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AOS 100: Weather and Climate. Instructor: Nick Bassill Class TA: Courtney Obergfell. Miscellaneous. New Homework “Forecasting” Assignment Exam Results. Review of December 8 th : Winter Weather. The three types of winter precipitation are snow, freezing rain, and sleet

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aos 100 weather and climate

AOS 100: Weather and Climate

Instructor: Nick Bassill

Class TA: Courtney Obergfell

  • New Homework
  • “Forecasting” Assignment
  • Exam Results
review of december 8 th winter weather
Review of December 8th: Winter Weather
  • The three types of winter precipitation are snow, freezing rain, and sleet
  • The type is determined by the profile of temperature in the atmosphere
  • Snow happens when the entire atmosphere is below freezing
  • Sleet happens when the falling snow melts, but refreezes in a deep freezing layer closer to the surface
  • Freezing rain happens when falling snow melts, then passes through shallow freezing layer lose to the surface, causing the rain to freeze on contact with the frozen ground
review continued
Review Continued
  • Snow ratios describe the ratio of amount of snow to the liquid equivalent of that snow
  • High snow ratios happen in cold environments and low ratios occur in warm environments
  • Major WI snowstorms require a source of warm, moist air
  • This air will rise above cold air (as in a warm front) and produce precipitation
  • Generally, stronger areas of low pressure are better at transporting the warm, moist air needed for big snowstorms
  • This is one of the reasons we often associate strong winds with snowstorms
  • Two common winter snowstorm tracks are the “Panhandle Hook” and the “Alberta Clipper”

Classic Wisconsin Snowstorm

Location of heaviest snow



Storm Track

review continued6
Review Continued
  • Once a forecaster sees precipitation in a model, he/she would look at the following, amongst others:
    • Is your location characterized by a 500-1000 mb thickness less than 540 dm? (This would indicate that the atmosphere is fairly cold, and it’s a rough approximation of the rain-snow line.)
    • Are the temperatures in the lower atmosphere below freezing? (So that snow doesn’t melt on the way down.)
    • Are ground temperatures going to be below freezing? (So that the falling snow can stick to the ground.)
    • How much is the liquid equivalent of that precipitation? What would the snow ratios be? (Once you know these answers, you can predict a snowfall amount.)
  • A “feedback” is what happens due to the result of some action (sort of like cause and effect)
  • Feedbacks are extremely important when discussing the Earth’s climate
  • “Positive feedbacks” (those that cause a continuation of the initial action) are especially important
  • Let’s look at some examples …
the ice albedo feedback
The Ice-Albedo Feedback
  • Recall that snow and ice have a high albedo (meaning it reflects a lot of sunlight)
  • If we warm the Earth a little (the “cause” in this example), then some of that snow and ice will melt, leaving bare ground or water
  • But ground and water do not reflect as much sunlight as snow or ice, meaning that it absorbs more radiation
  • This means that it will get even warmer!
  • This is an example of a positive feedback
oceans and co 2
Oceans and CO2
  • Water can absorb some CO2 from the atmosphere
  • However, cold water can hold more CO2 than warm water
  • These means that as the oceans warm, they can’t take in as much CO2, meaning that there’s more in the atmosphere
  • And more CO2 leads to a warmer Earth, meaning that this is another example of a positive feedback
volcanic eruptions aerosols
Volcanic Eruptions & Aerosols
  • When a volcano erupts, it puts a lot of “aerosols” (ash, smoke, and other particulates) into the atmosphere
  • These aerosols can help block sunlight from reaching the Earth’s surface when they are in the atmosphere
  • However, over time they gradually fall out of the atmosphere by themselves, or carried down by rain and snow
weather vs climate
Weather vs. Climate
  • Weather describes what is happening right now
  • Climate represents the average weather conditions over an extended period of time (think decades or longer)
  • Therefore, just because it may have been cold one month or rainy another, it’s not really possible to say the climate is getting colder or rainier, unless those conditions persist for a long time
  • This is why having a record snowfall doesn’t disprove global warming
  • The time-scales are so dramatically different that they’re practically unrelated (one snowstorm and climate)


  • As just shown, even the weather over an entire country as large as the United States for an entire month can be very different from the whole world
  • This is a perfect example of why it isn’t possible to extend local weather conclusions to the whole globe, or to global warming
ozone and the ozone hole
Ozone and The Ozone Hole
  • Ozone (O3) is a naturally occurring gas in the atmosphere which protects us from harmful ultraviolet (UV) radiation
  • Without ozone in the atmosphere, more UV radiation would make it to the surface, leading to more cancer and genetic mutations
chlorofluorocarbons cfcs
Chlorofluorocarbons (CFCs)
  • CFCs are a manmade gas (used in things like spray paint) that destroys Ozone in the atmosphere due to an interaction between the Chlorine in CFCs and Ozone
  • CFCs have an extremely long lifetime in the atmosphere (up to 100 years or more), so even a small amount is very damaging
  • Thankfully, most nations began phasing out CFCs in products beginning about 15 years ago, so they have been in slow decline since
the ozone hole
The Ozone Hole
  • Due to several factors, such as the long nighttime and global circulation patterns, the south pole is extremely susceptible to Ozone loss
  • This is where the “Ozone Hole” is located
  • However, since CFCs have begun to slowly diminish in the atmosphere, the Ozone Hole has generally stopped worsening