Hi there, Bea here! I'm just getting ready to head out for the day, and I'm trying to figure out if I really need to bring my umbrella along or not. You're probably thinking, "Bea, just check the weather forecast, right?" Except the weather forecast today looks like a modern art masterpiece – triangles, circles, lines, and squiggles everywhere. Does a triangle mean it's going to be sunny, or does it just mean we're going to have some really weirdly shaped rain today? And what's with the "H" and "L"? Maybe it's "H" for hot and "L" for less hot? I'm officially lost. Well, it looks like we're going to need to take a closer look at the patterns in Earth's atmosphere to understand this before I grab my umbrella. By the end of today's lesson, you'll be able to identify types of air masses, fronts, and pressure systems and explain how they impact the weather we experience. Let's get started! We already know that weather is influenced by factors like humidity and temperature and that global patterns of circulation and heat transfer cause huge amounts of air to move around the planet. Now, let's put this all together to talk about how these changes really happen – through air masses. An air mass is a large body of air with the same temperature and humidity throughout. They can be thousands of miles across and several miles deep, and they are moved around Earth's surface by global wind patterns. Air masses also tend to have consistent air pressure, which is the force air exerts on Earth's surface as it is pulled down by gravity. Air pressure is also directly connected to temperature. As we know, warm air rises and spreads out. This results in lower air pressure, since the air is less dense and exerts less pressure on the ground. And since cold air sinks, it brings higher air pressure, since the air is more dense and exerts more pressure. Air masses matter because they play a key role in determining the weather patterns we experience on Earth's surface. When an air mass moves into a new region, it brings its characteristic temperature, humidity, and air pressure with it, which influences the local weather. There are four kinds of air masses, and they are named for the area where they formed. Arctic air masses are very cold and form near the Arctic and Antarctic areas of the planet. Polar air masses are a little less cold, as they come from areas a bit past the frigid polar regions. Tropical air masses come from the tropical areas past that, and they are made of somewhat warm air. And equatorial air masses form around the equator, and they carry very warm air. We can also specify whether an air mass is maritime, meaning it formed over water, or continental, meaning it formed over land. However, there's one exception here: Equatorial air masses can just about only form over water, since there is so little land around, so we don't really need to specify for those ones. But for our other air masses, how do you think forming over land or water would change the characteristics of an air mass and the weather it brings? Pause the video here and record your thoughts in your guided notes. Maritime air masses tend to be more humid, while continental air masses tend to be more dry. So, a maritime Arctic air mass would be cold and humid and may bring freezing temperatures and snow. On the other hand, a continental, tropical air mass would be warm and dry and may bring clear skies and warm weather. Knowing where an air mass originated can help us predict the kinds of weather it will cause. Now, we know that air masses have a consistent temperature, air pressure, and humidity throughout. We also know that air masses are always moving around the Earth. So, what happens when two air masses meet? The boundary between air masses is called a front. Since a front typically means the air mass currently over an area is leaving and a new one is moving in, fronts often mean a change in weather is coming. There are a few different kinds of fronts that can occur based on how the meeting air masses are moving, and each brings different weather. A warm front occurs when a warm air mass moves into an area where a cooler air mass has been. As the warm air mass rises over the cooler air, it often cools and condenses into stratus clouds, leading to long periods of light rain. Then, the weather after a warm front passes tends to be warmer and more humid. A cold front occurs when a cold air mass moves into an area where a warmer air mass has been. The cold air mass sinks under the warm air, causing the warmer air to rise rapidly and cool, which can lead to the formation of towering cumulonimbus clouds and thunderstorms. Cold fronts often bring short but intense storms, and the weather after a cold front passes tends to be cooler and less humid. A stationary front occurs when two different air masses meet, but neither is strong enough to move into the other space, so it doesn't create a warm or a cold front. The boundary between the two air masses stays still. The weather along these kinds of fronts can vary, but they often create stratus clouds and bring light rain that lasts a while. The last kind of front is an occluded front. An occluded front occurs between the place where a warm front and a cold front meet. Basically, they happen when a warm air mass is moving between two cold air masses. Occluded fronts can cause pretty intense weather, like thunderstorms and even tornadoes, and they tend to bring less humid weather after they pass. You'll often see fronts color-coded on weather forecasts, and they'll have arrows or semi-circles that point in the direction the front is moving. This forecast is making more sense already, but what about those H's and L's? Remember, air masses each have their own consistent air pressure. Since fronts are the boundaries between air masses, they are often the site of changing air pressure too. We call these patterns pressure systems, and they are also an important part of understanding the weather. A high-pressure system is an area with higher air pressure than the air around it. High-pressure systems often occur when a lot of air is being pushed down towards the ground, then flows outward, which basically creates a stable zone of clear weather. High-pressure systems can be associated with warm fronts, but they can also happen in the center of an air mass. On the other hand, a low-pressure system is an area with lower air pressure than the air around it. These tend to happen in places where a large amount of air is rising, cooling down, and condensing into clouds. This results in less air pushing down on the ground, and it can also create a lot of movement in the atmosphere. Low-pressure systems are associated with stormy, changing weather and most commonly happen along fronts. That must be what those H's and L's meant on the weather forecast! I guess I'll be bringing my umbrella along today after all. So, which causes which? Do fronts cause pressure systems, or do pressure systems cause fronts? And what about the larger air masses they are a part of? Well, like most things in Earth science, this is a complex, interconnected process that takes place on a global scale. A change in any of these factors can impact all the others. For example, fronts can cause changes in air pressure that lead to the formation of pressure systems. But pressure systems can also intensify fronts and lead to changes in the weather. It's a lot like how moving air masses can shape the weather patterns, and in turn, the weather patterns influence how the air moves in the first place. Our atmosphere is a truly complex, interconnected system. That's why meteorologists pay close attention to air masses, pressure systems, and fronts. They all play a key role in determining the weather we experience. By tracking these patterns, meteorologists can predict changes in temperature, humidity levels, and upcoming storms. Then, they can share forecasts and warnings to help people prepare for changes in the weather. So, let's recap what we learned today. You now know that an air mass is a large body of air with the same temperature and humidity throughout; that fronts are the boundaries between air masses; that pressure systems are areas with different air pressure than the surrounding air; and that different kinds of air masses, fronts, and pressure systems are each associated with different changes in the weather. Be sure to complete the practice questions and extension activities that go with this lesson to learn more about the processes that influence our weather. And always remember, in Earth science as in life, you rock! See you next time.