What comes to mind when you hear about a change in the weather? What comes to mind when you hear about a change in the climate? If you think about these questions, you’ll realize that although the two words are closely related, there is an important difference in their meanings:

• Weather refers to the ever-varying combination of such things as winds, clouds, temperature, and pressure that makes some days hotter or cooler, clearer or cloudier, or calmer or stormier than others.
• Climate refers to the average of weather over many years. For example, we say that a desert has a warm, dry climate, even though it may sometimes rain or snow.

Figure 6.3.1–1 shows a simple example of this distinction. The weather is cloudy and rainy on the day the photo was taken, but the landscape shows that you are looking at a desert. In other words, the weather in this location will differ from day to day, but on average the region is dry enough to be a desert.

In this section, we’ll briefly discuss the different types of weather and climate that you might find around the world. We’ll then study climate and weather in more detail in the sections that follow.

By the way, while we won’t use the term much in this book, the scientific field that deals with weather and climate is often called meteorology , which is why weather forecasters are often known as meteorologists.

Describing Weather

Think about the kinds of words you might use to describe today’s weather , or what you might hear if you listen to a television weather report. Some of the words might be subjective, like “pleasant” or “terrible.” But a full weather report usually includes words to describe each of the following:

• temperature, which we usually measure in degrees Celsius or Fahrenheit
• wind, which we can describe by its speed and direction
• atmospheric pressure , so that you know whether your local conditions represent high or low pressure compared to their average (review the discussion in Section 6.2.1) and whether the local pressure is rising or falling
• cloudiness, such as how much of the sky is cloud-covered and the types of clouds we see
• humidity , which we commonly use to describe how wet or dry the air feels
• precipitation , by which we mean any water (such as rain or snow) that is falling from the sky

Cloud Types

However, not all clouds lead to precipitation. For example, thin, high clouds are common on sunny days. In fact, generally speaking, only thick, dark clouds are associated with storms.

Figure 6.3.1–2 shows several common types of clouds, along with their names (you do not need to memorize these names), and Slide Show 6.3.1–2 shows photos of some of these clouds. Notice that different types of clouds are found at different altitudes, and some span a much wider range of altitudes than others.

Humidity

The basic concept of humidity is very simple. As we discussed earlier in the chapter, humidity describes the amount of water vapor that is in the air in a particular place at a particular time. To understand the concept of humidity more deeply, we have to think about why water vapor is in the atmosphere. The answer is essentially as follows :

• This means that for liquid water at any temperature, there will always be at least a few water molecules near the surface that will have enough energy to break free and become gas (water vapor) molecules in the atmosphere. The warmer the temperature, the more molecules that will have enough energy to break free.
  This is the process we call evaporation , which essentially means “turning into a vapor” (and vapor is another word for gas) .
• At the same time, some water vapor molecules in the air will have low enough energy to return back to the water surface.
  This is the process of condensation , in which gas molecules return to the liquid state.
• The maximum number of water vapor molecules that can be in the air at any one time depends on the temperature: When it is warmer, the air can hold more water vapor.

Figure 6.3.1–3 illustrates these ideas. Note that even though warmer air can hold more water vapor, whether or not it does depends on how much water is available.

One way to quantify humidity is by stating exactly how much water vapor is in the air . However, in the context of weather, it is much more common to talk about the relative humidity , which tells us the percentage of maximum possible amount of water vapor that is in the air at a particular temperature. You can understand this concept as follows:

• As discussed above, the maximum possible amount of water vapor depends on temperature: Warmer air can hold more water vapor.
• If the air actually holds this maximum amount, we say that the air is saturated with water vapor and that the relative humidity is 100%.
  If you try to add more water vapor to saturated air, it won’t stay as vapor. Instead, it will condense into droplets of liquid water (or, if the temperature is low enough, into crystals of ice).
• In most cases, the air will contain less than the maximum possible amount of water vapor, and relative humidity tells us how close it is to the maximum. For example, a relative humidity of 25% means the air contains only 25% of the maximum amount of water vapor that it could hold.

