Download Weather - Fronts

Chap 4
Weather, Climate,
Atmosphere
Weather vs Climate
• Weather
• local area
• short-term physical conditions
• temperature and precipitation.
• Climate
• regional
• average weather conditions over a long time.
• Affected by
• Latitude and elevation
• The two most important factors are
• temperature and precipitation.
Weather
• condition in the atmosphere at a given place and time.
• temperature, atmospheric pressure, precipitation, cloudiness,
humidity, and wind.
• day-to-day winds, clouds, rains, storms, heat waves, droughts,
etc
Local Weather
• A weather front marks the boundary between two air-masses at
different densities.
• A front is about 100-200 km wide and slopes where warm and
cool air masses collide.
Cold front
Warm front
Weather - Fronts
• Warm Front –
• The boundary between an advancing warm air mass and the
cooler one it is replacing.
• Because warm air is less dense than cool air, an advancing
warm front will rise up over a mass of cool air.
Weather - Fronts
• Cold Front –
• The leading edge of an advancing air mass of cold air.
• Because cool air is more dense than warm air, an advancing
cold front stays close to the ground and wedges underneath
less dense, warmer air.
• A cold front produces rapidly moving, towering clouds called
thunderheads.
Weather: Stationary & Occluded Front
• Stationary front
• is a transitional
zone between two
nearly stationary air
masses of different
density.
• Occluded front
• air front established
when a cold front
occludes (prevents
the passage of) a
warm front.
Climate
• Average weather conditions over long period of time
•
•
•
•
Latitude
Altitude
Temperature
Rainfall
• Earth’ processes distributes heat & precipitation, creating
climate
• Unequal heating of Earth by Sun
• Rotation of Earth
• Earths orbit on tilted axis
• Atmospheric convection currents
• Ocean Currents
Earth’s climate system –
atmosphere/ocean interactions (driven by sun)
Climate - Changes with Latitude
From equator moving to the
poles
gets colder and
drier
There are
exceptions
particularly for
rainfall!
Biomes – observe latitude similarities
Climate - Changes with Elevation/Altitude
• As you go up in elevation:
• Colder & Drier
• Less soil and less
nutrients  less plants
• Less oxygen
• lower air pressure
 thinner
atmosphere  less
organisms
• Temperatures
decreases
• More UV rays –
limits plant growth
Solar Energy: Distributing Heat
• Global air circulation
• driven by the uneven
heating of the earth’s
surface by solar energy
• seasonal changes in
temperature and
precipitation:
• Angle of rays
• Surface area
• Reflective surfaces
Seasons: Earth’s tilted axis
• The Earth’s 23.5 degree incline on its axis remains the
same as it travels around the sun.
• As the earth spins around the sun the seasons
change.
VERY IMPORTANT: What causes the seasons?
• The ANGLE of the sun’s rays.
• Direct sunlight = summer
• Direct = more sunlight in a smaller area  more heat
stored
• Indirect sunlight = winter
• Indirect = sunlight spread over larger area  less heat
stored
Albedo
• Reflectiveness
• The higher the albedo of
a surface,
• more sun is reflected
• the less heat stored
• The lower the albedo of a
surface,
• more sun is absorbed
• the more heat stored.
Solar energy is transferred by
circulation/convection of
Masses of air and water
Wind connects all life on earth.
• Keeps tropics from
being unbearably hot.
• Prevents rest of world
from freezing.
• http://hint.fm/wind/
Figure 5-1
Wind
• Caused by the pressure gradient force.
• High pressure means more air,
• Low pressure means less air.
• The air moves from high to low, causing
wind.
Air Pressure
• pressure exerted by the weight of Earth’s atmosphere.
• Affected by
• Altitude
• temperature
• At sea level = 14.7 pounds per square inch.
• Measured with a barometer
Air Pressure
Pressure Gradient
• Changes from high to low.
• A higher pressure
gradient means stronger
winds
• the isobars on a weather
map would be drawn
closer together.
Wind
• A combination of the
• pressure gradient force
• the Coriolis effect.
• Friction at the Earth’s surface
Wind
Upper Level Flow
• There is little friction up in the upper troposphere
• Jet stream
• River of air
• fast flowing, narrow air currents found in the
upper atmosphere
• https://www.youtube.com/watch?v=C_HiBj0teRY
Ocean, Land, Valley, & Mountain Breezes
• Ocean –
• ocean-to-land breezes
• occur during the day.
• Land –
• land-to-ocean breezes
• occur at night.
