You’ve probably noticed that adults often greet each other with talk about weather, saying things like “don’t you just love this weather?” or “that was sure a crazy storm.” This should actually seem reasonable, because the ever-changing weather is one of the most important daily factors in how we go about our lives.

The importance of weather to our daily lives also explains why scientists spend so much effort trying to predict the weather. After all, reliable predictions can save property and lives by allowing us to prepare for coming storms. We’ve already discussed most of the concepts we need to think about weather prediction. Here, we’ll put those together so you can understand what you hear and see in weather forecasts. Video 6.3.3–1 (above) summarizes a few key ideas, some of which we have already discussed and some that we will discuss in this section.

Measuring the Weather

Recall that six important characteristics that we use to describe weather include temperature, wind, atmospheric pressure, cloudiness, humidity, and precipitation. We usually describe cloudiness in a qualitative way, such as by describing how much of the sky is cloud-covered and what types of clouds we see. All of the others can be measured quantitatively with devices used in weather stations (Figure 6.3.3–2). Use the following questions to review key ideas about these measurements.

Figure 6.3.3–2 – Devices for measuring local weather.

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. What do we call a device that measures the temperature, and what units to we commonly use to report the temperature?
   
   Answer

   A thermometer is used to measure temperature, which we usually state in degrees on either the Fahrenheit or Celsius scales. Remember that these scales are defined so that the freezing temperature of water is 0°C (32°F) and the boiling temperature is 100°C (212°F).

2. What do we call a device that measures atmospheric pressure? When we say that today’s weather has a high or low pressure, what is that being compared to?
   
   Answer

   A barometer is used to measure atmospheric pressure. When we talk about the pressure being high or low, we mean in comparison to the average atmospheric pressure for that location.

3. What two things do we have to measure in order to describe wind?
   
   Answer

   We must measure the wind speed and direction. (Fyi, the common device used for measuring wind speed and direction is called an anemometer . In Figure 4.26a, the anemometer is the part hanging out to the left.)

4. What is the common way of describing humidity in a weather report, and what does it mean?
   
   Answer

   The most common way of describing the humidity is with what we call the relative humidity , which tells us the percentage of the maximum possible amount of water vapor that is in the air at a particular temperature.

5. Precipitation that falls as rain is generally measured with a “rain gauge” (see Figure 6.3.3–2b), which simply measures how deep the rain fills the gauge. How would you measure precipitation that falls as snow? Can you think of a “fair” way of comparing the amounts of precipitation that come in different forms, such as rain or snow?
   
   Answer

   For snowfall, we usually measure the depth of the snow that collects. To compare rain and snow (or sleet or hail), you can let the snow collect in your rain gauge, then melt it so that you see how much actual water it represents.

Recall from Section 4.1.4 that these types of measurements, made directly on the ground, represent what we call ground truth data . In addition, we get data through remote sensing from satellites.

As you learned in this box, satellites can have different general types of orbits. Most weather data comes from satellites that are in one of these two types of orbit (Figure 6.3.3–3):

• A polar orbit, meaning that the satellite orbits around Earth, passing over the poles with each orbit. Weather satellites in these orbits typically orbit at an altitude of about 850 kilometers (530 miles). Because Earth rotates beneath them as they orbit, any single satellite will pass over a different “slice” of Earth with each orbit. Over the course of a full day, the satellite will typically pass twice over every region of Earth. This means that a single satellite can only get a weather snapshot of a region about twice a day. However, with multiple satellites, we can get satellite data for each location more often than that.
• A geostationary orbit, meaning that the satellite orbits at an altitude of about 36,000 kilometers (22,000 miles), where its orbital period matches Earth’s rotation period. This means the satellite will always view the same side of Earth. With multiple satellites, we can get images of the entire Earth.

Today, many satellites are collecting weather data from around the world at any one moment. These satellite observations give us a broad-based picture of global weather that has tremendously improved weather forecasting in recent decades.

Figure 6.3.3–3 – Satellites used to study weather are usually in either polar or geostationary orbits.Credit: Zofostro Science.

