Tue., Nov. 7 notes

First it can break into pieces, then each of the pieces can grow into a new snow crystal. Because snow crystals are otherwise in rather short supply, ice crystal multiplication is a way of increasing the amount of precipitation that ultimately falls from the cloud.

Several snow crystals can collide and stick together to form a snowflake. Snow crystals are small, a few tenths of a millimeter across. Snowflakes can be much larger and are made up of many snow crystals stuck together. The sticking together or clumping together of snow crystals is called aggregation (I frequently forget the name of this process and don't expect you to remember it either).

Riming (accretion) and graupel (aka snow pellets & soft hail)
The next process and particle are something that I hope you will remember.

Snow crystals can collide with supercooled water droplets. The water droplets may stick and freeze to the snow crystal. This process is called riming or accretion (note this isn't called collision coalescence even though it is the same idea). If a snow crystal collides with enough water droplets it can be completely covered with ice. The resulting particle is called graupel. Graupel is sometimes mistaken for hail and is called soft hail or snow pellets. Rime ice has a frosty milky white appearance. A graupel particle resembles a miniature snow ball. Or smaller finer grained version of the shaved ice in a "snow cone." Graupel particles often serve as the nucleus for a hailstone. You'll find lots of pictures on the internet (also several slides explaining the difference between hail, graupel, and sleet).

Hail is produced in strong thunderstorms with tilted updrafts. You would never see hail (or graupel) falling from a nimbostratus cloud. Here is a photo of a record setting 8" diameter hailstone collected in South Dakota. It is currently the national record holder. Here's another hailstone that is almost as big. It holds the record for Oklahoma. Click here to see a gallery of images showing hail damage to automobiles.

Essentially all the rain that falls in Tucson is produced by the ice crystal process. The left figure above shows how this happens. A falling graupel particle or a snow flake moves into warmer air and melts. The resulting drops of water fall the rest of the way to the ground and would be called RAIN.

In the middle picture graupel particles can survive the trip to the ground without melting even in the summer. Many people on the ground would call this hail but that wouldn't be quite right. Graupel is less common in the winter because it comes from thunderstorms and they don't form very often in the winter. Snow can survive the trip to the ground in the winter but not the summer. Snow does occasionally make it to the valley floor in Tucson.

Sometimes the falling raindrops will evaporate before reaching the ground. This is called VIRGA and is pretty common early in the summer thunderstorm season in Arizona when the air is still pretty dry. Lightning that comes from thunderstorms that aren't producing much precipitation is called "dry lightning" and often starts brush fires.

Rain will sometimes freeze before reaching the ground. The resulting particle of clear ice is called SLEET. FREEZING RAIN by contrast only freezes once it reaches the ground. Everything on the ground can get coated with a thick layer of ice. It is nearly impossible to drive during one of these "ice storms." Sometimes the coating of ice is heavy enough that branches on trees are broken and power lines are brought down (either by the weight of ice or falling tree limbs). It sometimes takes several days for power to be restored. Here's a gallery of images taken after ice storms.

This is the end of the material that will be covered on this week's quiz.

How and why surface and upper level winds blow the way they do.

Some real world examples are shown in the figure below (found on p. 121 in the ClassNotes). The two largest types of storm systems, middle latitude storms (extratropical cyclones) and hurricanes (tropical cyclones), develop around surface centers of low pressure

the term cyclone refers to winds blowing around a center of low pressure

Earlier in the semester we learned that winds spin counterclockwise around centers of low pressure in the northern hemisphere. Tuesday next week is the day we start to worry about what happens in the southern hemisphere. Winds change direction and spin clockwise around low pressure in the southern hemisphere.

Winds spin clockwise around "anticyclones" (high pressure) in the northern hemisphere and counterclockwise around highs in the southern hemisphere.

Why do winds blow in opposite directions around high and low pressure. Why do they even spin at all.
Why do the winds change directions when you move from the northern to the southern hemisphere.

