Although we notice the biggest shifts in weather patterns with changing seasons, weather can change rapidly from day to day and even from one hour to the next! The weather we experience each day depends a lot on the pressure and temperature of the air around us as well as many factors in Earth's atmosphere. Make your own barometer to learn more about air pressure and how it affects the weather.
*Note: Choose a room where the temperature doesn’t change much throughout the day, since temperature can affect air pressure!
When the barometric pressure of the air outside increases, so does the pressure of air pushing down on the balloon covering the jar. The balloon is pushed into the jar, causing the needle to point up above the line, letting you know there is high pressure. When the air pressure lowers, there is less pressure pushing down on the balloon so it puffs outward slightly. That’s because the air inside the jar has more room to expand. The swelling balloon causes the needle to point down (indicating lower pressure).
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Storms and Air Pressure
Our daily weather depends on lots of things that happen in the earth's atmosphere. The atmosphere surrounds Earth, going up for more than 300 miles until it blends in with space! It contains water vapor (moisture in the form of a gas, like steam), which can form clouds.
Have you ever heard that hot air rises? That's true! As air heats up, its molecules expand and spread out, making the air less dense than it was before. It floats up through the denser cooler air. As the warm air rises, it starts to cool off and its molecules move closer together, causing it to sink again. The air surrounding the Earth is constantly moving.
The air also has pressure, which is the weight of the atmosphere pressing down. Air pressure can be measured with a tool called a barometer. High pressure develops in areas where air cools and sinks. This usually causes fair weather with cool temperatures and few clouds. Low pressure tends to cause warmer stormy weather. Air masses with different temperatures and amounts of moisture (humidity) are sort of like oil and water – they don’t mix well! Rather than blending together, they push against each other creating clouds that can develop into storms.
Tornadoes, also called twisters, are violent pillars of spiraling air accompanied by powerful wind. They usually happen very quickly and only travel a few miles, but can destroy whatever is in their path. They can reach speeds of 300 miles per hour (mph) and tear roofs from houses and launch cars through the air! The central part of the U.S., from Texas through South Dakota, experiences so many tornadoes each year that it’s earned the nickname “Tornado Alley.”
Large thunderstorms are formed when cool, dry air mixes with warm, moist air. In “Tornado Alley,” cool, dry air is pushed down from the Rocky Mountain range or Canada while warm, moist air comes up from the Gulf of Mexico. Since warm air and cool air have different pressure, they swirl and spin when they meet, becoming unstable. If there’s also a lot of moisture in the air and rapidly swirling winds, a horizontal column of spinning air might form inside the thunderstorm. This swirling air can quickly become a funnel cloud and with the downward force of rain, hail, and wind, the funnel can get tipped or tilted (into a vertical position), sweeping across the ground as a tornado.
A hurricane begins as a thunderstorm over the ocean. When it passes over water with a temperature of at least 80 degrees (F), the storm becomes stronger and joins with other nearby thunderstorms. If the storm’s wind speed reaches 74 mph, it becomes a hurricane and will often move towards land.
Hurricanes, like tornadoes, are also spiraling masses that can cause lots of destruction. In addition to very strong wind, hurricanes bring another problem: lots of water. Huge amounts of rain and huge waves brought in from the ocean during a hurricane cause flooding. The center of a hurricane is called the eye – when they eye is passing over, the weather in that area is unusually calm. A hurricane’s storm clouds move around the eye counterclockwise and bring heavy rain and strong winds.
Earthquakes & Tsunamis
The earth's crust is made of large sections of land called plates. A fault is a break in the crust caused by plates moving next to each other. The plates can move apart, push toward each other, or grind against each other sideways. When two plates hit each other (or pull apart) hard enough, they cause an earthquake. Even though an earthquake occurs at the point where the faults meet, it creates waves that ripple through the layers of the Earth and can be felt from miles away (in fact, sometimes, waves from strong quakes are recorded by special equipment on the other side of the world!).
Earthquakes under the ocean floor can cause a tsunami (say it TSOO-nah-me), which is a powerful ocean wave caused by plates moving on their fault lines, not by wind and weather like other waves. Tsunamis move at speeds of around 600 miles per hour! Although powerful, a tsunami doesn't really become dangerous until it reaches shallow water near the shore. There the wave is forced to slow down, causing the wave's energy to sometimes build up into a wall of water up to 130 feet tall. Imagine an ocean wave as tall as a building rushing toward land at speeds as fast as an airplane!