homework
lym503829712
GEO344Lecture24-GreenhouseEffect.pdf
GEO 344 Weather and Climate Prof. Stuart Evans
Lecture 24 Greenhouse Effect
Homework #3 review
Pressure gradient force This is what causes the wind to blow in the atmosphere!
The PGF is directed from HIGH to LOW pressure.
H L PGF
High pressure (has more air overhead)
Low pressure (has less air overhead)
PGF
iso ba
rs
Coriolis effect Note: the Coriolis effect absolutely DOES NOT make water go in a circle in a toilet. It only affects things that travels 10s-100s of miles.
Storms are that big!
Storms have low pressure at their center. Low pressure sucks things in.
Coriolis effect: what I want you to know
1) It is caused by the spinning of the Earth 2) It only applies to thing moving at least 10s of miles 3) It bends the path of an object (or air or water) to their right
in the Northern Hemisphere (opposite in S. Hem.) 4) Coriolis force is stronger for faster objects and farther from
the equator
The surface – friction matters
PGF
Coriolisfriction
FRICTION: • Slows wind • Works in the opposite
direction of wind • Since friction SLOWS wind,
weaker Coriolis force
• Result of balancing PGF, Coriolis and friction is that wind flows ACROSS isobars TOWARDS the low
These forces are all balanced. End result: wind crosses the contours toward low pressure
wind
Homework #3 review
PGF creates wind from high to low pressure. (wind will be perpendicular to isobars)
Homework #3 review
Coriolis (in NH) bends wind to the right of the direction it was going. (wind will be along isobars)
Homework #3 review
Friction makes wind cross isobars at an angle, toward low pressure. (wind will be 30° - 45° across isobars)
Surface wind crossing contours
(N. Hemisphere)
More friction àgreater angle across isobars
Rule of thumb: Ocean: 15-30° angle Land: 30-45° angle
This is how much friction changes the direction.
Homework #3 review
All forces together mean: crossing the isobars at an angle, toward low pressure. (wind will be 30° - 45° across isobars)
COLD
Homework #3 review
Arrows are same size to demonstrate that contours are closer together in Burlington
Homework #3 review
Homework #3 review
Homework #3 review
Arrows are all the same length. Cross no contours in tropics, many in a mid- latitudes, a few in Arctic
Homework #3 review
Buys Ballot’s Law (geostrophic balance) • Take home points:
1. Wind flows parallel to pressure contours 2. Wind speed is proportional to pressure gradient (tighter contours à windier) 3. Near surface, wind crosses contours toward low pressure (because friction)
BUYS BALLOT’S Law: If a person in the Northern Hemisphere stands with her back to the wind, the atmospheric pressure is lower to her left and higher to her right.
Christophorus Henricus Diedericus Buys Ballot (seriously)
win d
so Coriolis
adjustsdirection
balances PGF
Clockwise around High
Counter-clockwise around Low
Faster with tighter contours
Slower with looser contours
Lower pressure to left
Higher pressure to right
Geostrophic balance Why does stronger PGF mean faster wind? • Stronger PGF requires stronger Coriolis • Strength of Coriolis is proportional to speed
PGF
Coriolis
wind PGF wind
A stronger pressure gradient (tighter contours) means PGF stronger, so Coriolis needs to be stronger, so wind faster.
Coriolis
Homework #3 review
Wind flow direction bending toward the left. or
Wind rotating clockwise around a low.
How can we forecast decades to centuries in advance if we can’t forecast the weather more than 10 days in advance?
Answer: we don’t try to forecast specific weather events a decade from now, instead we forecast things like global temperature.
Much simpler to understand, we just need to think about energy balance.
Reminders about Energy Balance
What does the rock teach us?
Power into rock from
hot plate
Hot plate
Power out of rock through infrared radiation (and convection)
• Energy balance is comparison of the energy going into something and the energy coming out.
