Air on surfaces with higher temperatures will then begin to rise because it is lighter less dense. As the air rises, it creates low atmospheric pressure. Air on surfaces with cooler temperatures sink do not rise. The sinking creates higher atmospheric pressure. This behavior or warm gases or liquids moving upward and replaced by cooler particles is called Convection. The energy moving during convection is called convectional current.
Important : Hot air rises, and cool air sinks. That brings about spatial differences in atmospheric pressure, caused by uneven heating. Let us see this illustration below showing pressure and wind direction:.
In the diagram above, notice how cool air falls, resulting in high pressure, and moving towards regions of low pressure. Mountains can force approaching air masses to rise to higher elevations. As the altitude of the air mass increases, the air expands because the air pressure is lower at higher elevations. When this happens, the volume of the air mass increases and the density decreases. At the same time, the temperature of the air mass decreases, causing water vapor in the air mass to condense and form clouds, and often precipitation.
Orographic precipitation is the fall of water on the side of the mountain range facing the direction of approaching air masses Fig. This mountain face is the upwind, or windward, side and is often green with vegetation. This phenomenon captures precipitation on one side of a mountain range, while leaving the other side of the mountain dry. A rain shadow is a dry area that forms on the downwind or leeward side of the mountain Fig.
The Mojave and Great Basin Deserts in the southwestern United States are both located in the large rain shadow cast by the Sierra Nevada mountain range. Over a year, about 60 times more sunlight falls on equatorial areas than on polar areas. This is due to the tilt of the earth on its axis The north and south poles only have sunlight during their summer months. Even in the summer, light is spread over a wider area at the poles than at the equator, reducing the intensity of light and heat.
Consequently, hot equatorial air masses, which are often humid, are less dense than cold polar air masses, which are often dry. At the poles, air masses sink as they become colder and denser. These sinking air masses push or displace other air masses away as they flow along the ground towards the equator Fig. Polar air becomes warmer as it moves away from the poles.
Air masses at the equator move in the opposite manner. At the equator, hot, moist, and less-dense air rises. At higher altitudes, this air gradually cools, forming precipitation, and dries out as it moves away from the equator and toward the poles.
The sinking of polar air and rising of equatorial air form a large-scale global circulation pattern and explains why winds generally travel from north to south in the Northern hemisphere Fig. Unequal heating of the earth affects pressure and density, and assists in driving wind flow patterns.
The earth constantly rotates, or spins, on its axis counter-clockwise, from the west towards the east. The earth has a greater circumference at the equator than near the poles, so in one rotation, a point on the equator will cover a greater distance than a point near the poles, in the same amount of time.
This means land and water near the poles is traveling from west to east at a few kilometers an hour, but land and water at the equator is moving in the same direction at more than two thousand kilometers an hour. The atmosphere surrounding the earth rotates at the same speed as the land and the ocean surface beneath it because air masses are loosely attached to the earth's surface by gravity. Air masses located about 50 kilometers south of the North Pole are moving at a rate of 7. The differential rotation of the higher latitudes compared to the equator affects the movement of air masses and water on earth.
Explore the effect of a rotating earth on air masses traveling north and south across the globe. Warm, dense, high-pressure air masses tend to flow away from the equator towards cool, less dense, low-pressure air masses at the poles. However, air masses do not move north or south in a direct, straightforward line due to the rotation of the earth and the Coriolis effect. The Coriolis effect is the deflection of air and water masses by the rotation of the earth.
Differences in rotational velocity between air masses located near the equator and those located near the poles deflect air circulation to the right in the Northern Hemisphere Fig. This deflective force of the Coriolis effect is greatest near the poles and weakest at the equator.
The Coriolis effect operates over a period of weeks because the movement of air masses is relatively slow. The Coriolis effect determines the movement of large-scale winds as well as localized high- and low-pressure systems.
This is all because of gravity, which is a driving force in controlling the vertical movement of air. A more significant force affecting wind patterns, though, is the Coriolis force. The wind is no exception and is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
The magnitude of the deflection is the least by the equator and the greatest around the poles. Other forces affect the behavior of winds as well. A commonly known one, friction, is a strong influence over winds near the ground. Friction always acts in opposition to the wind velocity and the flow of air in general. This also reduces the effects of the Coriolis force, and the atmosphere adjusts to that by turning the wind toward low pressure. The Coriolis force and friction combined, and balanced with the horizontal pressure gradient force, cause a balance in the atmosphere that explains the spiraling motion, instead of a straight inward or outward motion, around low- and high-pressure systems.
With winds directed toward low pressure and the fact that air rises in a low-pressure area, stormy weather results as water reaches its condensation point in the atmosphere.
Clouds and precipitation are the result.
0コメント