What would our weather be without water? Have you ever thought about what water, in all of its forms, contributes to the weather as we know it? Sure, there are the obvious things like clouds and precipitation, but would we still have "weather" without the water?
Mars is a good place to consider when answering this question. The Martian atmosphere is devoid of water. Does that mean that there is no weather on Mars? Certainly not! In fact, Mars can have some pretty cool weather--if you like dust devils (check out this series of images from the Spirit Rover of one such dust devil). So, water in the atmosphere is not a necessity for creating "weather," but it sure makes our weather more varied and interesting (not to mention allowing life to flourish on our planet).
In Lesson 4, we will begin to examine the varied roles that water plays in the atmosphere. We will begin by looking at the hydrological cycle and by becoming familiar with the various processes that move water through the Earth-Atmosphere system. Next, you will learn the physical principles behind cloud formation and (most importantly) why the idea that "warm air holds more water than cold air" is a fallacy. These physical principles will then be applied to a variety of different cloud formation scenarios (including orographic lifting and contrails). Finally, you will learn about the ways in which meteorologists keep track of water in the atmosphere and the variables that they use to describe the state of the atmosphere with respect to water.
Now it's time to get your "feet wet," so to speak. Before you dive into the text, make sure that you read (and print off) the learning objectives. They are very useful in helping you organize the information that you need to learn, and as you've seen, they tell you what kinds of questions to expect on the quiz.