Since the news has been almost non-stop following Tropical Storm and potential Hurricane Isaac, I thought I would continue the Weather 101 through this week. This may give followers of this Blog some insight into what meteorologists go through as they try to “accurately” predict the weather.
Weather and Weather Advisories
Everyone has heard the old saying, “if you don’t like the weather, wait ten minutes.” Actually, this can be truer than one might think. The weather is constantly changing and it appears in recent years that those changes are getting more drastic and subsequently dangerous. Even with all the most modern equipment, weather forecasting is still a “best guess” based on history, computer models, radar, barometers and other meteorological equipment.
What causes weather? The basic weather principles are fairly straightforward. If you understand the following principles and how they interact with each other you will have a basic understanding of weather.
- The earth is tilted on its axis by 23.5 degrees.
- The earth rotates one full rotation every 24 hours.
- The earth’s surface is composed of land and water that heat and cool unevenly.
- Hot air rises.
- Air over land and water absorb moisture at different rates.
How do these five principles interact to cause weather?
First let’s take the fact that the earth is tilted on its axis by 23.5 degrees. This tilt actually causes the seasons because parts of the earth’s surface are exposed to more sunlight than others. Those areas closest to the sun and exposed to the most direct sunlight are in summer season and those furthest from the sun and have the least sunlight are in winter season. As the sun’s relative position moves from 23.5 degrees north of the equator to 23.5 degrees south of the equator the seasons are created.
We know that the earth rotates 360 degrees in 24 hours that causes day and night. During the day the earth’s surface is warmed and this warmth is transferred to the air above the surface. At night the earth cools rapidly and subsequently the air above it is cooled as well.
How fast is the earth actually rotating? Since 360 degrees in 24 hours equates to 15 degrees per hour, and at the equator each degree equals 60 nautical miles, some simple math concludes that the earth is rotating at 900 miles per hour at the equator (15 x 60 = 900). North and south of the equator the distance around the earth is less than at the equator so the speed of rotation at points north and south of the equator would be less that at the equator. This situation causes Coriolis effect or horizontal deflection. The Coriolis effect causes winds to be deflected from the direction that they are attempting to move. It is the combination of the rising and falling of air and the Coriolis effect that makes weather systems in the United States generally move from west to east.
The uneven heating and cooling rates of the different surfaces of the earth (e.g. land and water) cause pressure systems and ties into the hot air rises principle. As one surface heats up rapidly and transfers that heat to the air surrounding it, that air rises causing a low-pressure area. As this air rises into the cooler atmosphere where, at some point, it becomes cool enough that now it falls back to earth creating a high-pressure area. When a low-pressure area was created, the cooler air from the high-pressure area moves in to fill the partial vacuum. Does the word circulation start to come to mind?
Warm air is able to hold more moisture than cool air. This difference in the ability to absorb water at different rates produces certain weather phenomenon. As moisture content builds in an air mass, clouds form and eventually rain and/or fog may follow.
To be continued…