Tue., Apr. 22 notes

Tuesday Apr. 22, 2008

Some Cuban music seemed appropriate for the few minutes before class started today since we would be spending part of the day covering hurricanes. A selection titled "El Cuarto de Tula" from the 1997 Buena Vista Social Club CD was played. The song tells an interesting story, you'll find the words in Spanish and English here. Wikipedia has some background information about the album.

The Experiment #3 revised reports were collected today.

The Experiment #4 reports have all been graded and can be picked up. Revised reports are due on or before Thur., May 1. Please return your original report with your revised report.

The Topic 6 (Rain Shadow Effect) 1S1P reports have been graded and were returned today together with the in class optional assignment from last Thursday. Here are page 1 and page 2 of the answers to a little bit longer assignment that was given to the MWF class. The MWF assignment contains all of the same questions that were on your assignment plus a few extra ones.

Remembering what you learned about cloud-to-ground lightning in class last Thursday, can you make any sense out of the following figure?

Which of these two discharges came first? What is the name of that initial discharge? Click here for the answer.

Here's about where we left off last Thursday

Lightning sometimes starts at the ground and travels upward. Upward lightning is generally only initiated by mountains and tall objects such as a skyscraper or a tower of some kind. These discharges are initiated by an upward leader. This is followed by a more normal downward leader and an upward return stroke.

Scientists are able to trigger lightning by firing a small rocket up toward a thunderstorm. The rocket is connected by a thin wire to the ground. When the rocket gets 50 to 100 m above the ground upward lightning will develop off of the top of the wire.

Scientists are able to take closeup photographs and make measurements of lightning currents using triggered lightning. Triggered lightning can also be used to test the operation of lightning protection devices. A short video showing rocket triggered lightning experiments was shown in class.

Near the end of the tape you will some cases where the lightning didn't follow the wire all the way to the ground (this is one reason why you need to be very careful doing experiments of this type). When the lightning strikes the sandy soil (instead of striking instruments on the ground) it sometimes will leave behind a fulgurite (the spelling in the figure is incorrect)
.

This is a drawing of a science fair project I once saw (I don't think it won a prize). If 10 to 20 Amps will cook a hotdog, imagine what the 10,000 to 30,000 Amps in a lightning return stroke can do to a person. It is best to try not to let that happen to you.

Stay away from tall isolated objects during a lightning storm. You can be hurt or killed just by being close to a lightning strike even if you're not struck directly.

An automobile with a metal roof and body provides good protection from lightning. The lightning current will travel through the metal and around the passengers inside (the people in Florida that were triggering lightning were inside a metal trailer and were perfectly safe). The rubber tires really don't play any role at all.

You shouldn't use a corded phone and electrical appliances during a lightning storm because lightning currents can follow wires into your home. Cordless phones and cell phones are safe.

To estimate the distance to a lightning strike count the number of seconds between the flash of light and when you first hear the thunder. Divide this by 5 to get the distance in miles.

The latest lightning safety recommendation is the 30/30 Rule. You 'll see that explained below.

The 30/30 rule
30/30 Rule graphic

30/30 Rule graphic

People should seek shelter if the delay in seconds between a lightning flash and its thunder is 30 seconds or less.

People should remain under cover until 30 minutes after the final clap of thunder.

(for further information see Jetstream An Online School for Weather )

Next a little introductory information on the next topic we will be covering - hurricanes.

The figure above (a copy was distributed in class) shows the two types of large synoptic scale storm systems: middle latitude storms (extra tropical cyclones) and hurricanes (tropical cyclones). The word cyclone is telling you that both types of storms have at least one characteristic in common, winds spinning around a surface center of low pressure. The winds spin counterclockwise around low pressure in the northern hemisphere. The winds reverse direction and spin clockwise around low pressure in the southern hemisphere. The Coriolis force is what causes this change. We will cover the Coriolis on Thursday.

You may already have been to the southern hemisphere or you may go there one day. You'll probably hear about how the Coriolis force or the Coriolis effect causes water to spin in a different direction in the southern hemisphere when it is draining out of a sink and toilet bowl (it's not true). That's another reason for covering the Coriolis effect in NATS 101.

We'll compare middle latitude storms and hurricanes in a little more detail below. But notice that middle latitude storms generally move from west to east in both the northern and southern hemispheres. Hurricanes, on the other hand move from east to west, again in both hemispheres. The next figure shows why this is true.

Winds blow from the west at middle latitudes (30 to 60 degrees latitude). These are called the prevailing westerlies. The winds blow from the southwest in the northern hemisphere and from the NW in the southern hemisphere. There is very little land at middle latitudes in the southern hemisphere. There is less friction when winds blow over ocean water and the prevailing westerlies can get very strong in the southern hemisphere especially in the winter. This is what the names "roaring 40s" and "ferocious 50s" are referring to.

In the tropics (30 N to 30 S) winds blow from the east, form the NE in the northern hemisphere and from the SE in the southern hemisphere. These are the trade winds. The trade winds converge at the equator and cause air to rise. This is referred to as the Intertropical Convergence Zone (ITCZ). Since the air is warm and moist clouds form and this is a rainy part of the globe. Surface winds weaken at the equator and back when ships depended on sails to move about the ships would sometimes get stuck near the equator. This is the origin of the name "doldrums" used to describe this part of the globe.

