My Science School

          As its name suggests, a warm front has an area of warm, moist air behind it. The warm air rises above the cold air, and clouds are formed along the front. From the ground, the first sign of a warm front approaching is the sight of high, wispy cirrus clouds and maybe some light rain. When the warm front has passed, there is usually a short period of dry weather.

          A warm front is the transition zone that marks where a warm air mass starts replacing a cold air mass. Warm fronts tend to move from southwest to southeast. Normally the air behind a warm front is warmer than the air in front of it. Normally when a warm front passes through an area the air will get warmer and more humid. Warm fronts signal significant changes in the weather. Here are some of the weather signs that appear as a warm front passes over a region.

          First before the warm front arrives the pressure in area start to steadily decrease and temperatures remain cool. The winds tend to blow south to southeast in the northern hemisphere and north to northeast in the southern hemisphere. The precipitation is normally rain, sleet, or snow. Common cloud types that appear would various types of stratus, cumulus, and nimbus clouds. The dew point also rises steadily

          While the front is passing through a region temperatures start to warm rapidly. The atmospheric pressure in the area that was dropping starts to level off. The winds become variable and precipitation turns into a light drizzle. Clouds are mostly stratus type clouds formations. The dew point then starts to level off.

          After the warm front passes conditions completely reverse. The atmospheric pressure rises slightly before falling. The temperatures are warmer then they level off. The winds in the northern hemisphere blow south-southwest in the northern hemisphere and north-northwest in the southern hemisphere. Cloudy conditions start to clear with only cumulonimbus and stratus clouds. The dew point rises then levels off.

          Knowing about how warm fronts work gives a better understanding of how pressure systems interact with geography to create weather. Looking at warm fronts we learn that they are the transition zone between warm humid air masses and cool, dry air masses. We know that these masses interact in a cycle of rising and falling air that alters the pressure of atmosphere causing changes in weather.

Picture Credit : Google

 

 

          Four major masses of air lie over different parts of the world. The tropical maritime mass is warm and moist; the tropical continental mass is hot and dry. The polar continental mass is cold and dry, and the polar maritime mass is cold and wet. These air masses are blown around by high-level winds, and their interactions have a major influence on the world's weather. The kind of weather experienced depends on the nature of the air mass — tropical masses bring warm, humid weather, and the polar masses tend to bring snow. In places where these masses meet, the weather can be very changeable indeed.

          Weather is controlled by a variety of factors. One of the most important is Earth's air masses. Air masses are huge parcels of air with specific characteristics. What's interesting about the characteristics of an air mass is that, not only do they describe the air mass, but they also tell you where you can find that air mass on Earth.

         Let's look at the different types of air masses found on Earth to see how this works. Air masses can be divided into two main categories based on whether they are found over land or water. If the air mass is found over land, this is a continental air mass. If the air mass is found over water, this is a maritime air mass. This makes sense: continental air masses occur over the continents, maritime air masses occur over the water, or marine environments. These categories are represented by a lowercase 'c' for continental or 'm' for maritime.

          The source region of the air mass helps us classify it even further, and for this, we have three categories. Arctic air masses occur over arctic regions, like Greenland and Antarctica. Polar air masses occur a little bit farther from the poles, like in Siberia, Canada and the northern Atlantic and Pacific Oceans.

          Finally, tropical air masses occur in the tropics, so along the equator and over Mexico and the Southwest U.S. Makes sense, right? These categories are represented by the first letter of the source region, but this time we use an uppercase letter. So, 'A' stands for arctic, 'P' for polar and 'T' for tropical. That's pretty easy to remember!

          Each source region can also be either continental or maritime, and to represent this, we simply combine the category letters. This gives us six total types of air masses on Earth: maritime arctic (mA), maritime polar (mP), maritime tropical (mT); and continental arctic (cA), continental polar (cP) and continental tropical (cT).

Air Masses and Weather

          You can understand a lot about weather from air masses just by looking at the name. Maritime air masses are going to produce moist weather because they occur over oceans, and oceans are filled with water! The air blowing over the ocean regions, either arctic, polar or tropical, picks up that moisture as it travels along. In maritime arctic and polar regions, this moist air is cool (as you probably expected), and the maritime tropical air mass produces the warm, humid conditions you would expect along the tropics, like Florida and the Caribbean.

