Atmospheric pressure
•	Pressure – force exerted against surface by molecular motion (weight of air)
–	Measure?
•	Barometer- mercurial; weight of air equal weight of mercury in vacuum tube
–	Why use mercury and not water?
•	Water would go to 33 ft!
•	Aneroid – evacuated chamber change shape with pressure changes (pop can)
–	Sea level? 14.7 lbs/in2, 29.92 in., 1013.2 mb

Pressure 
•	Influence of temperature on pressure
–	Can change density of air
•	And, of pressure on temperature
–	Higher pressure raises boiling temp
•	Boil water Flagstaff vs. Phoenix
•	Radiator cap

•	Need to correct for elevation
–	Why? 
•	Adjust for change in pressure from change in altitude
•	Lower?  = higher pressure
–	Why? More air above
•	Higher? = lower pressure
–	Why? Less air above

•	In terms of weight above you, think of high pressure as a ‘hill’ of air…
•	And of low pressure as a ‘valley’ of air 
–	Meaning, a valley of air would weigh less than the hill of air, OR 3ft of water over you would weigh less than 10ft of water over you in a pool
•	High pressure then descends…and low pressure ascends
–	Drinking thru a straw??

•	So, why do different pressure reading exist over the globe?
–	Differential heating of the earth
–	Air is a fluid, so changes in pressure can occur ‘laterally’ across the surface…
–	So, what is the product of unequal distribution of pressure?
•	Wind- the earths attempt to even out temps around the globe
–	Name the wind for the direction that it comes from

Winds
•	So, which direction does air flow?
–	Think of having a ball setting on the peak of the hill of air…it would roll to the valley!
–	So goes wind, flows from high to low
•	Meaning, out of the high, and into the low

•	But it is not that easy…
–	…If the only factor influencing wind direction was high and low pressure, wind would flow directly out of high and into low
•	Though the pressure gradient, or the ‘steepness’ of the hill of air determines the wind speed
•	Meaning; when the pressure difference between hi and lo are great, the winds move faster.
–	What else affects??

•	Coriolis effect
–	The apparent deflection of free-flowing objects over the face of the earth
–	To the right in N hemisphere
–	To the left in S hemisphere
–	Closer to poles? More Coriolis
•	Closer to axis
–	Closer to Equator? Less Coriolis
–	Faster winds/object? More Coriolis
–	Slower? Less Coriolis

•	So what does this do to our wind movement?
–	Let us add it to the gradient force…
–	Gradient force and Coriolis ‘compromise’
•	This occurs because these winds are aloft, or high above the surface
–	The Jet Stream (geostrophic wind)
–	This only happens in winds aloft…

•	On the surface there is one more factor
–	Friction 
•	In the form of earth’s surface
•	Friction adds another vector of influence to the gradient/coriolis wind
–	So, surface winds look more like…
•	So, Coriolis review
–	More coriolis when object move faster
–	Why? Object covers more ground, thus get experience more deflection
–	Right in N. Hemis. no matter the starting point in N. Hemis.
–	Jet stream product of coriolis/PG force (river)

Pressure 
•	High and low pressure centers development
–	Warm core/cold core; thermal/dynamic (handout)
•	Thermal high/low
–	Temperature-derived; local
•	Dynamic high/low
–	Winds aloft divergence/convergence, heat induced, global

•	So, high pressure:
–	Descends
–	Wind flows out
–	Spiral to the right (in N hem.)
•	Low pressure:
–	Ascends 
–	Wind flow into
–	Spiral into from the right
•	High pressure = anticyclone
•	Low pressure = cyclone