Differences	Similarities	Differences
generally larger than hurricanes		usually smaller than middle latitude storms
Found at middle latitudes Can form over land or water	Both storms have low pressure centers. (the low pressure becomes high pressure at the top of a hurricane) The term cyclone refers to winds spinning around low pressure. Winds spin CCW in NH, CW in SH	Found in the tropics (5 to 20 latitude) Only form over warm ocean water
Movement is from west to east	Upper level divergence can lower the surface pressure and cause both types of storms to intensity	Movement is from east to west
Fronts separate warm and cold air masses brought together by converging winds.		Warm moist air mass only
Storm winds intensify with altitude		Storm winds weaken with altitude
Strongest storms winter to early spring		Strongest storms late summer to fall
Produce rain, snow, sleet, freezing rain		Mostly just heavy rain

Most of the similarities and differences are pretty readily understood. The role that upper level divergence plays in the intensification of a hurricane will be discussed later.

The figure above shows the relative frequency of tropical cyclone development in different parts of the world. The name hurricane, cyclone, and typhoon all refer to the same type of storm (tropical cyclone is a general name that can be used anywhere). In most years the ocean off the coast of SE Asia is the world's most active hurricane zone. Hurricanes are very rare off the east and west coasts of South America.

Hurricanes form between 5 and 20 degrees latitude, over warm ocean water, north and south of the equator. The warm layer of water must be fairly deep to contain enough energy to fuel a hurricane and in order that mixing doesn't bring cold water up to the ocean surface. The atmosphere must be unstable so that thunderstorms can develop. Hurricanes will only form when there is very little or no vertical wind shear (changing wind direction or speed with altitude). Hurricanes don't form at the equator because there is no Coriolis force there (the Coriolis force is what gives hurricanes their spin and it causes hurricanes to spin in opposite directions in the northern and southern hemispheres.

Note that more tropical cyclones form off the west coast of the US than off the east coast. The west coast hurricanes don't generally get much attention, because they move away from the coast and usually don't present a threat to the US (except occasionally to the state of Hawaii). The moisture from these storms will sometimes be pulled up into the southwestern US where it can lead to heavy rain and flooding.

Hurricane season in the Atlantic officially runs from June 1 through to November 30. The peak of hurricane season is in September. In 2005, an unusually active hurricane season in the Atlantic, hurricanes continued through December and even into January 2006. Hurricane season in the Pacific begins two weeks earlier on May 15 and runs through Nov. 30.

Some kind of meteorological process that produces low level convergence is needed to initiate a hurricane. One possibility, and the one that fuels most of the strong N. Atlantic hurricanes, is an "easterly wave." This is just a "wiggle" in the wind flow pattern. Easterly waves often form over Africa or just off the African coast and then travel toward the west across the N. Atlantic. Winds converge as they approach the wave and then diverge once they are past it . The convergence will cause air to rise and thunderstorms to begin to develop.

In an average year, in the N. Atlantic, there will be 10 named storms (tropical storms or hurricanes) that develop during hurricane season. 2005 was, if you remember, a very unusual year. There were 28 named storms in the N. Atlantic in 2005. That beat the previous record of 21 names storms that had been set in 1933. Of the 28 named storms, 15 developed into hurricanes.

In some ways winds blowing through an easterly wave resembles traffic on a multi-lane highway. Traffic will back up as it approaches a section of the highway with a closed lane. Once through the "bottleneck" traffic will begin to flow more freely.

Another possibility is a lee side low.

Winds blowing over mountains on the west coast of Mexico will sometimes form a surface low on the downwind side of the mountains. Surface winds will spiral inward toward the center of the low. Note there are generally a few more tropical storms and hurricanes in the E. Pacific than in the N. Atlantic. They generally move away from the US coast, though the Hawaiian Islands are sometimes affected.

1. The converging winds pick up heat and moisture from the ocean. These are the two mains sources of energy for the hurricane.

2.   Rising air cools and thunderstorm clouds form. The release of latent heat during condensation warms the atmosphere. The core of a hurricane is warm.

3.   Pressure decreases more slowly with increasing altitude in the warm core of the hurricane. The result is that pressure at the top center of the hurricane is higher than the pressure further out from the hurricane (pressure at the top center is still lower than the pressure at the bottom center of the hurricane). Upper levels winds diverge and spiral outward from the top center of the hurricane.

4.   The upper level divergence causes the surface pressure to decrease. The speed of the converging surface winds increases and the storm intensifies. The converging winds pick up additional heat and moisture which warms the core of the hurricane even more. The upper level high pressure and the upper level divergence increase. The increased divergence lowers the surface pressure even more.

The increase in intensity is shown in the following figure (not shown in class)

The lower the surface pressure, the stronger the storm.

When meteorologist spot a cluster of thunderstorms, a tropical disturbance, on a satellite photograph they begin to keep an eye on it, as this might eventually develop into a tropical storm or a hurricane. If the storm develops, becomes more organized, and begins to rotate it is upgraded to a tropical depression. In order to be called a tropical storm, winds must increase to 35 knots. The storm receives a name at this point. Finally when winds exceed 75 MPH (easier to remember than 65 knots or 74 MPH) the storm becomes a hurricane.