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GEO 344 Weather and Climate Prof. Stuart Evans
Lecture 13 Wind
Updates
We’re online now!
• New syllabus – check the calendar • Reading quiz due next time AND next Tuesday
• Watch lecture on your own time, but you still have to keep up with reading quizzes and homeworks
• Leave questions in the discussion area of each lecture to get answers via short bonus video
• No more midterms – more homeworks instead • Office hours via Webex – hours adjusted slightly for people in
different time zones – Wednesday 3-4 and Friday 10-11. • Slides will still go online • Participation: TBD
Updates
Common concern: online learning is harder, how can you actually learn?
• Stick to a schedule of keeping up with the course • watching 4 lectures in a row when it’s time for a homework is much less effective
• Put more effort into the reading • the book is a great resource and will not lead you astray
• Treat the recording like a real lecture • “watching” it at 2x speed, or listening to it in the background, or having another
window open at the same time does not help you • do you normally take notes? Then keep doing that. The act of writing
something down helps you remember • Ask questions about the lecture content
• this is a really effective way to get something explained
Updates – Lectures
Lectures here Leave questions here
Our meeting room: https://ub.webex.com/meet/stuartev or search for me by UBIT name: stuartev
Updates – Office hours
Videoconference via Webex: http://www.buffalo.edu/ubit/service-guides/conference/webex.html
Can be run through browser or desktop app. I’m told desktop app performs better.
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What makes the wind blow?
Different forces push the air in different directions
How hard and in which direction the forces push the air determine how fast and in which direction the air flows
Forces that always matter • Pressure gradient force (PGF) • Coriolis force
Forces that sometimes matter • Friction (surface only) • Centrifugal force (curved flow only)
Focus on these for now. What causes them?
What do they do?
• Consider a thought experiment – overturned glass of water on table top
à Lift the glass… what will happen?
Pressure gradient force (PGF)
More weight overhead =
More pressure at surface
From high to low
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
Surface pressure is weight of air overhead. Lots of air overhead here
Less air overhead here PGF
PGF
PG F
PG F
PGF PG
F
PG F
PG F
Pressure gradient force
What causes it? • Change in pressure as you move horizontally (called pressure gradient)
What does it do? • Pushes things from high pressure to low pressure (it’s perpendicular to isobars) • PGF is stronger for bigger changes in pressure (it’s stronger when isobars closer together)
Surface pressure is weight of air overhead. Lots of air overhead here
Less air overhead here PGF
PGF
PG F
PG F
PGF PG
F
PG F
PG F
THIS WAS INACCURATE DRAWING OF PGF
Surface pressure is weight of air overhead. Lots of air overhead here
Less air overhead here PGF
PGF
PG F
PG F
PGF PG
F
PG F
PG F
PGF proportional to density of contours
Pressure gradient force On small scales (up to a few miles), wind will flow in the direction of the PGF
L
H
Low pressure inside vacuum
High pressure outside vacuum
L
H
Wind over mountain creates low pressure
PGF is created by pressure differences.
A good way to create low or high pressure: Make one place hotter or colder than the place next to it
A good way to create low or high pressure: Make one place hotter or colder than the place next to it
Cool ocean temps compared to warm land temps
Temperature differences to pressure differences
From left to right, there’s no change in pressure, so no
PGF, so no wind
Starting condition
Warm land warms the air above it
Warm air expands, takes up more vertical space
(see the contours spread upwards)
Temperature differences to pressure differences
Pressure at A is between 900 and
925 hPA
Pressure at B is less than 900 hPA
PGF
PGF pushes air from A to B à more weight overhead over ocean, less over land
Temperature differences to pressure differences
Less air overhead makes low pressure
at D
More air overhead makes high
pressure at C
PGF
Pressure at C greater than at D, so PGF pushes air inland at surface
Sea breeze is why it’s much cooler just along the coast
Pressure gradient force On small scales (up to a few miles), wind will flow in the direction of the PGF
This is a really important limitation!
Surface wind speed
Wind features that are many 100s of miles in size!
Need more than PGF to explain
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 (PGF).
Low pressure Low pressure
The Coriolis effect bends them to their right
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.
Low pressure
In the Southern Hemisphere the Coriolis effect bends wind to left. This means hurricanes spin the other way!
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
NOT the Coriolis effect! What’s the trick? https://www.youtube.com/watch?v=4IIVfoDuVIw
PGF + Coriolis (N. Hem.)
H L PGF
High pressure (has more air overhead)
Low pressure (has less air overhead)
PGF
iso ba
rs
1. PGF pushes air straight away from H and straight toward L
PGF + Coriolis (N. Hem.)
H L PGF + Coriolis produces
clockwise rotation around H
PGF + Coriolis produces counter-clockwise rotation around L
iso ba
rs
1. PGF pushes air straight away from H and straight toward L 2. Coriolis force bends the path of flow toward the flow’s right
PGF + Coriolis = Geostrophic balance
1000 hPa
960 hPa
970 hPa
980 hPa
990 hPa
PGF
Coriolis
Wind
See that PGF is trying to push toward low pressure
See that Coriolis is trying to push to right of the path
If wind is flowing along the isobars, with the low pressure on its left, then PGF and Coriolis are in balance
When PGF and Coriolis are balanced with each other we call it geostrophic balance.
“geo” = Earth “strophic” = turning (spinning) geostrophic balance = pressure balanced against the
Earth turning (the Coriolis force)
Coriolis effect
Let’s look at the PGF, Coriolis force, and geostrophic balance:
windy.com