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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