These are the kinds of questions we'll be addressing next week. And it's not just the wind. Ocean currents off the East and West Coasts of the US spin in a clockwise direction. They reverse direction and spin counterclockwise off the east and west coasts of South America.

Something else to notice in the figure. Storm systems in the tropics (0 to 30 degrees latitude) generally move from east to west in both hemispheres, in both hemispheres. At middle latitudes (30 to 60 degrees), storms move in the other direction, from west to east. That's not something we will cover in class, rather it will be the subject on an upcoming Optional Assignment.

We'll be able to learn most of what we need to know about surface and upper level winds in 10 relatively easy steps (though I've broken several of the steps into smaller parts)

Step #1 - Upper level and surface winds in the N. and S. hemisphere - summary

The next figure is on p. 122a in the ClassNotes.

Upper level winds spinning around high and low pressure in the northern and southern hemispheres are shown in the first set of four pictures. All the possibilities are here. The first thing to notice is that upper level winds blow parallel to the contours. Just 2 forces, the pressure gradient force (PGF) and the Coriolis force (CF), cause the winds to blow this way. Eventually you will be able to draw the directions of the forces for each of the four upper level winds examples. Here is an upper level wind example showing what you will be able to do.

The four drawings at the bottom of the page show surface winds blowing around high and low pressure in the southern hemisphere. Surface winds blow across the contour lines slightly, always toward low pressure. A third force, the frictional force is what causes this to occur. He is an example of what you will be able to say about surface winds.

Upper Level Charts Pt. 1 - Basic Features

We covered surface weather maps earlier in the semester but not upper-level charts. So we'd better learn some of the basics now. Also there is an Optional Assignment that accompanies some suggested reading on Upper Level Charts. The reading is split into three parts, we'll only look at the first part in class.

Upper level is just referring to atmospheric conditions at some level above the ground (weather maps are actually drawn showing conditions at multiple altitude above the ground. Upper level conditions can affect the development and movement of surface features (and vice versa).

In this first section we'll just learn 3 basic facts about upper level charts. First the overall appearance is somewhat different from a surface weather map. The pattern on a surface map can be complex and you generally find circular (more or less) centers of high and low pressure (see the bottom portion of the figure below). You can also find closed high and low pressure centers at upper levels, but mostly you find a relatively simple wavy pattern like is shown on the upper portion of the figure below (sort of a 3-dimensional view)

A simple upper level chart pattern is sketched below (a map view). There are two basic features: wavy lines that dip southward and have a "u-shape" and lines that bend northward and have an "n-shape".

The u-shaped portion of the pattern is called a trough. The n-shaped portion is called a ridge.

Troughs are produced by large volumes of cool or cold air (the cold air is found between the ground and the upper level that the map depicts). The western half of the country in the map above would probably be experiencing colder than average temperatures. Large volumes of warm or hot air produce ridges. We'll see why this is true in "Upper level charts pt. 2".

An actual example of an upper level map is shown below at left. Temperature data is shown in the figure at right. Colder than normal temperatures at right match up well with an upper level trough on the map at left. The warmer than average temperatures along the western US are associated with the eastern edge of an upper level ridge. Tucson is expecting near record high temperatures this afternoon.


A simple upper level pattern from February 10, 2016. The eastern half of the United States was under an upper level trough. There is a ridge over the western half of the country. (source of this image)	This is a "temperature departure from normal" map. The blue that covers much of the eastern part of the country indicates colder than normal. Orange and red mean warmer than normal temperatures. (source of this chart)

The winds on upper level charts blow parallel to the contour lines generally from west to east. This is a little different from surface winds which blow across the isobars toward low pressure. An example of surface winds is shown below.

That's it for the in-class coverage of upper level charts. Really all you need to be able to do is
1. identify troughs and ridges,
2. remember that troughs are associated with cold air & ridges with warm air, and
3. remember that upper level winds blow parallel to the contour lines from west to east.