Page 49 - ภาษาอังกฤษสำหรับสายวิทยาการอุตุนิยมวิทยา

44

7. Convection Currents

a. When two bodies of air next to each other are heated unequally, the warmer air
expands and becomes lighter or less dense than the surrounding cool air. The denser air is
drawn to the ground by its greater weight and 1ifts or forces the warm air upward. The flow
parallel to the ground between the two bodies is called advection

Convection current as a result of unequal heating of the atmosphere by different surface
temperatures. Cold air sinks forcing warm air upward. The horizontal flow in a convection

current is usually called advection.

Figure 5

b. These convection currents cause small-size wind systems that occur within the
larger overall system of circulation. These systems may influence local weather and be small
enough to affect only one airport, or large enough to affect an area of several thousand
square miles. But they do not include the large high- and low-pressure systems called
migratory highs and lows that migrate from one area of the earth to another.

8. Jet Stream

The jet stream is a narrow, high-speed, river of wind that moves at a shigh altitude around
the-earth. It does not flow directly in a straightpath, and it may not always be a continuous
flow. It moves through the normal wind patterns aloft. Wind speeds in the jet stream range
from