Have you ever blown up a balloon and let go of it without sealing it with a knot? What happens? That's right: it zips around in the air until the air is gone, and it is finally deflated. That occurs because there is a pressure difference between the air inside the balloon and the surrounding air. Even though we cannot always see or feel the air, it actually has a mass. It also has other physical properties, such as density and pressure. What does that mean for our atmosphere? Large bodies of air exist in our atmosphere where all the air within that region has a similar temperature, pressure and humidity. We call those bodies of air with similar characteristics air masses. Air masses change and shift all the time, but on any given day, there are many different air masses in our atmosphere, all with their own unique air pressure. Thinking back to the balloon example, we know that air pressure differences can cause air to move — similar to the balloon zipping around the room. The movement of those air masses in our atmosphere contributes to the weather we experience every day.

We know that the sun plays a significant role in the weather we experience on Earth. But how does the sun relate to air masses? The sun heats up our atmosphere and the Earth's surface, but the heating takes place unevenly because the sun's rays hit different areas of the Earth at different angles. Variations on the Earth's surface provide even more room for uneven heating. The air above water, for example, is typically cooler than the air above land, and the air above lighter colored surfaces is typically cooler than the air above darker ones (very simply, because dark color absorb more heat). Because the sun heats pockets of air to different temperatures, the sun actually helps create the different air masses in our atmosphere.

High- and low-pressure areas are affected by the interaction of air and temperature (among other things) in our atmosphere. Areas of high air pressure are often characterized by light winds and clear skies. With no clouds to reflect the sunlight, high-pressure regions also usually experience higher daytime temperatures. Low-pressure areas then, are regions where the atmospheric pressure is lower than that of the surrounding area. Low-pressure areas are associated with winds and cloudy, overcast skies. Due to the clouds, the sun's rays have a harder time penetrating low-pressure regions, usually leading to lower temperatures. Falling air pressure usually indicates that a storm of some sort is approaching. On the other hand, rising air pressure is usually an indication that the weather is clearing up.

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Have you ever heard of a barometer? Barometers are the instruments that meteorologists use to gauge changes in air pressure in order to make their forecasts. Engineers design the instrumentation that meteorologists use to help predict the changes in weather, including barometers. Engineers are always trying to improve the different instruments used to make weather measurements and predictions on Earth and in space. Weather satellites, orbiting our Earth, are designed by engineers to take pictures and measurements of moving air masses to help us understand how the weather is changing on our planet. Engineers develop and use new technologies to make weather instrumentation more accurate and helpful.

Changes in weather happen due to many reasons, including moving air masses and air pressure, as well as humidity and temperature. All of these things work together as parts of the system of weather that affects us every day.

Today we want to remember that hot air rises and cold air sinks. The sun heats up our atmosphere and the Earth's surface, but the heating takes place unevenly because the sun's rays hit the Earth's surface at different angles. The many different angles can be influenced by the topography and variations on the Earth's surface itself. The moving air that circulates in our atmosphere is called an air mass and can involve high or low air pressure.

Also, areas of high air pressure are often characterized by light winds and clear skies. Low-pressure areas are associated with winds and cloudy, rainy skies. Usually, falling air pressure usually indicates that a storm of some sort is approaching; rising air pressure is usually an indication that the weather is going to be fair.

Engineers design the instruments — including barometers — to measure air pressure, which meteorologists use to help analyze the Earth's weather systems. Engineers are always trying to improve these instruments to make them more accurate, more efficient, or to utilize new technologies.

Air Pressure and Flight: Have students investigate and write about how engineers have used the principles of air pressure to make things fly. For example, engineers design hot air balloons using their understanding of air pressure and knowing that hot air rises, while cold air sinks. Hot air balloons function using a flame to heat the air trapped in the balloon. As the balloon air gets hotter, its density decreases, causing the balloon to float on top of colder air, rising and lifting it up and away.

Acrostic Poem: Encourage students to synthesize and evaluate their learning by having them write an acrostic poem. To do this, ask them to write the word WEATHER vertically on a piece of paper. Then, have them use each letter in the word as the first letter of a word or phrase related to weather. For example, W = Wind, E = Elevation, A = Atmosphere, T = Tornado, H = Heat, E = Evaporation, R = Rain, etc.