| Polar Front Theory |
| How do storms in the middle and high latitudes form? | |
| How do upper level winds influence storm formation? | |
| What are the wind flow patterns in a developing storm system? |
| A wave cyclone begins along the polar front, where cold polar air meets warm subtropical air. | |
| The (stationary) front is locally a trough of low pressure. | |
| Air flows parallel to the front, but in opposite directions. |
| Because of the different wind directions, a frontal wave can form. | |
| The region of lowest pressure is at the junction between the two fronts. | |
| Green shaded region is where rain is. |
| Next an open wave forms with lower pressure at the center. | |
| Precipitation forms in a wide band in front of the warm front and a narrow band near the cold front. | |
| Energy comes from warm air rising, latent heat, and wind flowing toward low. |
| The faster moving cold front moves closer to the warm front, reducing shrinking the warm sector. | |
| Eventually, the front becomes occluded. At this point the storm is most intense. |
| The point where the cold, warm, and occluded fronts meet is called the triple point. | |
| A new wave may start at this secondary low. | |
| The center gradually dissipates as cold air lies on both sides of the original front. |
| Without the supply of energy from warm air rising and precipitation, the original front gradually disappears, leaving a low and a stationary front. |
| Low 1 is just forming, Low 2 is an open wave, and Low 3 is dissipating. The average speed of a cyclone is 25 knots. |
| Strengthening of a cyclone is called cyclogenesis. | |
| Regions in US where this frequently occurs: east slope of Rockies, the Great Basin, Gulf of Mexico, and Atlantic Ocean east of Carolinas. |
Typical Paths of Winter Anticyclones
| Air flow converges towards a low, acting to increase the pressure. | |
| Air flow diverges away from a high, acting to decrease the air pressure. | |
| How does a low intensify? |
| Where the isobars (and wind flow) get closer together the air is converging, while when the isobars get further apart the air is diverging. |
| A surface high will intensify if there is converging air above it. | |
| A surface low will intensify if there is diverging air below it. | |
| Note offset between surface and above highs and lows. | |
| Upper level wind influences the formation of surface highs and lows. |
| Large storm systems at middle and high latitudes form along the polar front. | |
| According to the polar front theory, storms start as a frontal wave, develop into an open wave, and eventually dissipate. | |
| Whether a storm intensifies is determined in part by upper level convergence and divergence. |