• Valley –
• from the plains into a valley between two mountains,
• the wind must divert into a smaller area.
• causes high winds through the valley.
• Mountain –
• Cool air coming from the top of the mountain
• sinks down on the eastern slope,
• causing increased winds on the mountain.
Rain shadow effect
• Topography - create local climate conditions known as
microclimate.
• For example mountains commonly result in
• high rainfall on the windward side and
• low rainfall in the rain shadow of the leeward side.
Air Masses and Storms
Continental vs. Maritime
• Continental fronts are
• generally cool and dry
• Maritime (ocean) fronts are
• generally warm and moist.
• When these two air masses converge,
• the result is usually rain.
Wind
Cyclones
• called
• hurricanes in the Atlantic
• typhoons in the Pacific
• Violent storms that form over warm
ocean waters and can pass over
coastal land.
• Giant, rotating storms with winds of
at least 74 mph.
• The most powerful ones have wind
velocities greater than 155 mph.
Wind – affected by spin of Earth
• Coriolis Effect
• Force created by the
rotation of the Earth on
its axis
• deflect winds
• to the right in the North
Hemisphere
• to the left in the South
Hemisphere.
Coriolis Effect
• The spinning of the earth
creates
• Different wind speeds at
different latitudes
• faster at equator,
• slower at poles
• clockwise winds in the
northern hemisphere
• counter clockwise winds in
the southern hemisphere
N. hemisphere
- CLOCKWISE
• This movement is called the
Coriolis Effect.
• Watch:
https://www.classzone.com/books/earth_scie
nce/terc/content/visualizations/es1904/es190
4page01.cfm?chapter_no=visualization
S. hemisphere COUNTER - CLOCKWISE
Convection Cells: with No spin
Wind
Coriolis Effect
• Global air
circulation is
affected by the
rotation of the
earth on its axis.
Figure 5-4
Coriolis and Convection cells
Without the Coriolis Effect,
• air would travel in 2 large cells.
• warm air at the equator would rise
and move toward the poles.
• at the poles, it would cool and sink,
then move back to the equator
along land, warming as it moved.
DOES NOT ACTUALLY HAPPEN
With the Coriolis Effect,
• air still travels from the equator
toward the poles,
• but due to the wind patterns, the 2
cells are broken into 6 global cells.
• Warm air rises in each cell and cools
in upper atmosphere, then sinks and
warms as it travels across the land.
Winds
Polar vs. Tropical
• The atmosphere has 3 prevailing winds.
• Prevailing winds that blow from the North or South Pole are
called Polar Easterlies.
• Winds that blow in the middle latitudes (between 30 and 60
degrees) are called the Westerlies
• Tropical winds that blow toward the equator are called Trade
Winds.
Convection
Currents
Figure 5-5
Convection Cells
• Heat and moisture are
distributed over the
earth’s surface
• by vertical currents
• which form 6 giant
convection cells at
different latitudes.
Figure 5-6
Circulation Patterns
Hadley Cells
• Warm moist air rises at the equator. Rain
• As air rises, it spreads out north & south, then cools and sinks at
30 degrees. Dry
• This is why most of the world’s deserts are found at 30 degrees.
• These are called the horse latitudes (3o degrees) because early
settlers would get stuck here in their boats & couldn’t move.
They would finally throw their horses overboard to lighten the
load & get moving again.
• Trade Winds blow towards equator
Hadley Cells
Circulation Patterns
Ferrell Cells
• Warm air rises at about 60 degrees. Rain
• and sinks at around 30 degrees, dry, both north and south.
• Westerlies. Predominant winds in US
Circulation Patterns
Polar Cells
• Air rises at about 60
degrees. Rain
• floats north, and
sinks at around 90
degrees, both north
and south. Dry
• Easterlies
Circulation Patterns
Ocean Currents:
• Ocean currents influence climate by
• distributing heat from place to place and
• mixing and distributing nutrients.
Earth’s Current Climate Zones
Figure 5-2
Ocean Currents: Transport nutrients and heat all over the
globe Also called the Ocean Conveyor belt
• Driven mostly by:
• Wind/Coriolis
• Temperature gradients (colder water sinks) THERMO
• Salinity gradients (saltier water sinks) HALINE
Upwelling
• Occur when ocean
currents pull water from
the deep ocean up to the
surface
• Upwellings pull cold,
nutrient rich water up to
the surface
• Good for fishing:
• increases NPP
• nutrients are often a
limiting factor
Gyre
Large scale
patterns of
water
circulation
• Direction driven by the Coriolis Effect and the continents
• 5 gyres:
• spin clockwise in the N. hemisphere and
• counter clockwise in the S. hemisphere
• Redistribute heat and nutrients
• Upwellings occur along the west coasts of continents
Ocean Currents
• There are two types of Ocean Currents:
1. Surface Currents / Surface Circulation
• These waters make up about 10% of all the water in the
ocean.
