Water in The Atmosphere

Humidity is the amount of water vapour in the air. Water vapour is the gaseous state of water and is invisible. Humidity indicates the likelihood of precipitation, dew, or fog. There are three main measurements of humidity: absolute, relative and specific. Absolute humidity is the water content of air at a given temperature expressed in gramme per cubic meter. Relative humidity, expressed as a percent, measures the current absolute humidity relative to the maximum (highest point) for that temperature. Specific humidity is a ratio of the water vapour content of the mixture to the total air content on a mass basis.

There is a limit to the amount of water vapour which can be held by the air. This limit changes with temperature. Warm air can hold more water vapour than cold air. Air containing maximum moisture it can hold at the given temperature is said to be saturated. Hence, the amount of water vapour that is needed to achieve saturation increases as the temperature increases. As temperature decreases, the amount of water vapour needed to reach saturation also decreases. As the temperature of a mass of air becomes lower it will eventually reach the point of saturation without adding or losing water mass.

When air reaches saturation, the water which it cannot hold gets precipitated as rain, snow hail etc. The temperature at which the air gets saturated without the addition of water vapour is called dew point.

The humidity is affected by winds and by rainfall. At the same time, humidity affects the energy budget and thereby influences temperatures in two major ways. One, water needs the energy to convert itself into vapour. This energy is absorbed from surroundings causing a cooling effect. This heat is returned to the surroundings when precipitation occurs.

Second, water vapour is the most abundant of all greenhouse gases. Water vapour, like a green lens that allows green light to pass through it but absorbs red light, is a “selective absorber”. Along with other greenhouse gases, water vapour is transparent to most solar energy, as you can literally see. But it absorbs the infrared energy emitted (radiated) upward by the earth’s surface, which is the reason that humid areas experience very little nocturnal cooling but dry desert regions cool considerably at night. This selective absorption causes the greenhouse effect.

Clouds are classified according to how they look and how high the base of the cloud is in the sky. This system was suggested in 1803. There are different sorts of clouds because the air where they form can be still or moving forward or up and down at different speeds. Very thick clouds with large enough water droplets can make rain or snow, and the biggest clouds can make thunder and lightning. There are five basic families of clouds based on how they look.
Cirrus clouds are high and thin. The air is very cold at high levels, so these clouds are made of ice crystals instead of water droplets. Cirrus clouds are sometimes called mares’ tails because they look like the tails of a horse.
Stratus clouds are like flat sheets. They may be low-level clouds (stratus), medium-level (altostratus), high-level (cirrostratus), or thick multi-level clouds that make rain or snow (nimbostratus).

Stratocumulus clouds are in the form of rolls or ripples. They may be low-level clouds (stratocumulus), medium-level (altocumulus), or high-level (cirrocumulus).

Cumulus clouds are puffy and small when they first form. They may grow into heap clouds that have a moderate vertical extent (nothing added to the name) or become towering vertical clouds (towering cumulus).
Cumulonimbus clouds are very large cumulus-type clouds that usually develop cirrus tops and sometimes other features that give them their own unique look.

Based on the height of formation of clouds, they are classified into different types.

High clouds form from 10,000 to 25,000 ft (3,000 to 8,000 m) in cold places, 16,500 to 40,000 ft (5,000 to 12,000 m) in mild regions and 20,000 to 60,000 ft (6,000 to 18,000 m) in the very hot tropics. They are too high and thin to produce rain or snow.

High-level clouds include Cirrus (Ci), Cirrocumulus (Cc), Cirrostratus (Cs)
Medium-level clouds or Middle clouds usually form at 6,500 ft (2,000 m) in colder areas. However, they may form as high as 25,000 ft (8,000 m) in the tropics where it’s very warm all year. Middle clouds are usually made of water droplets but may also have some ice crystals. They occasionally produce rain or snow that usually evaporates before reaching the ground.

Medium-level clouds include Altocumulus (Ac), Altostratus (As)
Low-level clouds are usually seen from near ground level to as high as 6,500 ft (2,000 m). Low clouds are usually made of water droplets and may occasionally produce very light rain, drizzle, or snow.

Low-level clouds include Stratocumulus (Sc) and Stratus (St). When very low stratus cloud touches the ground, it is called fog.

Moderate-vertical clouds are clouds of medium thickness that can form anywhere from near ground level to as high as 10,000 ft (3,000 m). Medium-level Cumulus does not have ‘alto’ added to its name. The tops of these clouds are usually not much higher than 20,000 ft (6,000 m). Vertical clouds often create rain and snow. They are made mostly of water droplets, but when they push up through cold higher levels they may also have ice crystals. They include Cumulus (Cu) and Nimbostratus (Ns)

Towering-vertical clouds are very tall with tops usually higher than 20,000 ft (6,000 m). They can create heavy rain and snow showers. Cumulonimbus, the biggest clouds of all, can also produce thunderstorms. These clouds are mostly made of water droplets, but the tops of very large cumulonimbus clouds are often made mostly of ice crystals.

