumulus (piled up), stratus (spread out)

18. Clouds
Form when a mass of air above the surface is cooled to its dew point temperature
Usually because air pushed into higher levels in atmosphere
Three major causes of upward air movement
Convection
Differential heating
Mountain Ranges
Barriers to moving masses
Colliding air masses with different densities
Cloud formation depends on atmospheric stability

19. Atmospheric stability
Stable atmosphere
Lifted parcel of air is cooler (and denser) than surrounding air
Lifted parcel returns to its original level
Unstable atmosphere
Lifted parcel of air is warmer (and less dense) than surrounding air
Moved to a higher level, it will continue to rise
“Thermals”

20. Upward mobility and moist air
Rising moist air cools and eventually reaches the dew point
Droplets condense around condensation nuclei in saturated air
(No condensation nuclei: supersaturated air)
Cooling of rising air slowed by release of latent heat of vaporization
Huge numbers of droplets appear as clouds

21. Origin of precipitation
Water returning to Earth’s surface
Dew and frost are surface processes, not precipitation
Precipitation forms in two ways
Coalescence of cloud droplets
Growth of ice crystals
Coalescence process
Takes place in warm cumulus clouds near the tropic oceans
Clouds contain giant salt condensation nuclei
Ice-crystal process
Takes place in clouds at middle latitudes
Ice crystals capture nearby water molecules and grow
Fall as snow in the winter; melt and turn to rain in summer

22. Weather producers Idealized model
Region 10ºN and 10ºS of equator receives more direct solar energy
Air heats up, rises and spreads toward poles
Air cools and becomes more dense as it rises, sinking back to the surface at latitudes 30ºN and 30ºS
End result
Band of low pressure near the equator, bands of higher pressure 30ºN and 30ºS of the equator
Large convective cells form to equalize pressure

23. Air masses Large, horizontally uniform bodies of air Four main types
Moisture and temperature conditions nearly the same
Four main types
Continental polar
Maritime polar
Continental tropical
Maritime tropical
Dictate air mass weather
Weather conditions remain the same over several days
Weather changes when a new mass moves in or when the air mass acquires local conditions

24. Weather fronts Boundaries between air masses at different temperatures
Cold front
Cold air mass moves into and displaces warmer air upward
Moist rising air cooled, leading to large cumulus and thunder clouds
Warm front
Warm air mass advances over a cooler air mass
Long, gently sloping front
Clouds and rain may form in advance of the front
Stationary front
Forces influencing warm and cold air masses become balanced

25. Waves and cyclones Mechanism Cyclone Anti-cyclone
Bulges or waves often form between oppositely moving air masses
Overriding, uplifted cold air produces a low pressure area
Further cold front motion leads to an occluded front and a cyclonic storm
Cyclone
Low-pressure area with inflowing, upward forced winds
Circulation pattern caused by Coriolis effect
Anti-cyclone
High-pressure area
Air sinks, is warmed; relative humidity is lowered

26. Major storms Rapid, violent weather changes
Often associated with frontal passage
Three major types
Thunderstorms
Tornadoes
Hurricanes

27. Thunderstorms Usually develop in warm, very moist, and unstable air
Three stages
Cumulus
Associated with convection, mountain barriers or a cold front
Mature
Updraft can no longer support growing ice crystals and snow flakes
Falling frozen water melts and becomes rain
Hail formed through ice accumulation cycles
Final
All updrafts are exhausted

28. Lightning and thunder
Updrafts, downdrafts and circulating precipitation separate electrical charges
Charges accumulate in different parts of the thunderhead
Lightning
Discharge between charge centers
Discharge can be cloud to ground, ground to cloud or cloud to cloud
Expanding pressure wave from heated air produces crack of thunder

29. Hail Frozen precipitation
Irregular, spherical or flattened forms of ice
Alternating layers of clear and opaque ice
Believed to form as hailstone cycles through falling and returning to upper parts of thundercloud

30. Tornadoes Smallest, most violent weather disturbance
Rapidly whirling column of air
Diameter of meters
Wind speeds up to 480km/h
Damage produced by high winds, drop in pressure at the center, and flying debris
Associated with intense thunderstorms

31. Hurricanes Tropical depression Tropical storm Hurricane
Area of low pressure
Winds generally moving 55km/h or less
Tropical storm
More intense low pressure area
Winds between 56 and 120 km/h
Hurricane
Very intense low pressure
Winds in excess of 120 km/h
Fully developed hurricane has a calm eye surrounded by intense rain and thunderstorms

32. Weather forecasting Based upon mathematical models of the atmosphere
Billions of calculations necessitate use of supercomputers
Fairly accurate forecasts up to three days possible
Major uncertainty: insufficient technology to connect small and large scale events
Ultimately oceanic influences need to be better understood

33. Climate
Composite, larger weather patterns occurring over a number of years
Determining factor in
Types of plants and animals at a given location
Types of houses built
Lifestyles
Influences
Shape of the landscape
Types of soil
Agricultural type and productivity

34. Major climate groups Two primary factors
Intensity of incoming solar radiation
Determined by angle of incidence
Number of daylight hours
Tilt of Earth’s rotation axis
Equatorial regions receive more solar radiation
Intensity changes on a yearly basis
Number of daylight hours varies annually

35. Climate and latitude Low latitudes Middle latitudes High latitudes
High solar radiation
Yearly variation small
Temperatures uniformly high
Middle latitudes
Higher solar radiation during one part of the year; lower during the other
Overall temperatures lower with greater variation than low latitudes
High latitudes
Maximum amount of radiation during one part of the year; none in the other
Overall temperatures are lowest with widest variation

36. Climate zones Defined in terms of yearly temperature averages
Tropical climate zone
Near equator
Receives most solar radiation; hot
Polar climate zone
Least solar radiation; cold
Constant daylight part of summer; constant darkness part of winter
Temperate climate zone
Intermediate between others
Rainfall patterns shift seasonally

37. Regional climatic influences
Four major factors
Altitude
Higher altitude air radiates more energy into space
Mountains
Cooler air at higher altitudes
Upwind slopes receive more precipitation; downwind slopes less

38. Regional climatic influences
Large bodies of water
High specific heat of water moderates temperature changes
Ocean currents
Can bring water nearby that has a different temperature than the land

39. Describing climates Problems No sharp boundaries
No two places have exactly the same climate