• These waters are the upper 400 meters of the ocean.
• Driven mostly by wind
2. Deep Water Currents
• These waters make up the other 90% of the ocean
• move around the ocean basins by density and gravity.
• The density difference is a function of different temperatures and
salinity…THERMOHALINE
• These deep waters sink into the deep ocean basins at high
latitudes where the temperatures are cold enough to
cause the density to increase.
El Nino Southern Oscillation (ENSO)
• El Nino – warming event in the tropical Pacific Ocean
• Caused by weak/no trade winds blowing WEST
• The upwelling along the west coast of the
Americas stops
• Occurs in cycles every 2-7 years
• La Nina – cooling event in the tropical Pacific Ocean
• The opposite end of the spectrum as an El Nino
• Trade winds and upwelling is strong
• Both extremes cause extreme weather events globally
El Nino Southern Oscillation
ENSO effects – El Nino
• During an El Nino event
• Warmer ocean temps in
the Pacific
• Warmer and wetter in the
western Americas
• Suppresses hurricane
activity in the Atlantic
ocean
• Major declining in fishing
populations during an El
Nino
• Drought in the western
Pacific and Australia
ENSO effects – La Nina
• During a La Nina event
• Pacific ocean temps are
colder than normal
• Droughts along the west
coast of the Americas
(more snow in
northwest US)
• Active hurricane season
in the Atlantic
• More rain in the
Western Pacific and
Australia
The Atmosphere
• The mixture of gases known
as air
• protects life on Earth
• by absorbing ultraviolet
radiation and
• reducing temperature
extremes between day and
night.
• The atmosphere is not static.
• Interactions involving the
Weather occurs in the troposphere. Gaseous water molecules
held together by intermolecular forces cause the formation of
clouds.
amount of sunlight,
• the spin of the planet and
• tilt of the Earth’s axis
• cause ever changing
atmospheric conditions.
The auroras occur in the thermosphere and are caused by
interactions between the Earth’s atmosphere and charged
particles streaming from the Sun.
Formation
of the Atmosphere
• Most of the Earth’s early
atmosphere was lost due to
the vigorous solar wind from
the early Sun.
• Continuous volcanic
eruptions built a new
atmosphere of:
water vapor
carbon dioxide
nitrogen
methane
Air Density
• The atmosphere
consists of layers
• with different
• temperatures
• pressures,
• compositions.
The Atmosphere
• Earth's atmosphere contains roughly:
78% nitrogen
20.95% oxygen
0.93% argon
0.038% carbon
dioxide
Trace gases
The Earth’s atmosphere (where pressure becomes negligible)
is over 140 km thick. Compared to the bulk of the planet, this is
an extremely thin barrier between the hospitable and the
inhospitable.
1% water vapour
All images: NASA
Troposphere
• 75% of mass of
atmosphere
• 0 to 11 miles in altitude
• 78% nitrogen, 21%
oxygen
• Location of Earth’s
weather
• Temperature decreases
with altitude until the
next layer is reached,
where there is a sudden
rise in temperature
Stratosphere
• 11 miles to 30 miles in altitude, calm
• Temperature increases with altitude
• Contains 1000x the ozone of the rest of the
atmosphere; ozone forms in an equilibrium
reaction when oxygen is converted to O3 by
lightning and/or sunlight
• 99% of ultraviolet radiation (especially UVB) is absorbed by the stratosphere
• Mesosphere
•
•
•
30 to 50 miles in altitude
Temperature decreases
with increasing altitude
Meteors burn – shooting
stars
Exosphere
• Above 75 miles
• Outer space
• Satellites orbit
 Thermosphere
 50 to 75 miles in altitude
 Temperature increases
with increasing altitude
 Northern Lights
 Space station
Heat Transfer
• Conduction
• Warm air holds more moisture than cold air. During
conduction, heat & moisture from the ocean or land
moves into the atmosphere.
• Ex. cold air moving over warm water (like a lake),
forming steam fog.
• Radiation
• Radiation drives weather. Heat from the sun
warms the earth, which radiates the heat back
into the atmosphere.
• Convection
• Air & water movement