Earth Speak

Tide
Everyone who's visited an ocean has probably heard the terms "high tide" and "low tide." But what does that really mean? Tides are formed by the moon's gravitational pull on the Earth. When it's high tide, the ocean rises. And during low tide, the ocean recedes. If you're on the coast and the moon is directly overhead, you should experience a high tide. Learn more about tides and the moon's pull over the ocean in this article from HowStuffWorks.com.

Tornado
A tornado is one of those amazing, awesome acts of nature that simply leave you dumbfounded -- a huge, swirling, 200-mph beast of a storm that appears to have a mind of its own. You have to actually see one with your own eyes to believe it. In certain places, tornadoes appear with amazing regularity. That's why we see them in the news all the time. If you've ever seen a whirlpool form in your bathtub, sink or toilet when the water is draining, you have seen the basics of a tornado at work. Learn some more fundamentals of these swirling beasts in this HowStuffWorks article.

Tropical depression
Everyone has weathered a bad thunderstorm before. But when does that thunderstorm move into a more dangerous category? The next step up on the danger meter is tropical depression. Tropical depressions happen when wind storms are 38 mph or below, and they occur only in tropical climates. But when a storm's winds hit more than 38 mph, the storm becomes something else entirely. And it could eventually make its way to hurricane status. Read what it takes to become a hurricane, and see some of the damage hurricanes have caused in this article from HowStuffWorks.com.

Tropical Storm
Everyone has weathered a bad thunderstorm before. But when does that thunderstorm move into a more dangerous category? The next step up on the danger meter is tropical depression, and then tropical storm. Tropical storms occur when winds are 39 to 73 mph, and they happen only in tropical climates. Above 73 mph, the storm becomes something else entirely. And it could eventually makes its way to hurricane status. See the devastation from the 10 most destructive hurricanes in this HowStuffWorks article.

Troposphere
The troposphere is the layer of the Earth's atmosphere closest to the ground -- the air we breathe. Within that layer there are good particulates (oxygen) and bad particulates (ozone). We want the ozone layer, but we don't want ozone pollution. Good in the atmosphere but bad on the ground? Find out about ozone pollution, how it affects you and what you can do about it. Read more about the troposphere and how it plays into ozone pollution at HowStuffWorks.com.

Urban Heat Island
The urban heat island effect is a condition in which city and suburban developments absorb and trap heat. Anyone who's walked across a scalding parking lot on a hot summer day has felt one effect of an urban heat island. But there may be a reprieve for city dwellers -- green rooftops. Long popular in Europe, green rooftops have begun to appeal to homeowners, businesses and even cities as an attractive way to promote environmentalism while solving the problems of conventional roofs. Green roofs supplement traditional vegetation without disrupting urban infrastructure -- they take a neglected space and make it useful. Read more about these urban gardens and what exactly they do in this article from HowStuffWorks.

Volcanoes
People today share many of the feelings that volcano observers have had throughout human history: We're in awe of the destructive power of nature -- and unsettled by the thought that a peaceful mountain can suddenly become an unstoppable destructive force. In its simplest definition, a volcano is any place on the planet where some material from the inside of the planet makes its way through to the surface. This material, or magma, moves through the Earth’s crust -- unless its upward pressure is exceeded by the downward pressure of the surrounding solid rock. At this point, the magma collects in magma chambers below the surface of the Earth. If the magma pressure rises to a high enough level, or a crack opens up in the crust, the molten rock will spew out at the earth's surface. If this happens, the flowing magma (now called lava) forms a volcano -- a familiar sight to most of us. Learn more about volcano formation, volcano sizes and shapes, and the different types of eruptions in this article from HowStuffWorks.

Water Cycle
The water cycle is the continuous movement of water in and around the Earth. Water never really goes away -- it just changes form. The sun drives the entire water cycle and is responsible for its two major components: condensation and evaporation. When the sun heats the surface of water, the water evaporates and ends up in the atmosphere as water vapor. It cools and rises, becoming clouds, which eventually condense into water droplets. Depending on the temperature of the atmosphere and other conditions, the water precipitates as rain, sleet, hail or snow. Read more about the water cycle in this article from HowStuffWorks.

Wildfires
An average of 5 million acres burns every year in the United States, causing millions of dollars in damage. Once a fire begins, it can spread at a rate of up to 14.29 mph and destroy everything in its path. On a hot summer day, when drought conditions peak, something as small as a spark from a train's wheel striking the track can ignite a wildfire. Sometimes, fires occur naturally, ignited by heat from the sun or a lightning strike. However, the majority of wildfires are the result of human carelessness. Among the most common causes for wildfires are: arson, campfires, improperly discarded cigarettes, burning debris, and irresponsible use of matches or fireworks. There are three components needed for ignition and combustion to occur. A fire requires fuel to burn, air to supply oxygen, and a heat source to bring the fuel up to ignition temperature. As a fire begins to burn, there are several factors that determine how it may spread, including fuel, weather and topography. Find out more about how wildfires work and the variables that contribute to their damage in this article from HowStuffWorks.com.

Wind Power
Wind power starts with the sun. When the sun heats up a certain area of land, the air around that landmass absorbs some of that heat. At a certain temperature, that hotter air begins to rise very quickly because a given volume of hot air is lighter than an equal volume of cooler air. Faster-moving (hotter) air particles exert more pressure than slower-moving particles, so it takes fewer of them to maintain the normal air pressure at a given elevation. When that lighter hot air suddenly rises, cooler air flows in quickly to fill the gap the hot air leaves behind. That air rushing in to fill the gap is wind. Find out how a wind power works, and how wind turbines capture the kinetic energy in wind to generate electricity, in this article from HowStuffWorks.