The fact that relative humidity depends on temperature leads directly to the processes that form clouds and dew . Use the following questions to explore these ideas.

Discuss the following questions with a classmate. Then click to open the answers to see if they agree with what you came up with.

1. Suppose that your local air temperature is 20°C (68°F) and the relative humidity is 75%, and the air then suddenly became warmer (without any change in the total amount of water vapor in the air). Would the relative humidity go up, go down, or stay the same? Explain why.

2. Now, suppose instead that the air suddenly became cooler. What would happen to the relative humidity in that case?

4. Your answer to question 3 implies that at some altitude, the air is likely to reach a relative humidity of 100%. What will happen as the air rises beyond that altitude?

5. Why does dew tend to form when it is cool in the morning rather than when it is warmer during the day?

There’s one more idea we need to discuss about humidity, which is the way it makes us feel. In particular, you are probably aware that a hot, humid day tends to be much more uncomfortable than a hot, dry day. Interestingly, the reason is not because of the water vapor itself, which we can’t really feel. Instead, the reason has to do with what we call evaporative cooling , in which the fact that evaporating molecules are carrying away energy leads to cooling of the surface where they were previously located . This is how sweating works: Our bodies sweat when we are hot, because the evaporating water (sweat) cools us down. The next two questions explore how higher humidity therefore makes us less comfortable.

Continue from the previous question set, discussing these with a classmate. Then click to open the answers to see if they agree with what you came up with.

6. How would you expect the relative humidity to affect the rate at which your sweat can evaporate on a hot day? That is, would higher humidity make your sweat evaporate faster or slower, and why?

7. How does the change you found in question 6 explain why a hot, humid day is less comfortable than a drier day of the same temperature?
Hint: Think about the above discussion of how sweating cools your body through evaporative cooling .

Types of Precipitation

Remember again that clouds are made of droplets or ice crystals. These droplets or crystals are held aloft by air pressure and winds, but they can fall to the ground if they grow large enough. As we’ve discussed, any form of water that falls toward the ground is called precipitation . The most common forms of precipitation are rain and snow, but they are not the only forms. Precipitation can also fall in the forms that are usually called sleet and hail, and you may also be familiar with freezing rain. Let’s briefly discuss the circumstances under which we may experience each of these forms of precipitation.

We’ll start with the most familiar: rain and snow. Figure 6.3.1–4 summarizes how rain and snow are formed:

• Snow forms when water vapor condenses to make ice crystals in clouds. If these crystals fall all the way to the ground, then it is snowing.
• Rain can form in either of two ways: (1) Rain can form when water vapor condenses to make liquid droplets in clouds. (2) Rain may start out as snow, but if the snow melts into liquid before it reaches the ground, then we will see it raining rather than snowing.

Sleet and freezing rain are variations on these themes that occur in winter storms (Figure 6.3.1–5a). The difference between them and ordinary snow is that they fall through a layer of relatively warm air that lies between the ground and the clouds.

• Sleet is essentially small “ice pellets” that fall to the ground. Sleet usually starts out as snowflakes in clouds. But as it falls through a warm air layer, the snow melts into raindrops, then refreezes to make the “ice pellets” before hitting the ground.
• Freezing rain forms similarly to sleet in that it starts out as snow and melts into raindrops as it falls through a warm air layer. In this case, however, the raindrops remain as rain until they hit the cold ground (or the cold surfaces of other objects), at which point they freeze on contact. Freezing rain tends to be especially dangerous, because it coats the ground and other objects with a layer of slippery ice (Figure 6.3.1–5b).

Hail may look similar to the ice pebbles of sleet (Figure 6.3.1–6), but it is made quite differently. Hail forms only in thunderstorms, which generally occur in warm weather. Note that while most hailstones are fairly small (like the ice pebbles of sleet), some extreme thunderstorms can produce much larger ones.

Figure 6.3.1–6 – Hail may look similar to the ice pebbles of sleet, but it forms very differently, being made in thunderstorms that generally occur in warm weather. These photos show that hailstones come in a wide range of sizes.

