1. EASC 11 The Final FRONTier Fill in your note outline as you follow along with fronts…. A front is the boundary between two air masses – where the temperature and humidity changes An approaching front means a change in the weather At the front, the colder, denser air mass will slide under the warmer, less dense air mass Fronts usually bring precipitation because warm air is forced to rise, so then it cools, condenses, and forms clouds

2. EASC 11 Types of Fronts Figure 1: A front 1.Cold Front: cold air is advancing (moving forward) and displacing warmer air and bringing intense rain and wind Cold fronts have steep slopes, with the steepest part near the ground because friction (with the ground) slows the air mass

3. EASC 11 2.Warm Front: warm air is advancing (moving forward) and displacing colder air and bringing light to moderate rain Warm fronts have gentler slopes Figure 2: Cold and Warm Fronts

4. EASC 11 3.Stationary Front: neither air mass is being displaced, and the front does not move 4.Occluded Front: when a cold front overtakes a warm front Formation of Mid-Latitude Low (Cyclone) In the area of continental North America from Alaska/Yukon/Northwest Territories/Nunavut to Florida, the main weather-producers are called Mid-Latitude Lows, or Mid-Latitude Cyclones Mid-Latitude Lows are large centres of low pressure that generally travel from west to east over a period of a few days to over a week A low starts as a kink or wave in the polar front, possibly caused by a kink or wave in the jet stream The wave moves west to east and ripples the polar front

5. EASC 11 Figure 3: Formation of a Mid-Latitude Cyclone

6. EASC 11 The polar front is the boundary between cool air masses in the polar easterlies and warm air masses in the prevailing westerlies Cold polar air masses lie to the north of the front and warm tropical air masses lie to the south of the front The front bulges southward on the west side of the wave, and polar air pushes tropical air southward (this is a south moving COLD front) On the east side of the wave the polar front bulges northward as a WARM front Between these two fronts is a large pocket of warm air that forms a region of low pressure called a “low” or cyclone

7. EASC 11 In the northern hemisphere, winds blow COUNTERCLOCKWISE (anticlockwise) and toward the centre of a low This air converges in the low pressure area, rises, and forms clouds and precipitation This condensation releases heat energy, which lowers the pressure at the surface, making the low even stronger; this makes the winds stronger Typical weather sequence as a low passes to the North: 1.A long period of steady precipitation in advance of the warm front 2.Warming and slow clearing AFTER the warm front passes; if the air is humid, showers may occur, especially near the centre of the low

8. EASC 11 3.Showers around the time the cold front passes 4.Cooling and rapid clearing, with a change toward the weather characteristic of the newly-arrived cold air mass Sequence of clouds and weather as a warm front passes 1.Cirrus and cirrostratus – no precipitation 2.Altostratus – sun (or moon) blocked 3.Heavy nimbostratus – steady rain (or snow) Clouds along a warm front are usually stratiform because the air is stable – only horizontal movement, no upward movement Cold fronts are steeper and move faster than warm fronts, so air is forced upward quickly by the cold front, and is usually unstable – clouds are usually cumuliform

9. EASC 11 Cumuliform (such as cumulonimbus) clouds grow quickly around the cold front Often rain showers turning into heavier precipitation and even thunderstorms When a cold front passes, weather changes are abrupt In a HIGH pressure area (an anticyclone), weather is usually bright and clear, because dry air is sinking at the centre (pressure is greatest) – wind moves outward from the centre Air moves in a CLOCKWISE direction away from a high Small cumulus clouds (fair weather) may form over heated ground during the day – no precipitation At night heavy dew, frost, and radiation fog may form Temperature inversions may also form