Air Masses and Fronts

If you walk to school, you’ll almost always find that the weather at school is essentially the same as the weather at your home. If you get in a car and drive to a nearby town, the weather will probably also be very similar. But if you get on an airplane and fly a long distance, you may land in a place where the weather is completely different from yours. This tells us that, at any one moment, the weather tends to be similar over particular regions, but different from the weather in other regions. Of course, the weather also moves, so the weather that is in one region right now may move into a neighboring region over the coming days.

The places where different air masses meet are called weather fronts . There are four main types of weather front, called cold, warm, stationary, and occluded. Let’s briefly look at each.

Cold Fronts

A cold front is place where a mass of cooler, drier air is pushing into a mass of warmer, moister air (Figure 6.3.3–4). The cooler air is denser than the warmer air, so it forces the warmer air upward. This, in turn, means that moisture in the warm air will condense to form clouds as it rises upward. Weather maps show cold fronts with a line of blue triangles. Temperatures are lower behind the front (that is, in the cooler air mass) than ahead of it (where there is still a warmer air mass).
Important note: Despite its name, the “cold” air moving with a cold front is not necessarily very cold; it is simply cooler than the air it is replacing as it advances. For example, in Figure 6.3.3–4b, notice that the “cold” temperatures are still well above freezing. In the summer time, the air behind a cold front may actually be quite warm, as long as it is pushing into an even warmer air mass.

Figure 6.3.3–4 – The process of and symbols for a cold front , where a mass of cooler, drier air pushes into a mass of warmer, moister air. Notice that the front is moving to the right in these illustrations. Credit: Adapted from Windows to the Universe, Lisa Gardiner.

The clouds that form along a cold front are often thick and therefore very likely to produce storms with heavy rain, hail, or snow. As a result, cold fronts are often clearly visible to our eyes both from the ground and in satellite photos (Figure 6.3.3–5). This also explains why cold fronts are often associated with severe weather such as thunderstorms and tornados.

Figure 6.3.3–5 – Cold fronts are typically marked by the advance of stormy weather that can be seen both from the ground and from space.

Warm Fronts

A warm front occurs where a mass of warmer air pushes into a mass of cooler air (Figure 6.3.3–6). The warmer air is less dense than the cooler air it is pushing into, so the warm air tends to move both forward and upward, leading to cloud formation. You’ll therefore often get rain as a warm front passes through, but once it pushes ahead, your weather will warm up with the warmer air that comes behind it.
Important note: Again, the air in a warm front is not necessarily very warm; it is simply warmer than the air it is replacing as it advances.

Figure 6.3.3–6 – The process of and symbols for a warm front , where a mass of cooler, drier air pushes into a mass of warmer, moister air. Notice that the front is moving to the left in these illustrations. Credit: Adapted from Windows to the Universe, Lisa Gardiner.

Keep in mind that weather is complex, and warm fronts and cold fronts can sometimes be close together or even overlapping. Figure 6.3.3–7 shows a remarkable satellite image and video in which we see both a warm front and a cold front, both advancing on a high pressure air mass.

Occluded Fronts
Cold fronts generally move faster than warm fronts. As a result, a cold front can sometimes catch up to a warm front that was moving ahead of it. The result is called an occluded front (Figure 6.3.3–8). As with other fronts, you’ll often get rain or other precipitation as the front passes by you, with clearer skies after it passes. Video 6.3.3–9 summarizes some of the ideas we’ve discussed about cold, warm, and occluded fronts.

Figure 6.3.3–8. This diagram shows a cold front that is overtaking a warm front, creating an
occluded front in the place where it has already overtaken it. Notice that the symbol for an
occluded front is the purple line of alternating triangles and semicircles, which all point
in the direction that the front is moving. Credit: Windows to the Universe, Lisa Gardiner.

Stationary Fronts

A fourth type of front occurs when two air masses of different temperature are advancing from opposite directions, so that they collide with one another. If neither is strong enough to push the other out of the way, the result can be a stationary front , which is essentially a place where a front has stopped moving (Figure 6.3.3–10). Winds often blow in opposite directions along the two sides of a stationary front, helping it to stay in place until the winds shift. Stationary fronts can sometimes stay in place for days, and because fronts are often marked by storms, these fronts can produce long-lasting rain and flooding.

Figure 6.3.3–10 – A stationary front forms where two air masses of different temperature collide, and neither can push ahead of the other. This can sometimes lead to many days of severe weather along the front.

