Erica M. Mentone
Before students can really understand storms, their schema has to include some knowledge of how the earth's hydrologic cycle works. The hydrologic cycle includes not only the evaporation and condensation of the earth's water supply to form precipitation, but also the energy source by which the hydrologic system is fueled: the sun.
In order to understand the hydrologic cycle, we first must examine the energy source: the sun. The sun provides 1.7x10
17
watts of power to the earth each year (Herring). The temperature is warmer at the equator and cooler at the poles because the equator is exposed to more direct sunlight. Some heat is absorbed by the earth but the most heat is held in our planet's oceans. What can not be contained is emitted back into space. The air is not able to hold much heat at all. When the earth's atmosphere is letting out as much energy as it is taking in, the heat budget is balanced. If the earth is absorbing more or less heat then the atmosphere is letting back out into space, the global temperature will be rising or falling slightly (Herring).
During the day, the sun warms the earth. As the earth's temperature increases, water turns from the liquid state to vapor in the process called evaporation. 90% of evaporation occurs from bodies of water on the earth including oceans, lakes, rivers, and frozen water such as ice and snow. The other 10% evaporates from the leaves of plants in a process called transpiration (US Department of the Interior and US Geological Survey, 2006). When water evaporates, it rises into the atmosphere in a process called convection. Convection is the effect of warmer matter rising and cooler matter sinking. Water vapor is swept up into the atmosphere in warm air currents that are rising.
Once the water vapor rises and cools, it condenses. Condensation is the reverse of evaporation. The temperature of our atmosphere gradually cools from the ground through the troposphere. The vapor continues to rise until it reaches an area of the atmosphere where it can cool. When it does, the vapor turns back to the liquid state in the form of water droplets; this is what forms clouds. Water droplets are also in the air itself, but clouds are groupings of water particles that are large enough to see. Eventually, when the clouds become heavy with water droplets, they are no longer able to be held up, and fall to the earth in the form of precipitation.
The precipitation that falls to the earth causes erosion. Rocks are broken down by water. Heavy rainfall and storms can cause rivers, lakes, and the ocean to overflow their banks causing erosion on the shores. The earth is constantly reshaping itself through erosion.
Then, the cycle starts over again.
