High School Earth Science/Changing Weather

The weather in a location often depends on what type of air mass is over it. Another key factor revolves around whether or not the spot is beneath a front, the meeting place of two air masses. The characteristics of the air masses and their interactions can determine whether the weather is constant over an area, or whether there are rapid changes in air temperature, wind, precipitation and even thunderstorms.
Air Masses.
An air mass is a batch of air that has nearly the same temperature and humidity (Figure 16.14). An air mass is created above an area of land or water known as its source region. Air masses come to have a distinct temperature and humidity when they remain over a region for several days or longer. The heat and moisture leave the ground and move into the air above it, until the overlying air takes on the temperature and humidity characteristics of that particular region.
Air masses are created primarily in high pressure zones. They most commonly form in polar and tropical regions, which have very distinctive temperature and humidity. The temperate zones are ordinarily too unstable for air masses to form. Instead, air masses move across them, making the middle latitudes the site of very interesting weather.
Air masses can be 1,600 km (1,000 miles) or more across and several kilometers thick.
Temperature and humidity may change a bit horizontally across the air mass, but not too much. An air mass may have more changes with altitude.
Meteorologists use symbols to describe the characteristics of an air mass. The first symbol tells whether the air mass had its origin over a continent (c) or over an ocean (m, for maritime). As you might expect, air masses that form over oceans contain more water vapor than those that form over land. The second symbol tells the general latitude where the air mass gained its temperature and humidity traits. The categories are arctic (A), polar (P), tropical (T), and equatorial (E). Of course, air masses that form over polar areas are colder than those that form over tropical regions.
Globally, the major air masses are continental arctic or continental antarctic (cA or cAA); continental polar (cP); maritime polar (mP); continental tropical (cT); maritime tropical (mT); and maritime equatorial (mE). Maritime arctic and continental equatorial air masses rarely form.
A third symbol takes into account the properties of an air mass relative to the ground it moves over. If the air mass is colder than the ground, it is given the designation k, for cold. If it is warmer than the ground, it is given the designation w. For example, a cPk is an air mass with a continental polar source region that is colder than the region it is now moving over.
Air Mass Movement.
Air masses are pushed along by high-level winds, although they move slower than the winds. An air mass gets its characteristics from the ground or water it is above, and it also shares those characteristics with the regions that it travels over. Therefore, the temperature and humidity of a particular location depends partly on the characteristics of the air mass that sits over it.
If the air mass is very different from the ground beneath it, storms may form. For example, when a colder air mass moves over warmer ground, the bottom layer of air is heated. That air rises, forming clouds, rain, and sometimes thunderstorms. When a warmer air mass travels over colder ground, the bottom layer of air is cooled. This forms a temperature inversion, since the cold air near the ground is trapped. Inversions may form stratus clouds, advection fogs, or they may trap a layer of pollution over a city.
In general, cold air masses tend to flow toward the equator and warm air masses tend to flow toward the poles. This brings heat to cold areas and cools down areas that are warm. It is one of the many processes that act towards balancing out the planet’s temperatures.
Fronts.
Two air masses meet at a front. Because the two air masses have different temperature and humidity, they have different densities. Air masses with different densities do not easily mix. Ordinarily, when fronts meet, one air mass is lifted above the other. Rising air creates a low pressure zone. If the lifted air is moist enough, there will be condensation and precipitation. Fronts usually also have winds in them. If the temperature difference between the two air masses is high, then the winds will be strong. Fronts are the main cause of stormy weather.
The map symbols for the different types of fronts are shown in (Figure 16.15): (1) cold front, (2) warm front, (3) stationary front, (4) occluded front, (5) surface trough, (6) squall line, (7) dry line, (8) tropical wave.
The direction that fronts move is guided by pressure gradients and the Coriolis Effect. In the Northern Hemisphere, cold fronts and occluded fronts tend to move from northwest to southeast. Warm fronts move southwest to northeast. The direction the different types of fronts move in the Southern Hemisphere is the mirror image of how they move in the Northern Hemisphere. Fronts can be slowed or stopped by a barrier such as a mountain range.
The rest of this section will be devoted to four types of fronts. Three of these fronts move and one is stationary. With cold fronts and warm fronts, the air mass at the leading edge of the front gives the front its name. In other words, a cold front is right at the leading edge of moving cold air and a warm front marks the leading edge of moving warm air.
Stationary Fronts.
Most fronts move across the landscape, but at stationary fronts the air masses do not move. A front may become stationary if an air mass is stopped by a barrier. For example, cold air masses may be stopped by mountains, because the cold air mass is too dense to rise over them.
A region under a stationary front may experience days of rain, drizzle and fog. This weather may be present over a large area. Winds usually blow parallel to the front, but in opposite directions. This results in shear stress. Shear stresses result when objects are pushed past each other in opposite directions.