          In contrast, continental air masses produce dry weather. This is because the continents just can't compete with the oceans when it comes to moisture! The continental arctic and polar air masses produce dry, cold weather in the winter and pleasant weather conditions in the summer.

Picture Credit : Google

          When different air masses meet, varying pressure differences cause two things to happen. Warm air either bulges into the cold air, or the cold air pushes into the warm air. The collision causes the warm air to rise rapidly over the cold air, creating an area of low pressure called a frontal depression. The weather in this area becomes very unsettled and is worse when the differences in pressure and temperature are greatest. Depressions cover huge areas but tend to pass over in less than a day.

          Cloud formation occurs when humid or water vapor-filled air rises to the point where cooler temperatures force condensation. This often involves the movement of air masses, which are large bodies of air with similar temperatures and moisture content. Air masses are typically at least 1,000 miles (1,600 km) wide and several miles thick.

Four naturally occurring mechanisms on Earth cause air to rise:

Orographic lifting: This phenomenon occurs when an airflow encounters elevated terrains, such as mountain ranges. Like a speeding car heading toward a hill, the wind simply powers up the slope. As it rises with the topography, water vapor in the airflow condenses and forms clouds. This side of the mountain is called the windward side and typically hosts a great deal of cloud cover and precipitation. The other side of the mountain, the leeward side, is generally less lucky. The airflow loses much of its moisture in climbing the windward side. Many mountain ranges virtually squeeze incoming winds like a sponge and, as a result, their leeward sides are home to dry wastes and deserts.

Frontal wedging: When a warm air mass and a cold air mass collide, you get a front. Remember how low-pressure warm air rises and cold high-pressure air moves into its place? The same reaction happens here, except the two forces slam into each other. The cold air forms a wedge underneath the warm air, allowing it to basically ride up into the troposphere on its back and generate rain clouds. There are four main kinds of fronts, classified by airflow momentum. In a warm front, a warm air mass moves into a cold air mass. In a cold front, the opposite occurs. In a stationary front, neither air mass advances. Think of it as two fronts bumping into each other by accident. In an occluded front, a cold front overtakes a moving warm front, like an army swarming over a fleeing enemy.

Convergence: When two air masses of the same temperature collide and neither is willing to go back down, the only way to go is up. As the name implies, the two winds converge and rise together in an updraft that often leads to cloud formation.

Localized convective lifting: Remember the city example? This phenomenon employs the exact same principle, except on a smaller scale. Unequal heating on the Earth's surface can cause a pocket of air to heat faster than the surrounding air. The pocket ascends, taking water vapor with it, which can form clouds. An example of this might be a rocky clearing in a field or an airport runway, as both absorb more heat than the surrounding area.

Picture Credit : Google

          Swirling masses of high- and low-pressure air are constantly moving around the Earth. When two masses of air with different characteristics meet, they do not mix, and a boundary develops between them. This boundary is called a front. On the ground, the arrival and departure of a front is felt by sharp changes in the weather.

          A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena outside the tropics. In surface weather analyses, fronts are depicted using various colored triangles and half-circles, depending on the type of front. The air masses separated by a front usually differ in temperature and humidity.

          Cold fronts may feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Warm fronts are usually preceded by stratiform precipitation and fog. The weather usually clears quickly after a front's passage. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift.

          Cold fronts and occluded fronts generally move from west to east, while warm fronts move poleward. Because of the greater density of air in their wake, cold fronts and cold occlusions move faster than warm fronts and warm occlusions. Mountains and warm bodies of water can slow the movement of fronts.When a front becomes stationary—and the density contrast across the frontal boundary vanishes—the front can degenerate into a line which separates regions of differing wind velocity, known as a shearline. This is most common over the open ocean.

Picture Credit : Google

 

 

          Severl Major bands of high-and low-pressure areas exist in different parts of the world. Air moves from the areas of high pressure to the low-pressure areas. The movements between these areas contribute to the world's winds and weather patterns.

