This lecture talks about fronts associated with mid-latitude cyclones. A mid-latitude cyclone is a low pressure system that forms and moves in the mid-latitudes. That makes it extra tropical.
Cold fronts can bring extreme weather in the form of heavy intense precipitation, hail, thunder, lightning, damaging winds, and even tornadoes. Warm fronts can bring sleet and freezing rain and fog. A mid-latitude cyclone has attendant fronts that are whipped around the low pressure center. In the northern hemisphere, the circulation around a low pressure system is counterclockwise.
Fronts are simply boundaries between two different air masses. When the warm air is advancing, a warm front boundary is formed. When the cold air is advancing, a cold front is formed.
If the cold front catches up to the warm front, an occluded front is formed. A mid-latitude cyclone usually starts with a stationary front. where neither air mass is advancing.
The symbols for the fronts show the direction that the front is advancing. For example, in the mid-latitude diagram, the cold front is advancing to the east. The front is advancing to the east because of the westerly winds behind the cold front. If you look at the winds in the diagram, you can see they're moving counterclockwise around the low pressure Cyclogenesis is the development or strengthening of a mid-latitude cyclone.
A mid-latitude cyclone usually begins as a stationary front, where cold and warm air masses meet but aren't yet advancing. A kink forms as a frontal wave because of circulations around the air masses, and upper-level divergence can strengthen low pressure. Most mid-latitude cyclones dissipate within a few days of formation.
There are a few main source regions for mid-latitude cyclones. The lee side lows develop on the leeward side of the Rockies and the Appalachians. Cape Hatteras, North Carolina is a major source when the warm Gulf Stream waters deepen the warm sector. Remember, a front is just a boundary between two different air masses.
A warm front occurs when the warm air is advancing into the cooler air mass. Typically, the MT air mass is advancing into the MP air mass. Since the warm air mass isn't dense enough to push the cooler air mass, the warm air overruns or moves up and over the cooler wedge.
This causes more horizontal lift, resulting in stratus clouds. Stratus clouds aren't usually able to grow large cloud droplets, so warm fronts usually have light drizzly precipitation. However, the precipitation happens well ahead of the front line.
Winds behind the warm front are typically southerly, and winds ahead of the warm front are often easterly. A cold front occurs when the cold air is advancing into a warmer air mass. Typically, the CP air mass is advancing into an MT air mass. The CP air mass, being colder and more dense, can cause vertical movement.
Tall cumulus clouds can form with enough moisture that large droplets can grow in the cloud. Cold fronts are associated with heavy intense precipitation, hail, lightning, and even more severe weather such as flash floods and tornadoes. The precipitation typically occurs just along the cold front boundary.
Since cold air is more dense, it will also move faster around a mid-latitude cyclone. When a cold front catches up to a warm front, extreme lifting occurs and the storm occludes. At occlusion, the storm is usually most intense and sometimes extreme weather can occur.
However, the occlusion marks the end of the mid-latitude cyclone as the air masses start to mix. The warm air at the surface is lifted, essentially stabilizing the atmosphere. Nor'easters are mid-latitude cyclones that move along the east coast of the United States. They entrained moist winds from the northeast.
A typical nor'easter begins as a hatteras low pressure system. Its path can be controlled by cold air damming. Cold air gets trapped between the leeward side of the Appalachians and the warm Gulf Stream current. As moist air moves north, it overruns the trapped cold air.
Nor'easters can bring rain, sleet, freezing rain, or snow depending upon the temperature profile and the thickness of the cold air layer. Nor'easters are often the most intense storm systems. They can develop and intensify quickly, increasing the danger.