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From Chapter 20.2 Fronts, Earth Science, Tarbuck & Lutgens

Recall that air masses have different temperatures and amounts of moisture, depending on their source regions. Recall also that these properties can change as an air mass moves over a region. What do you think happens when two air masses meet?

When two air masses meet, they form a front, which is a boundary that separates two air masses. Fronts can form between any two contrasting air masses. Fronts are often associated with some form of precipitation,

A warm front forms when warm air moves into an area formerly covered by cooler air

The slope of the warm front is very gradual, as shown in Figure 10.
As warm air rises, it cools to produce clouds, and frequently precipitation.

The sequence of clouds shown in Figure 10 typically comes before a warm front. The first sign of the approaching warm front is the appearance of cirrus clouds. As the front nears, cirrus clouds change into cirrostratus clouds, which blend into denser sheets of altostratus clouds. About 300 kilometers ahead of the front, thicker stratus and nimbostratus clouds appear, and rain or snow begins

formation of a warm front
A cold front forms when cold, dense air moves into a region occupied by warmer air

On a weather map, the surface position of a cold front is shown by a blue line edged with blue triangles that point toward the warmer air mass.

Figure 11 shows how a cold front develops. As this cold front moves, it becomes steeper. On average, cold fronts are about twice as steep as warm fronts and advance more rapidly than warm fronts do.

These two differences—rate of movement and steepness of slope— account for the more violent weather associated with a cold front.
The forceful lifting of air along a cold front can lead to heavy downpours and gusty winds. As a cold front approaches, towering clouds often can be seen in the distance. Once the cold front has passed, temperatures drop and wind shifts. The weather behind a cold front is dominated by a cold air mass. So, weather clears soon after a cold front passes. When a cold front moves over a warm area, low cumulus or stratocumulus clouds may form behind the front.
Formation of a cold front

Stationary fronts

Occasionally, the flow of air on either side of a front is neither toward the cold air mass nor toward the warm air mass, but almost parallel to the line of the front. In such cases, the surface position of the front does not move, and a stationary front forms.

Occluded fronts
When an active cold front overtakes a warm front, an occluded front forms. It develops as the advancing cold air wedges the warm front upward. The weather associated with an occluded front is generally complex.  Most precipitation is associated with the warm air’s being forced upward. When conditions are suitable, however, the newly formed front is capable of making light precipitation of its own.

It is important to note that the descriptions of weather associated with fronts are general descriptions. The weather along any individual front may or may not conform to the idealized descriptions you’ve read about. Fronts, like all aspects of nature, do not always behave as we would expect.

Formation of an occluded front

Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework

8.MS-ESS2-5. Interpret basic weather data to identify patterns in air mass interactions and the relationship of those patterns to local weather.

HS-ESS2-6. Use a model to describe cycling of carbon through the ocean, atmosphere, soil, and biosphere and how increases in carbon dioxide concentrations due to human activity have resulted in atmospheric and climate changes.

ESS2.D Weather and climate: Complex interactions determine local weather patterns and influence climate, including the role of the ocean. Human activities affect global warming. ALSO The role of radiation from the Sun and its interactions with the atmosphere, ocean, and land are the foundation for the global climate system.
College Board Standards for College Success: Science

Objective ES.1.4 Weather Processes. Students understand that weather is the result of short-term interactions (days) among the atmosphere, hydrosphere, lithosphere and biosphere.

ESH-PE.1.4.2 Describe, in terms of temperature, pressure and moisture conditions, the formation of severe weather conditions such as tornadoes, hurricanes and thunderstorms.

ESH-PE.1.4.2a Construct an illustration of a mature thunderstorm that shows how air movement inside the storm leads to the formation of hail and tornadoes.
ESH-PE.1.4.2b Describe how, within storm systems, thermal energy is converted into both mechanical energy (wind) and electrical energy, and link these phenomena to the law of conservation of energy.

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