Collision

 

When two air masses with different temperatures and moisture content collide, they cause atmospheric disturbances. When the warm air rises, its cooling causes water vapor to condense and the formation of clouds and precipitation. A mass of warm

and light air is always forced upward, while the colder and heavier air acts like a wedge(=a piece material with a pointed edge at one end and a wide edge at the other, either pushed between two objects to keep them still). This cold-air wedge undercuts the warmer air mass and forces it to rise more rapidly. This effect can cause variable, sometimes stormy, weather.

Cold Fronts

These fronts occur when cold air is moved by the wind and collides (=bumps) with warmer air. Warm air is driven upward. The water vapor contained in the air forms cumulus clouds, which are rising, dense white clouds. Cold fronts can cause the temperature to drop by 10° to 30° F (about 5°-15° C) and are characterized by violent and irregular winds. Their collision with the mass of ascending(=going upwards) water vapor will generate rain, snow flurries(=brief light snowfalls), and snow. If the condensation is rapid, heavy downpours(=fall of rain), snowstorms (during the cold months), and hail may result. In weather maps, the symbol for a cold front is a blue line of triangles indicating the direction of motion.

Warm Fronts

These are formed by the action of winds. A mass of warm air occupies a place formerly(=before) occupied by a mass of cold air. The speed of the cold air mass, which is heavier, decreases(=becomes less) at ground level by friction(=rubbing), through contact with the ground. The warm front ascends and slides above the cold mass. This typically causes precipitation(=rain or snow) at ground level. Light rain, snow, or sleet are typically produced, with relatively light winds. The first indications of warm fronts are cirrus clouds, some 600 miles (1,000 km) in front of the advancing low pressure center. Next, layers of stratified clouds, such as the cirrostratus, altostratus, and nimbostratus, are formed while the pressure is decreasing.

Entire Continents

Fronts stretch over large geographic areas. In this case, a cold front causes storm perturbations(=a small change in the regular movement) in western Europe. But to the east, a warm front, extending over a wide area of Poland, brings light rain. These fronts can gain or lose force as they move over the Earth's surface depending on the global pressure system.

Rossby Waves

Large horizontal atmospheric waves that are associated with the polar-front jet stream. They may appear as large undulations in the path of the jet stream. The dynamics of the climatic system are affected by these waves because they promote the exchange of energy between the low and high latitudes and can even cause cyclones to form. accentuate = make more noticeable meander = curve