On New Year’s Eve 2010 a rare outbreak of tornadoes devastated parts of Arkansas, Missouri and Mississippi. The little town of Cincinnatti, Arkansas was leveled. Seven people lost their lives, three in Arkansas and four in Missouri. In larger towns tornado sirens warned residents to seek shelter well in advance of the tornadoes. In those areas residents survived with minor injuries though homes and businesses were annihilated.
Peak tornado season usually strikes from March or April in the Deep South to May and June in the southern Plains and as late as June or July further north. This New Year’s Eve, however, a frigid and fast-moving polar air mass collided with a much warmer and slower moving air mass in the over the southeastern states. This collision sparked violent thunderstorms which spawned the devastating tornadoes.
An air mass is a large area of air, usually covering thousands of square miles, that has roughly the same temperature and humidity. For example, in a cold and dry air mass all of the air will be cold and dry. There will not be areas that are hot and humid. In the United States there are five kinds of air masses that affect weather: continental arctic, continental polar maritime polar, maritime tropical and continental tropical.
- Continental Arctic (cA), or simply Arctic, air forms in the Arctic region in the northern reaches of Alaska and Canada. It is extremely cold and dry air. This air mass forms when a high pressure area becomes stationary north of latitude 60. The air becomes extremely cold due to a lack of warming sunlight and the constant snow and ice cover on the ground during winter. Temperatures can drop to -60 degrees Fahrenheit, or colder. Arctic air masses do not form in the summer when sunlight warms the air. In the winter, however, these air masses plunge south sweeping across southern Canada and the northern US, bringing bone-numbing cold, ice and snow storms, and sometimes record-breaking temperatures.
- As Arctic air moves south, it becomes Continental Polar (cP) air. Continental Polar air masses are also cold and dry but are not as cold as Arctic air. These air masses form over southern Canada and dominate the winter weather of the US. When cP air forms in the summer, it brings pleasant clear weather to the northern US states.
- Maritime Polar (mP) air masses form over high latitude ocean waters and bring cold, moist weather to the Pacific Northwest when they form over the Pacific Ocean and to the Northeast when they form over the North Atlantic. Typical weather associated with a Maritime Polar air mass can be foggy, drizzling, cloudy, and days-long rain. When mP air sweeps in from the Pacific and encounters mountain ranges such as the Rockies, the air mass produces significant rain and snow. On the eastern side of the mountains the air mass modifies into a continental one, though not polar as the air is not dry or cold enough.
On the other end of the spectrum are the tropical air masses which influence summer weather – maritime tropical and continental tropical air masses.
- Maritime Tropical (mT) air masses dominate the weather in the southeastern US during summer months. These air masses are responsible for the oppressive heat and humidity found in Florida and the Deep South. Though Maritime Tropical air can form year-round, it is pushed south to the equator during winter and is not much of a factor on US weather during that time of the year. This air forms over the warmer Atlantic Ocean and Gulf of Mexico waters.
- The last category of air masses for the US is Continental Tropical (cT), hot and very dry air which forms over the Desert Southwest and northern Mexico during summer. These air masses bring record-breaking heat to the Plains and Mississippi Valley, and are largely responsible for the drought that affects the Plains states some years. When cT air meets Maritime Tropical air, thunderstorms form along the boundary. Continental Tropical air moves eastward, and as it does it’s properties become more like Maritime Tropical air. Moisture evaporates into the air making it more humid. Continental Tropical air masses rarely form in the winter.
From bitterly cold winters to hot and humid summers in Florida, drought conditions in Texas to hurricanes, the five different air masses found in the United States dictate the nation’s weather. Even something as unpredictable and unlikely as the New Year’s Eve tornadoes can be predicted by knowing what the air masses are, where they form, what their properties are and how they behave. Knowing these pieces of information helps meteorologists predict the weather and issue warnings that can help save lives.
“Air Masses.” www.geo.msu.edu
“Air Mass Classification.” www.theweatherprediction.com/basic/airmass
“Air Masses.” www.noaa.gov