science
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Nov16_weather_part2.pptxWeather in your textbook
Section 24.4: Components of Earth’s atmosphere- shows the
different layers
Section 24.5: Solar energy-
Just the first 2 pages. About seasons. You should find
this to be review
Page 683, in Section 24.6-
Has discussion of Coriolis and relates it to playing catch on
A merry-go-round (which would be very hard!) (Figure 24.27)
Figure 24.26 is like what I showed in class
More atmosphere/weather in the textbook
Figure 24.28. Good illustration of circulation around
high’s and low’s
Section 25.1- Atmospheric moisture
Goes into more depth than we had time for. But the
cartoon on page 699 is good illustration why condensation
occurs when it gets colder. Molecules move more slowly- less
energy
Page 700 has good discussion of condensation.
Makes the good point that fog is condensation- basically a cloud on
the ground
Today’s lecture: Section 25.4
Fronts and storms
Section starts off by talking about rising air
(“air must be lifted”)
What way did we discuss last time to lift air,
to cause it to rise?
Convection: 3 places in class
Another good way to get air to rise: mountains
See Figure 25.11 in text book
From last time: life is not this simple
Tropical convection
(where the most heating is)
Fronts and Air Masses
Consequence of Coriolos
Global pattern of highs and lows are created
Alternating patterns of rising and sinking air
This simple circulation becomes This
Circulation Patterns Affecting North America
We see two high pressures in the Northern Hemisphere.
One is near latitudes of 20-30 and is from the equatorial
Convection cell
The other is from cold sinking air at the north pole
And then there is this red curvy line…… what is that?
H
L
Pattern of Highs and Lows Alternating
Cold N
Wind
(wind from the N)
warm S
Wind
(wind from the S)
Cold N
wind
Wind flow matches up where they meet
From last time: air rising at lows, sinking at highs
Sinking air at anticyclones dries air out. Highs are clearer and dry
| Vertical Motion | |||
| Rising (low pressure) | Sinking (high pressure) | ||
| Horizontal Motion | Southerly | Warm and cloudy/rain | Warm and dry/sunny |
| Northerly | Cold and cloudy/rain/snow | Cold and dry |
4 combinations of air motion and weather
What happens if two high pressures approach
each other?
High #1
High #2
Warm S wind
Cold N wind
They fight!
(you think I’m kidding. I’m not)
High #1
High #2
Warm S wind
Cold N wind
The boundary is called a “front”
High #1
High #2
Warm S wind
Cold N wind
(TV weathercasters love to talk about “warm air” battling
“cold air”. The term comes from World War I the front lines)
Who wins?
it depends
High #1
High #2
Warm S wind
Cold N wind
If High #1 pushes High #2 out of the way, the Cold N wind
wins cold front. Dotted arrow shows the movement
Who wins?
it depends
High #1
High #2
Warm S wind
Cold N wind
But sometimes High #2 wins. Warm S winds takeover the
battlefield. a warm front
Fronts
A front represents a boundary between two air masses that contain different temperature, wind, and moisture properties.
When cold air meets warm air, warm air rises because its
Less dense.
Another way air is forced to rise at fronts
As we’ll see its similar to air rising at cyclones
The Polar Front
The red wavy curve is the boundary between cold polar air
and warm subtropical air
Often a sharp temperature gradient across this boundary.
(This is why we care about gradients in isotherms!)
Fronts comes in several “flavors”
1. Cold front (cold air overtaking warm air)
2. Warm front (warm air overtaking cold air)
Very different.
Cold fronts: dense cold air pushes warm air away
This is how they are drawn on weather maps
Air rises abruptly.
Brief intense showers
or thunderstorms
In warm front- less dense warm air has a hard time pushing cold air out
But regardless who wins….. Rain tends to occur near fronts
Why?
But regardless who wins….. Rain tends to occur near fronts
Because when cold and warm air masses meet, the warm
air rises. Rising air means it gets colder.
If it gets colder, it holds less moisture saturation
Then condensation. Clouds and rain
Fronts are the 4th way we’ve learned to cause precipitation
But its really variation of precipitation and lows (cyclones)
Why?
Differences in weather between cold and warm fronts
C
A
D
B
E
Differences in weather between cold and warm fronts
C
A
D
B
E
Cold fronts produce more severe weather. Air rises suddenly.
Thunderstorms common. Windy.
Warm fronts move slower gentler steady rain, drizzle
Finally: Fronts and Cyclones are close cousins
specifically mid-latitude cyclones
The wavy polar front will distort and a cyclonic circulation can
spin up. Thus low pressure occurs in the middle of the
two high pressures- one polar, one tropical.
It shows 3 different variations to a mid-latitude cyclone and the polar front
Some times the battle
is a draw. The Front
doesn’t move
Don’t confuse mid-latitude cyclones with tropical cyclones!
(also known as hurricanes or in the Pacific, typhoons)
Smaller, much more intense
No fronts- its all tropical air
H
General pattern to fronts and mid-latitude cyclones
Warm front to the East because that’s where the south or
south-westerly winds are
Cold front to the West- that’s where cold air is being pulled down
by northerly or northwesterly winds
Where precipitation occurs with Mid-latitude cyclones
Warm rain
Cold rain
Snow?
In winter- snow can occur on the “back side” of the low. Winds are
northeasterly (cold) but near enough to the low for rising air.
Classic pattern for a East Coast winter snow storm. Low just off the coast
so we get the NE winds on the “back side”
Note: cold fronts move so much
faster than warm fronts they can
“catch up” and when they merge its
an occluded front (you’re not
responsible for this- its just so you
recognize the symbols)
By tracking the movement of highs and lows. From west East
2. Dragged along by the jet stream
3. Winds at airplane altitude are very fast (100 mph)
and drag the highs and lows with them
4. Much of weather forecasting is using computers to
simulate the jet stream
So this is another variable which changes when you go up in altitude….. Winds are faster!
How do we forecast the weather?
Summary: How to get Precipitation
you need rising air condensation
2nd: blow air over mountains
How and where does rising air occur?
One way- from convection. Like thunderstorms
Cold fronts: air rises quickly. Stormier conditions
Warm fronts: air rises more slowly. Steadier/lighter rain
3rd way: rising air at center of cyclones
3a: when cold air meets warm air
at a front and warm air is forced to rise up