Rain is when water falls from clouds in droplets that are bigger than 0.5 mm. Droplets of water that are about 0.2mm to 0.45mm big are called drizzle. Rain is a kind of precipitation. Precipitation is any kind of water that falls from clouds in the sky, like rain, hail, sleet and snow.

When the Sun heats the Earth’s surface, the ground heats the air above it. Convection makes the air rise and cool. When it cools to the dew point, clouds form and rain follows. This type of rainfall often causes summer showers and thunderstorms. This type of rainfall is called convectional rainfall.

Relief rain usually occurs along coastal areas where a line of hills runs along the coast. When the wet onshore wind from the sea meets a mountain, hill or any other sort of barrier, it is forced to rise along the slope and cools. When the air temperature falls to its dew point, water vapour condenses to form clouds. When the clouds can no longer hold the water droplets, relief rain begins to fall on the windward slope of the mountain. On the leeward slope, air sinks, it is warmed and further dried by compression. Therefore, the leeward slope is known as the rain shadow. Moist winds blow in from the sea and are forced to rise over the land. The air cools and the water vapour condenses, forming raindrops. Relief rain is also a very dense and cold mixture of precipitation.

Relief or orographic rain is formed when the air is forced to cool when it rises over relief features in the landscape such as hills or mountains. As it rises it cools, condenses and forms rain. The highest annual rainfall totals occur in mountain areas. There is often a rain shadow effect whereby the leeward (downwind) slope receives a relatively small amount of rain.

Frontal rain happens when cooler and warmer, humid air meets in a weather front. The less dense warm air rises and condenses forming clouds. These clouds grow and eventually create rain. In some places in the northern temperate zone, the cold air front tends to come from the North West and the warm air front comes from the south-west.

Other forms of precipitation include snow, sleet, dew, frost, and hail. Fog and mist are not precipitation but suspensions. In that case, the water vapour does not condense sufficiently to precipitate.

Snow forms when water in the atmosphere becomes frozen. When the dew point is below the freezing point of water, water crystallises as snow and falls down. Sleet is a type of precipitation where ice pellets fall down from the sky. When drops of rain fall through a mass of air below freezing point, the drops get solidified to become ice pellets.

Dew is a thin film of water that has condensed on the surface of objects near the ground in the morning or evening. These objects cool in the night. When they cool, the thin layer of air around them cools too. This makes some water vapour condense on the object. Frost is ice that is formed when water vapour freezes onto a surface. It has a white, powdery appearance. It forms on cold surfaces when the temperature of the air is very low.

A piece of hail (called a hailstone) is a lump of ice that falls out of a storm cloud. A hailstone begins as a small water droplet or as a rounded snow pellet in a cloud. The drop grows by collecting many cloud drops. The little drop is blown by a strong wind inside the cloud to where it meets with some extremely cold water drops. These supercooled drops are still liquid water even though the temperature is below freezing. When the little drop mixes with these extremely cold drops, they join, and the little drop has now become a hailstone.

The little hailstone is thrown up inside the cloud, still collecting other cold drops. The hailstone gets bigger and bigger until it goes to the top of the cloud. Then, because there is no more wind, it falls back down through the cloud. While it is falling it gets even bigger as it bangs into more supercooled drops. If it goes down very fast it can hit the Earth at up to 90 mph (144 kph), bouncing like popcorn. If the hailstone hits the dirt, it can actually bury itself.
The patterns of precipitation and wind decide the climate on different places of the earth. Climate means the usual condition of the temperature, humidity, atmospheric pressure, the wind, rainfall, and other meteorological elements in an area of the Earth’s surface for a long time. Climate is different from weather. Weather is the condition of these elements right now, for shorter periods of time that are up to two weeks.

The latitude, ground, and height can change the climate of a location. It is also important to note if oceans or other large bodies of water are nearby. Climates are most commonly classified by temperature and precipitation.
Weather is the day-to-day or hour-to-hour change in the atmosphere. Weather includes wind, lightning, storms, hurricanes, tornadoes, rain, hail, snow, and lots more. Energy from the Sun affects the weather. Climate tells us what kinds of weather usually happen in an area at different times of the year. Changes in weather can affect our mood. We wear different clothes and do different things in different weather conditions. We choose different foods in different seasons, like ice cream in the summer, or hot chocolate in the winter.

Hence geography and climate decide the lifestyle and economic activities in an area and study of geography is essential for understanding the distribution and utilisation of resources.

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