Lesson 6 - Weather and Climate
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To do:
- Check the schedule for this week's reading & upcoming assignments
- Read the lecture and assigned reading in the text
- Participate in discussions to prepare for the midterm
- Take the Week 6 Quiz
By the end of this lesson you should be able to:
- Describe at least three mechanisms that will cause clouds to form
- Discuss the effects of violent weather such as hurricanes and tornadoes on people
- Apply geographic principles of climate introduced in class to predict climate in Modelland
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Air Masses and Weather
In lecture 3, we discussed how warm air had more energy, and therefore formed areas of low pressure in the atmosphere, while cold air was more dense and formed areas of high pressure in the atmosphere. In lecture 4, we discussed atmospheric and oceanic circulation and global temperatures. In the previous lesson, we talked about the water cycle and water resources on earth. In this unit, we are going to bring together these discussions about the atmosphere/hydrosphere interactions in our exploration of weather and global climate.
First, let's get some very important definitions out of the way. Weather and climate are not the same thing. Weather is the short-term, day to day condition of the atmosphere, while climate is the long term (over decades) average of weather conditions in a region. Climate change can only be documented by noting a rise the average annual global temperature over many years. Another way to think about is that weather is a series of short-term events, while climate is a long-term trend. A person who studies and predicts the weather is called a meteorologist. A person who studies long-term trends in weather and how climate changes over many years is called a climatologist.
Air masses are classified based on their moisture and temperature. Air masses pick up moisture when they are over bodies of water, and they loose that moisture when they are over land. Therefore, wet air masses are designated m for maritime, and dry air masses are designated c for continental. The temperature of an air mass is designated as follows: A for arctic, P for polar (high latitude, but not as high as arctic), T for tropical, E for equatorial and AA for Antarctic.
The following air masses affect the United States. Write down their abbreviations in the parenthesis below:
- Maritime Polar (___) affects the north west and north east coasts all year.
- Continental Polar (___) affects the north central USA in the summer and more severely in the winter.
- Maritime Tropical (___) affects the south west and south east coasts all year.
- Continental Tropical (___) affects the south central US during the summer.
Clouds and violent weather occur when air masses collide. When a cold mass pushes under a warm air mass, the warm air is cooled adibatically (recall the average lapse rate of 6.4 degrees C per kilometer from lecture 3). The moist warm air quickly reaches its dew point and clouds form. If you watch the weather report on the evening news, you will note that the cold fronts are denoted by lines with triangles pointing in the direction that the front is advancing. Warm fronts are denoted by lines with half-circles pointing in the direction that the front is advancing.
Another important atmospheric lifting mechanism is orographic lifting. Orographic lifting occurs when air is forced up to high altitudes as it is pushed up a mountain front. The air cools adibatically, and clouds form, and precipitation often results. The windward side of the mountain, therefore, receives much more precipitation than the leeward side. This is known as the ‘rainshadow effect’. One example of orographic lifting is the Sierra Nevada mountains. Moist air blows off of the Pacific ocean and is forcibly lifted upslope. This results in significant rain and snow on the western side of the Sierras (Tahoe, for example), and very little of the precipitation makes it over to the east side of the Sierras. This results in a dry desert on the eastern slope. If you have ever driven over highway 80 to Reno, or highway 120 past Yosemite, you will have seen this transformation first hand. The western side of the Sierras boast lush forests, while the eastern slope is a dry, sage brush desert.
Violent Weather
Two types of violent weather affect the United States each year: tornados and hurricanes. The U.S. has more tornadoes each year than any other country in the world. There are two conditions necessary for tornadoes to form. First, fast winds in the upper troposphere are slowed at the ground because of friction. This condition, of fast winds in the upper troposphere, and slower winds in the lower troposphere, causes a tube of horizontally rotating air to form. For a tornado to form, something must make that column of rotating air turn vertical. A strong updraft of rising hot air can be powerful enough to twist the horizontally rotating column of air into a vertical column of low pressure air.
Thunderstorms that precede tornadoes usually form in the late afternoon after the sun has warmed the ground. The air heated over the ground is buoyant and it rises and rapidly reaches the dew point causing the formation of massive cumulonimbus clouds. The updraft of the rising air is often enough to turn the horizontally rotating air in the troposphere into a vertical funnel cloud. In the late 1960’s, Dr. T. Theodore Fujita, an authority on tornadoes at the University of Chicago, proposed a scale (called the Fujita scale) for classifying tornadoes according to their rotational wind speed and the damage done by the storm.
Browse this web page for more information: NOAA FAQ about Tornadoes
Tropical cyclones are another awesome expression of nature’s power. Tropical cyclones go by several different names, depending on their location. Hurricane is the term used in the Atlantic and East Pacific, Typoon is the term used in the Western Pacific, and Cyclone is the term used in the Indian Ocean or near Australia. Cyclones are zones of intense low pressure. The Atlantic basin cyclones that you investigated in Lab 3 generally started on the western coast of Africa as zones of low pressure resulting from superheated land. These low pressure zones then moved out over the central Atlantic and gathered the abundant energy stored as latent heat in the warm water. By the time that they reached the eastern coast of the Americas they were fully developed, powerful cyclones. Read the following web sites on Hurricanes carefully:
Note that tornadoes can be a bi-product of hurricanes!
Global Climate Patterns
The interaction of the atmosphere with the hydrosphere and the lithosphere create weather and climate. The study of weather and climate is an entire course unto itself. Therefore, in this section I will be going over the bare essentials of how latitude and location on a continent affects the weather and climate of a place.
You may recall our discussion of how Hadley cells create bands of low pressure (due to the rising air masses) at the equator, and bands of high pressure (due to the falling air masses) at the tropics (23.5º North or South). You may also recall that high pressure is associated with dry, sunny weather, and low pressure is associated with wet, cloudy weather. In Africa, large bands of desert are approximately 23.5º North, and to a lesser extent in North America. You might want to research the bands of equatorial rainforest across Africa and South America.
Climate is classified by both average annual precipitation and average annual temperature ranges. Note the graphs at the bottom of the world climate map.These are called climatograms. They are a type of graph that shows you annual temperature ranges and precipitation totals for an average year (would this be weather or climate?). The x-axis represents the months of the year. The left y-axis represents the temperature in degrees F. The right y-axis represents precipitation in inches (can you find the typo in the atlas?).
Modelland is an imaginary continent on our earth. All of the patterns of ocean circulation and predominate winds apply to it. You might think of Modelland as a picture of the earth past, or a future vision of the earth. The letters denote locations whose climate will be discussed below. Test yourself (good practice for the midterm!) by matching the climate descriptions with the letters below. Answers will appear at the end of this lecture.
- These are deserts because they are located in a zone of high pressure.
- This is a lush tropical forest because it is located in a zone of low pressure.
- This is located in a rainshadow.
- These are cool polar currents.
- These are warm tropical currents.
- This is an example of continenality.
- This is an example of oceanality.
- This is much more cool than its neighbor at the same latitude because __________.
B & C are deserts because it is located in a zone of high pressure.
A is a lush tropical forest because it is located in a zone of low pressure.
F is located in a rainshadow.
D are cool polar currents.
E are warm tropical currents.
J is an example of continenality.
K is an example of oceanality.
I is much more cool than H because it is at a higher altitude.
Below are examples of temperature profiles for H & I (effect of altitude on temperature) and J & K (oceanality/continality effects)
Review learning outcomes.
Please complete the Assignments and Exams section for each lesson before proceeding to the next lesson.
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