After several days, the front will break apart. The temperature gradient or temperature difference across the front may decrease, so the air masses start to mix. Shear stresses may force the front to break apart. Conditions may change so that the stationary front is overtaken by a cold front or a warm front. If the temperature gradient between the air masses increases, wind and rainy weather will result.
Cold Fronts.
When a cold air mass takes the spot of a warm air mass, there is a cold front (Figure 16.16). Since cold air is denser than the warm air, the cold air mass slides beneath the warm air mass and pushes it up. As the warm air rises, there are often storms.
When cold air moves underneath warm air, the ground temperature drops. The humidity may also decrease since the colder air may also be drier. Winds at a cold front can be strong because of the temperature difference between the two air masses. When a cold front is on its way, there may be a sharp change in dew point, changes in wind direction, changes in air pressure, and certain characteristic cloud and precipitation patterns.
Cold fronts often move rapidly across the landscape. Fast-moving cold fronts create a line of intense storms over a fairly short distance. A squall line is a line of severe thunderstorms that forms along a cold front (Figure 16.17). If the front moves slowly, the storms may form over a larger area.
Imagine that you are standing in one spot as a cold front approaches. Along the cold front, the denser, cold air pushes up the warm air, causing the air pressure to decrease. If the humidity is high enough, some types of cumulus clouds will grow. High in the atmosphere, winds blow ice crystals from the tops of these clouds to create cirrostratus and cirrus clouds. At the front, there will be a line of rain or snow showers or thunderstorms with blustery winds. Behind the front is the cold air mass. This mass is drier and so precipitation stops. The weather may be cold and clear or only partly cloudy. Winds may continue to blow into the low pressure zone at the front.
The weather at a cold front varies with the season. Thunderstorms or tornadoes may form in spring and summer, when the air is unstable. In the spring, the temperature gradient can be very high, causing strong winds to blow at the front. In the summer, thunderstorms may be severe and may also include hailstorms. In the autumn, strong rains fall over a large area. If the front moves slowly, enough rain may fall to cause flooding. Cold fronts in winter may bring frigid temperatures and heavy snows. The cold air mass is likely to have formed in the frigid arctic.
When the temperature gradient across a cold front is low, a cold front has little effect on the weather. This may occur at some locations in the summer. Along the western United States, the Pacific Ocean warms and moistens cold air masses so that the temperature gradient across a cold front is small.
Warm Fronts.
A warm front is found where warm air mass slides over a cold air mass (Figure 16.18). Since the warmer, less dense air is moving over the colder, denser air, the atmosphere is relatively stable. Warm fronts travel much more slowly than cold fronts because the leading cold air mass is dense and sluggish.
Imagine that you are on the ground in the wintertime under a cold winter air mass with a warm front approaching. The transition between the cold air and the warm air takes place over a long distance. This means that the first signs of changing weather appear long before the front is actually over you. In fact, weather changes may appear hundreds of kilometers ahead of the front. Initially, the air is cold: the cold air mass is above you and the warm air mass is above it. High cirrus clouds mark the transition from one air mass to the other.
Over time, cirrus clouds become thicker and cirrostratus clouds form. As the front approaches, altocumulus and altostratus clouds appear and the sky turns gray. Since it is winter, snowflakes fall. Soon the clouds thicken and nimbostratus clouds form. Snowfall increases. Winds grow stronger as the low pressure approaches. As the front gets closer, the cold air mass is just above you but the warm air mass is not too far above that. The weather worsens. As the warm air mass approaches, temperatures rise and snow turns to sleet and freezing rain. Warm and cold air mix at the front, leading to the formation of stratus clouds and fog (Figure 16.19).
As the front passes over you, the temperature and dew point rise and the rain likely ends. Winds change direction. The transition is not nearly as dramatic as when a cold front passes over, since there is more mixing of the two air masses occurring in a warm front.
The Pacific Ocean also plays a role in modifying the warm fronts that reach the west coast of the United States. These storms are so broad that it is very difficult to spot exactly where the warm front is!
Occluded Fronts.
An occluded front or occlusion usually forms around a low pressure system (Figure 16.20). The occlusion starts when a cold front catches up to a warm front. The air masses, in order from front to back, are cold, warm, and then cold again. The boundary line, where the two fronts meet, curves towards the pole because of the Coriolis effect. If the air mass that arrives third is colder than either of the first two air masses, that air mass will slip beneath the other two air masses. This is called a cold occlusion. If the air mass that arrives third is warm, that air mass will ride over the other air mass. This is called a warm occlusion.
Occluded fronts can cause drying or storms. Precipitation and shifting winds are typical. The weather is especially fierce right at the occlusion. The Pacific coast has frequent occluded fronts. All of these fronts are part of the mid-latitude cyclone. These weather systems will be discussed in the next lesson.