Tuesday, August 21, 2007

Hurricanes and Latent Heat

Whether we like it or not, it is hurricane season. These giant storms generate strong winds and heavy rains that are capable of extreme destruction.

This is a picture of Hurricane Dean from www.wunderground.com.
hurricane-dean.jpg

Do you know what fuels a hurricane?

Most hurricanes begin as small storms that form in the warm waters of the Atlantic Ocean, as far away as the western coast of Africa. These small storms grow in both size and intensity at an alarming pace when they are exposed to enough heat, moisture, and unstable air.

This track of all the named Atlantic storms in 2006. Storms get named only when it reaches the size of a Tropical Depression. Some storms grow, some lose strength. Take a look!
2006atlantictrackmap.gif

A hurricane cannot form on land because it needs an open body of warm water – like the Atlantic Ocean at the end of summer – for it to grow. Instead, dangerous thunderstorms and smaller spinning storms, like tornadoes, will form on land.

Heat from the sun helps to warm the ocean’s waters to provide energy to establish the best conditions to create a hurricane. The latent heat (see what this is below!) of water helps the feed a hurricane’s strength and intensity once it forms.

What is Latent Heat?

Things in our everyday lives are almost always either solid, liquid, or gas. These are the three phases of matter. Depending on how much the temperature changes, the phase can change. Water is an excellent example of something that we have seen in three phases:

Solid – Ice in the freezer
Liquid – Water from the tap
Gas – Steam above a boiling pot of water

When matter changes phases heat is either given off or absorbed. This is called latent heat.

Check out this illustration from http://www.physicalgeography.net.
latent.gif

Let’s take a closer look at latent heat so we can understand why it is so important.

According to the illustration, heat is absorbed when a solid changes into a liquid. Even more heat is absorbed when a liquid changes into a gas.

Let us imagine that we fill a pot with ice cubes. We know that ice will melt outside of the freezer. The ice is literally absorbing heat from the air! To turn a pot of water into a gas (steam), it has to be heated on a stove and boiled.

melting-ice.jpg

What happens when liquid changes into a solid or a gas changes into liquid?

We know that heat must be absorbed to melt ice or boil water. So if we want to make ice or condense steam, then this means that heat has to be given off or removed.

Making Ice
If you fill a cup with water and place it in the freezer, after a few hours it will turn into ice. How does this happen?

Although a freezer/refrigerator is cold, it is a heat pump: it absorbs heat from the things inside it (making it cold) and pumps the heat outside! Have you ever noticed how hot it gets behind a refrigerator?

This means that heat is given off when changing from a liquid to a solid.

Condensing Water Vapor
When a gas turns in to a liquid, heat is also given off. If you take a glass of ice water outside on a hot and humid day, water vapor from the air will touch the cool surface of your glass and immediately turn into liquid water. The heat given off by this phase change causes the ice in the glass to melt faster than if was sitting in warm air alone.

condensation-glass.jpg

So what does this have to do with hurricanes?

As I mentioned before, hurricanes need heat, moisture, and unstable air to grow and become powerful.

At the end of summer, the water of the Atlantic Ocean reaches its highest temperature. This causes the ocean’s water to evaporate more than usual, putting more water vapor in the air. Eventually, the water vapor will cool and condense to form clouds. This means that heat is given off in the sky.

This occurs over huge area of the ocean, meaning that the sky is being heated more than usual. From the “How does a hot air balloon work” discussion, we know that hot air rises. To a weather scientist, too much hot air is a sign that the sky is unstable.

Did you know that an easy way to tell if the air is unstable is to look for tall puffy clouds in the sky? This is often a sign of bad weather!

When a storm system enters this heated area, it causes the storm to grow both outwards and up into the sky. The hotter the air gets, the stronger the storm becomes. If the heating continues, a small storm can grow into an extremely powerful hurricane many hundreds of miles wide!

To learn more about hurricanes, NASA has a great website on hurricanes with links to interesting videos and information.

NASA’s Hurricanes Main Page

This link has a short 8 minute video with cool animations that is very informative.

This link has an amazing video of Hurricane Katrina. Notice how the hurricane looks small at the beginning of the video before it hits Florida and then grows to a huge size over the warm waters of the Gulf of Mexico. I am amazed and scared by the power of nature!

Dr. Dave

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