An air mass is a vast body of air that covers many thousands of square miles and maintains relatively uniform temperature and moisture characteristics. The Maritime Tropical (mT) air mass is a significant component of North American weather, especially during warmer months. Defined by its origins over warm waters near the equator, this system results in air that is both hot and rich in moisture. Understanding the mT air mass is important because it dictates the severity of summer weather across much of the continent.
Defining the Maritime Tropical Air Mass
The Maritime Tropical air mass is characterized by its two defining properties: “maritime” and “tropical.” “Maritime” indicates the air mass forms over a large body of water, making it inherently moist or humid. “Tropical” signifies that the air mass originates in low-latitude regions near the equator, resulting in warm to hot temperatures. This combination makes the mT air mass warm, moist, and typically unstable in its lower layers.
These air masses transport the most moisture and produce the most rain in the regions they affect. The air in an mT mass is generally less dense than colder air masses, which contributes to its tendency to rise. Meteorologists denote this air mass with the abbreviation “mT” on weather maps.
The primary source regions impacting the United States include the Gulf of Mexico, the Caribbean Sea, and the western Atlantic Ocean. When these masses move inland, their warm, moisture-laden properties contrast sharply with continental air, leading to significant weather events. High dew point temperatures measure the moisture content and are a defining characteristic of this air.
How Maritime Tropical Air Masses Form
Maritime Tropical air masses acquire their specific characteristics through prolonged contact with warm ocean surfaces. They require a long residence time over tropical or subtropical oceans to fully absorb heat and moisture. In North America, this typically occurs over the Gulf of Mexico and the Gulf Stream.
The formation process is driven by the transfer of heat and water vapor from the ocean to the overlying air. Warm waters readily evaporate large volumes of water, saturating the low levels of the air mass with moisture. Solar heating combines with this moisture infusion, warming the air column.
These air masses often develop in association with large, stable high-pressure regions, such as subtropical high-pressure systems. These systems feature light winds, allowing the air to remain relatively stagnant over the source region for days or weeks. This extended period ensures the air mass adopts the uniform temperature and moisture properties of the underlying ocean.
The Weather They Bring
When a Maritime Tropical air mass moves over land, it transports warm, moist, and often unstable conditions far from its source region. The most noticeable effect is the oppressive humidity, caused by the very high dew points within the air mass. This high moisture content makes the air feel much hotter than the actual temperature, leading to a significantly higher heat index.
The instability of the mT air mass makes it prone to lifting, which easily triggers the formation of clouds and precipitation. As the warm, moist air rises, it cools, and the abundant water vapor condenses, leading to widespread shower and thunderstorm activity. This process is responsible for the majority of summer thunderstorm activity across the eastern and central United States.
The energy provided by this warm, moist air is a factor in more extreme weather systems. Maritime Tropical air masses fuel the development and intensification of tropical storms and hurricanes over the ocean. When an mT air mass collides with a colder, drier air mass along a weather front, the substantial contrast in properties can lead to the formation of severe thunderstorms and heavy rainfall.