To understand how hail forms, we need to understand the thunderstorms that make them. Thunderstorms form when warm surface temperatures and cool upper atmosphere temperatures promote strong convection , so that the warm air rises rapidly upward. Because the air at high altitude is very cold, the water vapor in the rising air condenses to make clouds of ice crystals. The strong convection can make thunderclouds quite tall and thick, with tops that sometimes reach the stratosphere , where they flatten out to make an anvil shape (Figure 6.3.1–7).

Figure 6.3.1–7 – The strong convection in thunderstorms often allows their cloudtops to extend all the way to the stratosphere ,
where the layered air makes their tops flatten out. This flattening gives thunderclouds their characteristic anvil shape, seen here
in a view from an airplane. Credit: Hussein Kefel.

Thunderstorms can produce either rain or hail as follows:

• If the ice crystals in the clouds fall while still fairly small, they will melt on their way down, making rain.
• However, the strong convection that makes thunderstorms also gives them strong, upward winds that can keep the ice crystals aloft. If they stay aloft long enough, the crystals can keep growing larger, becoming hailstones that are large enough to stay solid when they finally fall to the ground (Figure 6.3.1–8).

Extreme Weather

The weather is always changing, but most of the time we are able to live with it fairly easily. On occasion, though, the weather can become severe enough to count as what we often call extreme weather , meaning weather that can cause substantial damage or loss of life.

We’ve already seen examples of some extreme weather events, and you are probably familiar with many others. Study slide Show 6.3.1–9 to review some of the many different types of extreme weather that can occur on Earth.

Slide Show 6.3.1–9 – These slides show a few of the many types of extreme weather events.

The fact that extreme weather events can cause so much damage explains why scientists spend a great deal of time studying them. For most types of extreme weather, scientists now have a fairly detailed understanding of how and why the events occur. Prediction of these events is still difficult, however, a topic we’ll discuss further in Section 6.3.3. For now, it’s worth noting just a couple of important issues about extreme weather events.

First, while some events like thunderstorms or tornados may seem fairly small and local, they usually occur over a large region. In other words, if you are experiencing thunderstorms, it is likely that similar storms are occurring over a fairly large region around you.

Second, it’s important to realize that the same types of storms sometimes have different names. One common example is with the tropical storms that form over low pressure regions in tropical oceans (that is, near the equator):

• A tropical storm first forms as what is called a “tropical depression” (the word “depression” is used because it is over a low-pressure area).
• It officially becomes a tropical storm when it has sustained wind speeds above 62 kilometers per hour (39 miles per hour).
• If sustained winds reach 119 kilometers per hour (74 miles per hour), then we call it a hurricane if it is in the North Atlantic or Northeast Pacific Ocean, a typhoon if it is in the Northwest Pacific, and a cyclone if it is in the South Pacific or Indian Ocean. In other words, hurricanes, typhoons, and cyclones are all the same type of storm.

Describing Climate

Remember that climate refers to the average of weather over many years. Generally speaking, we can look at climate in two ways:

• When we speak of the global climate, we are talking about a general average for the whole world.
• When we speak of local or regional climate, we are talking about how the climate in that particular place compares to that of other places around the world.

In the rest of this chapter, we will focus on local and regional climates. (We’ll focus on global climate and how it is changing in Chapter 7.) Because this means comparing a particular place’s climate to the rest of the world, a description of climate generally focuses on the three major factors:

• Temperature: How do average temperatures in the region compare to averages around the world?
• Humidity and precipitation: Is the region generally wet or dry compared to averages around the world?
• Seasonal variation: How does the regional climate vary with the seasons?

Slide Show 6.3.1–10 gives a few examples of these ideas. Use the slide show and the discussion that follows to help you start thinking about ways in which you might describe climate. Then, in the next section, we’ll discuss climate patterns in a little more detail.

Slide Show 6.3.1–10 – These slides show three major factors that we use to characterize local or regional climates.