When the atmosphere comes into touch with a big, generally regular land or sea surface for a long enough period of time, it acquires the temperature and moisture qualities of that surface. The largest air masses on Earth originate in polar or subtropical latitudes. They are forced over large distances by horizontal wind patterns called circulations.
The primary source region for the majority of the world's clouds is the tropical ocean, particularly the Indian Ocean. A secondary source region is located in southern Africa. A much smaller amount forms in mainland North America near the border between Canada and Mexico.
Tropical cyclones can also influence cloud formation through their effects on the surrounding environment. For example, hurricanes may increase the probability of precipitation by disrupting the normal flow pattern of a region that would otherwise be prone to drought. These disruptions can last for days or even weeks after the storm has passed.
Finally, some clouds form in high-altitude regions away from any major landmasses. It is here that ice crystals can grow directly from the moisture in the air. When these crystals melt in springtime, they contribute water vapor to the atmosphere.
The main reason why so many clouds occur in the tropics is because it is there that you find the highest temperatures and lowest levels of oxygen. This is exactly what is needed for molecules to join together and form particles that will eventually become clouds.
An air mass is a volume of air described by its temperature and water vapor concentration in meteorology. Air masses travel hundreds or thousands of kilometres and adapt to the properties of the land underneath them. They are categorised based on latitude and whether they originate on land or in the sea. The three main air masses at high altitude are polar, continental and maritime.
Polar air masses are cold and dry. They arise over the Arctic Ocean and usually stay there for several months due to lack of moisture from the South Atlantic ocean. Polar air masses can reach temperatures as low as -80°C or lower and have very little water vapor in them because it gets evaporated away by the cold atmosphere around it.
Continental air masses are warm and wet. They include the temperate zones air masses which originate over Europe and Africa and flow into Asia. Continental air masses can be divided into two categories based on their position relative to the Earth's axis of rotation: western and eastern. Western continental air masses flow north-west to south-east across the mid-latitudes, while eastern continental air masses flow east-northeast over Russia and China toward the Pacific Ocean. These two types of air masses differ mainly in their temperature and precipitation patterns. Western continental air masses are generally warmer than eastern ones and may contain up to 10% more water vapor per unit mass.
An air mass is a huge volume of air with uniform temperature and moisture qualities. Large flat areas where air may stay stationary long enough to take on the properties of the land below are the greatest source locations for air masses. Mountains, deserts, and oceans all produce winds that flow over their surfaces creating air currents that can travel great distances.
The word "atmosphere" comes from Greek astronomy. The Greeks observed that many things in life behaved like planets, including the moon, the sun, and even people. They believed that all these celestial bodies were made of earth and sky mixed together and they called this mixture atmosphere. Today we know that people are made of water, earth, and oxygen; stars are made of hydrogen, helium, and other elements. But before our knowledge, astronomers used whatever was available at the time and assumed it was correct.
So the ancient Greeks saw earth and sky as two parts of one thing. We know today that earth and sky are separate entities that only interact through natural phenomena such as wind, rain, snow, and ice but people first started thinking this way hundreds of years ago!
Since air is almost invisible and consists of tiny particles that are hard to measure, scientists use instruments to learn more about it.
The temperature and humidity attributes of air masses define them. The underlying surface qualities of the location where air masses arise determine their attributes. When air masses develop, they travel within the overall circulation. They may be advected or entrained by other flow elements such as winds or ocean currents.
Air masses are classified according to their source region. An air mass that originates over a warm ocean will be warmer than one that originates over a cold ocean. An air mass that is advected or entrained by a strong wind will become cooler than its source region due to heat loss to the environment. Cooler offshore waters release more latent heat than warm coastal waters, so an air mass that originates over the ocean will tend to be warmer than one that does not originate over the ocean.
An air mass that is advected or entrained by a large body of water will become wetter than its source region because it acquires moisture from the ocean. An air mass that is advected or entrained by a large body of land will become drier because it expels moisture into the environment.
Air masses can also be classified by their composition. An air mass that is high in pressure will have more density than one that is low in pressure.