Metr 302

Class Notes for October 1 – 3, 2003

Ami Murray

Son-Hui Wong

October 1, 2003

Assignments

No new reading

Housekeeping

Exam has been moved to October 8, 2003

High & Low Pressure

Area of low pressure with at least one isobar around it = cyclone

Area of high pressure=anti-cyclone (?)

Coriolis Effect

Explains why air doesn’t move exactly from high to low pressure areas.

Coriolis was a mathematician.

4 key things to remember regarding CE:

1.CE deflects all frictionless moving objects to the right in the northern hemisphere and to the left in the southern hemisphere. We are not frictionless because we are grounded to the earth by the soles of our shoes and gravity. Examples of frictionless things are airplanes, bullets, sea currents, ships, and wind. (This part is the most important for this class.)

2.CE is maximum at the poles and nonexistent at the equator. As you move just a little bit north or south from the equator CE increases gradually until you reach it’s max at the poles. This because of the geometry of the planet. CE is the reason why no hurricanes ever form at the equator even though that is where the warmest ocean temperatures exist.

3.The faster the object is moving at any given latitude and time, the more it is deflected by CE.

4.CE is greater the longer the time interval of an object in motion i.e. wave motion per one month is affected more than a plane in the air for one hour.

CE came out of equations.

An object moving 20 mph would have to travel over 100 miles before it even feels CE.

Coriolis as a factor only effects things on a very large scale. For example, the jet stream (18,000 ft) is profoundly affected by CE. On the other hand, tornadoes are not affected

Air from San Francisco wouldn’t be affected until they reached about Sacramento.

In order to have Coriolis, air must be moving.

There is a tug of war happening between air moving from areas of high to low pressure and CE, so there is a compromise. That compromise is a spiral in the air current.

Bathtub-Old Wives Tale

People used to be believed that CE affected the vacuum of the water going down a bathtub drain. From what we know of the facts of CE, it is easy to see that this is incorrect. The bathtub is too small a scale for CE tow work. The water going down the drain is neither traveling at least the speed of 20mph nor is it moving over at least the span of 100 miles.

Wind Direction - Aloft

There is a difference between air moving on the ground and air moving aloft. Air moves faster aloft because there is less geography i.e. mountains etc. As air moves across rugged topography it is physically slowed down. This affect is minimal at the ocean, Great Plains and places of very flat geography. There is a lot more friction at the ground; therefore wind speed is much greater aloft.

CE is greater aloft. Therefore wind trying to move into low-pressure area at the 500mb (18,000 ft) can’t because of the CE. The air moves parallel to the low-pressure area.

1.At sea level, air turn counter clock wise around low-pressure areas and inward. It moves clock wise and outward at high-pressure areas.

2.Air flows faster the greater the pressure/closer the isobars.

A.

On a weather map there are many flags in a low-pressure system; the isobars are very crowded, which means faster wind speed.

When aloft, the wind moves parallel to the isobars because the Coriolis Effect is so strong.

Polar jet stream (aka storm track) = large clouds, storms appear to move along jet stream

Example question:

A.

L

L

What direction is the wind flowing at two identical points but at different levels? One is at the surface and the other is 18,000ft?

	Surface weather map	Weather at 18,000ft

The air is flowing clock wise and inward at the surface level, and parallel at 18,000ft.

(The reason 18,000 ft. is chosen to compare with the surface is because that is the level that the jet stream is in and is also the 500mb level, which holds about ½ of the atmosphere.)

Hurricanes are a collection of cumulonimbus clouds that form in the tropics. The track of hurricanes depends on the wind and not the sea temperature.

Wind drifts from east to west in the atmosphere.

Rain Showers vs. Rain

The difference between a rain and a rain shower has to do with the surface area covered.

October 3, 2003

Assignments

No new assignments

Housekeeping

Our exam will be on information from the beginning of the semester to today’s lecture (9/3/03).

# 822 Scantron is needed.

Review for the exam will be on Monday (9/6/03).

Our class website has been rearranged. Graphics that are repeatedly used in class, have been given it’s own area.

Remember to check the completed lecture topic links from time to time.

Handout is given so that we can have a hard copy of the surface and 500mb maps to write on.

It is more important to understand the concepts and not memorization.

The tests are not made to be tricky.

Surface Weather Map - handout

First thing when looking at map, is to look at the legend.

Regarding wind flow, the first thing you have to see is whether it is a surface map or not.

Isobars are written in whole numbers.

There are 2 anticyclones, 1 cyclone and 1 trough

Trough – (not normally labeled on a weather map) Low pressure area not enclosed by an isobar.

Mountains will mess up wind flows.

Sometimes it will make the wind flow look as if it is moving into an area of high pressure but actually it’s just the wind deflecting the wind flow. Eventually the wind will flow into an area of low pressure.

When looking at weather maps, it’s important to remember not to look at everything at once as this can become confusing. Try looking at one weather station at a time and work from there.

500mb Chart – handout

Solid lines are isobars

The dashed lines are isotherms (This is not important to know for this class).

The airflow is parallel to the isobars.

The air is faster.

The air flows counter clockwise and inward towards low-pressure areas

Weaker lows do not have many isobars around it.

Polar Jet Stream

A fat arrow is used to indicate the jet stream.

The arrow is drawn in the middle of the flow.

The Jet Stream flows around the North Pole.

The Jet Stream relates to thunderstorms.

Understanding Isobars will help you understand wind and understanding wind will help you understand local weather.

For example, if wind is coming from the north, the air is generally colder and if it is coming from the south, the air is generally warmer.

When looking at maps, focus on separate pieces of information.

For example, picking one station from either side of the low.

Wind does not only flow from side to side.

Air also sinks and rises.

This is what creates substantial cloud development.

Clouds are created by condensed water vapors.

Air cooled enough to reach dew point temperature will create precipitation.

Weather Balloons

They are filled with helium.

The local weather balloon site is at the Oakland Airport.

As pressure surrounding the balloon decreases, the balloon will expand.

It will eventually pop.

Weather Balloons with usually pop at about 100mb, which is at 100, 000 ft.

In the weather balloon the molecules strike each other.

At the surface the molecules are rapidly striking each other, causing the gases in the balloon to heat.

As the balloon rises and expands, the molecules strike each other less frequently causing the gases in the balloon to cool.

There is the same amount of molecules inside the balloon as when it was at the earth’s surface now they just have more room to move around. They don’t bounce off of each other as much thus cooling the gasses in the balloon.

Effects of upward motion

Rule of Thumb

Air that rises, cools.

Significant in knowing if the weather will be fair or foul, also in knowing about cloud development.

At the surface level, air is going to a common point (at a low pressure area). When the air reaches that point, where does it go? It goes up.

In cumulonimbus clouds the air is moving up rapidly.

It can obtain speeds to 70mph to 80mph.

On surface weather maps, you will see that high-pressure areas do not have many clouds.

Low-pressure areas usually have many clouds.

2 new map vantage points:

1- Planview = looking down on the map (birds eye view)

2- Cross section = viewing map from the side