• If something is in energy balance à the temperature does not change
• If something is NOT in energy balance à the temperature is changing
Balanced
Unbalanced Unbalanced
Unbalanced
Energy balance explains temperature change
Bank account analogy: if deposit = spend à balance stays same
deposit spend
if deposit > spend à balance goes up if deposit < spend à balance goes down
The rock Earth
Power out (mostly infrared)
Power in from hot plate the Sun
temperature
Energy balance (or lack thereof) explains the daily cycle
During the day, power in (sunlight) is greater than power out (mostly infrared)
At night, power in (no more sunlight) is less than power out (mostly infrared)
Energy in > energy out Energy in <
energy out
Energy balance (or lack thereof) explains the daily cycle
Energy going out (infrared)
sunrise sunset
Energy coming in (sunlight)
• When the Earth is getting more energy than it’s losing, it warms up. Keeps warming until late in afternoon.
• When the Earth is losing more energy than it gets, it cools down (starts cooling late afternoon, continues until morning)
COOLING
COOLINGWARMING
Energy balance (or lack thereof) also explains the seasons
Power in from Sun
Power out (mostly infrared)
Difference between power in and out
month
One hemisphere gains energy (summer) while the other loses energy (winter)
Sunlight is the energy in Infrared is the energy out
Energy balance (or lack thereof) explains whether the entire Earth changes temperature (i.e. global warming compared to a stable climate).
What could change the energy balance of the Earth?
Change how much energy comes in • Make the Sun brighter/dimmer
• only trivial changes on human time scales • Make the Earth more/less reflective
• happens (mostly through ice and clouds), but not the most important
Change how much energy goes out • Make the greenhouse effect stronger/weaker
• This is the major cause of modern global warming
The Earth and Moon are the same distance from the Sun Why is the Earth so much warmer?
Average temp = -18 °C
Average temp = 14 °C
The Earth and Moon are the same distance from the Sun Why is the Earth so much warmer?
Atmosphere that causes a greenhouse effect
No atmosphere
How an actual greenhouse works
Energy gets in
Energy can’t get out
Blankets work the same way as an atmosphere
Blankets work the same way as an atmosphere
1) Blanket warmed up by body heat
Blankets work the same way as an atmosphere
1) Blanket warmed up by body heat
2) Blanket radiates infrared energy to your body
Blankets work the same way as an atmosphere
1) Blanket warmed up by body heat
2) Blanket radiates infrared energy to your body
3) Body heat + infrared energy = warmer than just body heat
Blankets work the same way as an atmosphere
1) Blanket warmed up by body heat
2) Blanket radiates infrared energy to your body
Earth
Atmosphere
3) Body heat + infrared energy = warmer than just body heat
The atmosphere provides most of the energy to the surface!
Greenhouse Gases (why does the atmosphere trap infrared?)
38
§ The atmosphere is made up of § 78% nitrogen § 21% oxygen § 1% argon
§ 0.4% water vapor § 0.04% CO2 (~400 ppm) § 0.00017% methane (~1.7 ppm) § other stuff, like ozone
What’s the atmosphere made of ?
§ The atmosphere is made up of § 78% nitrogen § 21% oxygen § 1% argon
§ 0.4% water vapor § 0.04% CO2 (~400 ppm) § 0.00017% methane (~1.7 ppm) § other stuff, like ozone
super boring, don’t do anything to infrared
Important ones for radiation are all a tiny fraction of atmospheric mass. Called greenhouse gases (GHGs).
When something is this rare, humans can make a big difference in how much of it is in the atmosphere.
What’s the atmosphere made of ?
What happens when light hits an object / gas / liquid?
Infrared
Carbon dioxide (and other GHGs) absorb infrared light, but do not absorb visible light
Red lightBlue light
carbon dioxide
CO2 is a greenhouse gas
Its ability to absorb infrared was discovered by John Tyndall in 1859.
Other greenhouse gases (GHGs): methane water vapor nitrous oxide chlorofluorocarbons (CFCs)
The greenhouse effect
Energy gets in through glass
Energy can’t get out through glass
Energy gets in through atmosphere
Energy has hard time getting out through atmosphere
More GHGs = thicker / more layers of atmosphere harder for infrared to escape surface gets hotter
equivalent to more or thicker blankets
Greenhouse gases strengthen the greenhouse effect
Venus 98% carbon dioxide 735 K
Earth 0.04% carbon dioxide 289K
45
What’s been happening with GHGs?
More and more GHGs in our atmosphere à stronger greenhouse effect à trapping more infrared à out of energy balance à warming planet
Where the measurements come from
Very high up (13,678’) in the middle of the Pacific means very clean air to sample.
Ice cores trap ancient air bubbles whose composition can be analyzed