Weather Maps

Before you continue, make sure you can find and identify all the above features on the two weather maps. To further help clarify the ideas, watch Video 6.3.3–12.

Figure 6.3.3–11 – Two examples of typical weather maps. Credit: NOAA, National Weather Service.

Some weather maps show additional details. Figure 6.3.3–13 shows three common examples. The map in Figure 6.3.3–13a has the same general features as the maps above, but also adds infrared satellite views that reveal cloud cover and color-coded radar data that reveals regions of precipitation. Figure 6.3.3–13b shows a map with added contours that show pressure changes more precisely, allowing us to get a sense of surface wind speeds. Figure 6.3.3–13c shows a map of winds at high-altitude, allowing us to see patterns of jet streams (review the jet stream discussion in Section 6.2.1). Weather forecasters use a combination of all of these types of weather maps, and many more.

Weather Prediction

For most of human history, the weather came unexpectedly and often suddenly, sometimes leading to tragedy on a massive scale. That began to change as weather stations spread around the world, and people began to learn about the effects of changing pressure and advancing weather fronts . For example, people learned that barometer measurements can often tell us about the general trend we should expect in the weather.

To be sure you understand how changing pressure aids in weather prediction, 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. When you hear a weather forecaster say that “the barometer is rising” or “the barometer is falling,” what does that mean?
   
   Answer

   Remember that a barometer is used to measure air pressure. So a rising barometer means the local air pressure is rising, and a falling barometer means the air pressure is falling.

2. Based on what you have learned about high (H) and low (L) pressure regions on weather maps, what does a “rising barometer” suggest about how your weather will be changing in the coming hours to days?
   
   Answer

   Remember that high (H) pressure is typically associated with clearer skies (see Figure 6.2.1–10). Therefore a rising barometer suggests that a higher pressure air mass is moving into your region, which means a good chance that your skies will be clearing up, bringing nicer weather.
   

   View Figure 6.2.1–10

3. What does a “falling barometer” suggest?
   
   Answer

   A falling barometer suggests that a low (L) pressure air mass is moving into your region, in which case you may get rain or snow as it moves in.

Weather measurements and communication between people at different weather stations made it possible to make educated guesses about how weather might change up to a few days ahead of time. This knowledge was very useful even if imperfect. It also saved lives, by making it much more likely that people would get advance warning of dangerous storms.

The dawn of the space age brought much more improvement in weather forecasting, because weather satellites allow us to actually see weather systems as they move on our planet. Today, scientists can do even better by using computer models to aid in weather prediction. These models combine the available weather data with known physical laws affecting air and water on Earth to make predictions about how the weather will change in coming days. Video 6.3.3–14 gives a one-minute overview of how models are used in weather forecasts, and Video 6.3.3–15 summarizes many of the ideas we’ve discussed in this chapter, using a famous hurricane disaster to show how important weather prediction is to our lives.

• Geography also plays an important role. Generally speaking, it is easier to predict weather in places where the terrain is smooth (or over the oceans) than in places where there are more mountains and valleys, primarily because of the way those geographical features can interact with winds.
• Size scale is also very important. It is easier to predict the general weather over a large region than to predict exactly what will happen in one small area. For example, scientists can make reasonable predictions about the general region in which thunderstorms are likely to occur, but cannot predict exactly where the rain will be heaviest or where a tornado might hit.

We’ll close this chapter by returning to one important idea that we briefly introduced earlier (in the Discussion “Weather and Climate Predictability”): Climate is much more predictable than weather. The reason goes back to the fact that climate is a long-term average of weather, and averages are always easier to predict than individual events. As a simple example of why, consider the following:

• If you choose a single 6-year-old child and a single 7-year old child at random, there’s no way to know in advance which one will be taller.
• However, there’s no doubt that the average height of 7-year-old children is taller than that of 6-year-old children.

This example illustrates the general principle that it is always easier to predict averages than individual events. (You can explore further examples in this Chapter 7 activity.) It is therefore easier to predict local climate than local weather. It is also easier to predict future changes in global climate than in individual local climates, an idea that will be very important as we discuss climate change in Chapter 7.

Back Next