Читать книгу Weather For Dummies - John D. Cox - Страница 48

When a cold front moves into space occupied by warmer air, it forces the warmer air to rise up and over the leading cold wedge. This is because cold air is denser, or heavier, than warm air and sort of bullies it out of the way. As the warm air rises, it mixes with the cold air and condenses into big clouds. Figure 3-3 shows what happens with the passage of a typical cold front.

FIGURE 3-3: These diagrams illustrate what usually happens when cold fronts and warm fronts come along.

Often, these cold fronts are carried along by westerly winds, which tear away the tops of these clouds and carry them out far in advance of the approaching front. The first sign of an advancing cold front can be the appearance of these high clouds, followed quickly by a thickening sky. And then a line of heavy thundershowers develops as the front passes, followed by steadier and gradually lighter rain. A typical cold front moves about 30 miles an hour, often traveling to the south and east.