| To understand a weather pattern, no matter | | | | have given this air movement the name "trade |
| where in the world it might occur, it is necessary | | | | winds." Eventually the movement of the air mass |
| to know about the interaction of the sun with the | | | | loses its steam and the "dead" areas around the |
| Earth, its atmosphere and the air masses that | | | | equator at this time were known as doldrums. |
| move around the globe. Much of the weather | | | | The label for this general type of air movement is |
| change that we experience on Earth is caused by, | | | | Hadley cell, because George Hadley, a scientist |
| or includes, the uneven heat distributed | | | | from England, first described the phenomenon in |
| throughout the atmosphere. If the sun heated the | | | | the 1700s. |
| land, the oceans and the air at the same level all | | | | But some of the air at the equator rises and, |
| over the world, there would be no need for the | | | | instead of dropping back toward the ground 30 |
| atmosphere to attempt a state of equilibrium. | | | | degrees north or south of the equator, continues |
| Simply stated, weather forecasts look at how | | | | on toward the North Pole or South Pole. Even the |
| warm air masses and cold air masses interact, | | | | strongest air mass won't always make it all the |
| even collide, to create certain conditions. For | | | | way to one of the poles, however. The warmer |
| example, warm air will rise until it comes in | | | | air comes into contact with much cooler air at |
| contact with a boundary called the tropopause. | | | | about 60 degrees north of the equator and south |
| This is basically a level separating the troposphere | | | | of the equator. Thus the name "polar front." |
| and stratosphere that warm air cannot break | | | | In this case, the warmer air rises above the polar |
| through. | | | | air mass and much of it makes a return trip |
| When an air mass bumps against this barrier it | | | | toward the equator. The air mass may then sink |
| generally spreads in the direction of the poles. As | | | | toward the Earth at the 30 degrees latitude |
| the air cools it moves back toward the Earth at | | | | mentioned earlier. This weather phenomenon, |
| about 30 degrees latitude north and south of the | | | | called Ferrel cells, was first identified by William |
| equator. The drop of such an air mass usually | | | | Ferrel about 150 years ago. In a slight twist on all |
| results in an area of high pressure, with its | | | | this air mass movement, some air does continue |
| common clear skies and little or no precipitation. | | | | on to the North Pole or South Pole, cooling and |
| While it is not easy to predict weather, over the | | | | dropping to the ground, then moving toward |
| years meteorologists and other scientists have | | | | slightly warmer climes at 60 degrees. This subset |
| discovered that the sinking air mass 30 degrees | | | | of air movement is known as a polar Hadley cell. |
| from the equator pushing out whatever air mass | | | | Figure in the effect of the earth's tilt and rotation |
| is there. The displaced air returns to an area | | | | with the movement of air masses and you have |
| closer to the equator and to low pressure. Sailors | | | | most of the ingredients for weather change. |