Transcript September 29th: Cyclones Intro (we won`t cover all of this today)

AOS 100: Weather and
Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell
Miscellaneous
• Homework’s are due by the end of map
discussion!
• Exam Reminder
• Note-takers
Review of September 24th: Mostly
Moisture
• There are three important moisture variables to
consider:
- Relative humidity is defined as the ratio of the
amount of water vapor in the air to the amount of
water vapor the air can hold
- Dewpoint is defined as the temperature the air
would have to be cooled to reach saturation
- Mixing Ratio is the ratio of the mass of water to the
mass of dry air
• Warmer air can hold more water vapor than colder
air
Review Continued
• There are two types of heat: Sensible heat and
Latent heat
• Sensible heat is the type you can measure with
a thermometer
• Latent heat is the heat required (or released) to
change a substance from one phase to another
• Gases are more energetic than liquids, which
are more energetic than solids, so to move up in
energetic states, energy is taken from the
environment, and vice versa
• This means that heat is either taken from the
environment, or given to the environment
depending on the type of phase change
Review Continued
• If saturated air is forced to rise, it will cool,
causing condensation to occur
• This releases latent heat, which warms the
air a little
• This is the reason why dry air cools faster
as it rises than saturated air
• Therefore the lapse rate of moist air is less
than the lapse rate of dry air
• Remember that the lapse rate is simply a
measure of how much the atmosphere
cools with height
Adiabatic Lapse Rate Mixing Ratio
Moist Adiabatic
Lapse Rate
Temperature
Dewpoint
Temperature
Cyclone Overview
• Cyclones are areas of low pressure
• In the Northern Hemisphere, winds rotate
counter-clockwise around them and
clockwise in the Southern Hemisphere
(this can also be deduced from force
balances)
• Two primary forms of cyclones are:
- Extratropical cyclones
- Tropical cyclones (i.e. Hurricanes)
Extratropical Cyclones
http://cimss.ssec.wisc.edu/goes/blog/wp-content/uploads/2008/04/080410_g12_vis_anim.gif
Why Do Extratropical Cyclones
Exist?
• Recall that the equatorial regions of the planet
are warmed by the sun more than the poles
• However, the equator doesn’t continuously get
warmer, and the poles don’t continuously get
colder
• This is because extratropical cyclones act to
transport heat poleward (i.e. they advect cold air
towards the equator and warm air towards the
poles)
• Therefore, they are found most commonly in the
mid-latitudes where the temperature gradient is
strongest
Extratropical Cyclone Structure
• Generally, cold air flows southward on the
western portion of the cyclone, and warm air
flows northward on the eastern portion (in the
Northern Hemisphere)
http://www.uwsp.edu/gEo/faculty/lemke/geog101/images/08j_midlat_cyclone_3.gif
Fronts
• The intersection of different airmasses are
called fronts
• The most common type are Warm Fronts
and Cold Fronts
• Fronts typically
move in the direction
the triangles or
semicircles are
pointing
Fronts Continued
• Fronts are commonly located near regions
of precipitation
• This is due to the fact that fronts are often
areas where warm and moist air is forced
to rise
• There are four types of fronts:
- Cold Fronts
- Warm Fronts
- Occluded Fronts
- Stationary Fronts
Cold Fronts
• Cold fronts are often sharp boundaries
between warm and cold air
• Cold air is more dense than warm air, so it
easily replaces the warm air
• This often acts to focus upward vertical
motion, leading to a narrow band of
precipitation
• Severe weather is commonly observed in
advance of the cold front
Cold Fronts Continued
Warm Fronts
• Warm fronts are frequently less sharply
defined than cold fronts (but not always)
• Since warm air is less dense than cold air,
it has a greater difficulty displacing cold air
• Therefore, a warm front has a much
shallower slope compared with cold fronts
• Precipitation associated with warm fronts
is often lighter and has a greater coverage
than with cold fronts
Warm Fronts Continued
A Comparison
Occluded Fronts
• Due to the different densities of
cold and warm air, cold fronts
typically move faster than warm
fronts
• Therefore, sometimes the cold
front “catches up” to the warm
front, especially with strong, longlived cyclones
• When this occurs, an occluded
front is formed (often depicted in
purple)
Occluded Fronts
Continued
• Recall that
precipitation is
produced in regions
where warm moist air
is forced to rise
• Once an occluded
front has formed,
there is no source of
warm air, so
precipitation often
weakens in intensity
From: http://www.ux1.eiu.edu/~jpstimac/1400/FIG09_008.jpg
Stationary Fronts
• As the name implies, stationary fronts are very
slow moving (if they move at all)
• Stationary fronts separate cold air from warm
air, and can be associated with precipitation
• Stationary fronts frequently turn into cold fronts
or warm fronts
From:
http://www.ux1.eiu.edu/~jpsti
mac/1400/FIG09_008.jpg
Finding Fronts
• Fronts will often be associated with all or most
of:
–
–
–
–
–
A locally strong temperature gradient
Relatively lower pressure
Clouds and precipitation
Converging winds
Positive vorticity (i.e. a sharp turning of the winds
cyclonicly)
– Locally strong moisture gradients
• It is often helpful to first locate the location of
lowest pressure, since fronts normally lead from
it
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Fronts in Other Images
• Besides looking at surface observations,
fronts can often be seen in other ways
• If precipitation is associated with the front,
they can be seen on radar images
• The clouds associated with fronts can be
seen on satellite images also
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