          If an isobar chart is observed, it can be seen that pressure is not distributed uniformly in the atmosphere around our planet: there are areas with a lower pressure than the surrounding areas and areas where the pressure is higher. Due to a characteristic of gases, air tends to move from high pressure areas towards those with low pressure in an attempt to balance the difference. The presence of high and low pressure areas is therefore the principal motor of all meteorological phenomena, in other words, of the ‘weather’. Hence, it is important to understand how air circulates close to these areas (see graph) and how they are distributed in the atmosphere.

Anticyclones

          In high pressure zones, air tends to sink towards the ground causing the air that is present to move away with a divergent movement. The air gets compressed while descending and tends to disperse the clouds, and in fact high pressure conditions are associated with settled and calm weather. As a result of the Coriolis effect, air tends to move away from the high pressure system, clockwise in our hemisphere and anticlockwise in the Southern Hemisphere (anticyclonic circulation).

Cyclones

          A low pressure area, instead, tends to attract air from the surrounding region where the pressure is higher. Near the centre of the cyclone, air tends to rise higher attracting a growing amount of air from the neighbouring areas. On rising, air expands and cools with the subsequent formation of clouds and precipitation: it is for this reason that low pressure areas are usually associated with bad weather. Air tends to converge towards the low pressure centre with an anticlockwise movement in our hemisphere and a clockwise movement in the Southern Hemisphere (cyclonic circulation).

Circulation cells

          Temperature and pressure differences are not distributed casually in the atmosphere but permanent and stable low and high pressure areas can be identified, which are organized so as to form big circulation cells around the world (Mean Annual Isobars, Isobars in the month of July, Isobars in the month of January). This situation, obviously, is not static and unchangeable. During the year the circulation cells move towards the North or the South, depending on the unequal amount of solar energy that the different regions of the Earth receive in each season: in our hemisphere, they move towards the Equator in winter and towards the Poles in summer.

          Three main circulation cells can be identified in each hemisphere that are placed symmetrically respect to the Equator.

Picture Credit : Google

 

          The equtor receives the greatest amount of the Sun's heat, making the land very hot. This heats up the air, creating a large area of mainly low pressure. This area is known as the Intertropical Convergence Zone (ITCZ).

          The ITCZ (Intertropical Convergence Zone) play important role in the global circulation system and also known as the Equatorial Convergence Zone or Intertropical Front. It is a basically low pressure belt encircling Earth near the Equator. It is a zone of convergence where the trade winds meet. Here, we are giving the concept, causes and impact of ITCZ (Intertropical Convergence Zone) for general awareness.

          It is a zone between the northern and southern hemisphere where winds blowing equator-ward from the mid latitudes and winds flowing poleward from the tropics meet. It shifts from north and south seasonally according to the movement of the Sun. For Example- when the ITCZ is shifted to north of the Equator, the southeast trade wind changes to a southwest wind as it crosses the Equator. The ITCZ shifts only between 40° to 45° of latitude north or south of the equator based on the pattern of land and ocean.

          ITCZ (Intertropical Convergence Zone) is caused by the convergence of northeast and southeast trade winds in the area encircling Earth near the Equator. For better understanding, we must know about the trade winds and air masses.

1. Trade Winds: Easterly winds that circle the Earth near the equator.

2. Air Masses: A volume of air defined by its temperature and water vapour content. In tropical latitudes this air mass is hot to very hot, with high relative humidity, bringing unstable weather.

         It appears as a band of clouds consisting of showers, with occasional thunderstorms, that encircles the globe near the equator due to the convergence of the trade winds.

Picture Credit : Google

 

 

          At the altitude at which many jet aircraft fly, the air pressure is extremely low —less than the pressure inside the human body. This makes it impossible for the body to take in air. There is also very little oxygen, so the air inside the plane has to be pressurized in order to simulate the level of air pressure on the surface.

          Although aircraft cabins are pressurized, cabin air pressure at cruising altitude is lower than air pressure at sea level. At typical cruising altitudes in the range 11 000–12 200 m (36 000–40 000 feet), air pressure in the cabin is equivalent to the outside air pressure at 1800–2400 m (6000–8000 feet) above sea level. As a consequence, less oxygen is taken up by the blood (hypoxia) and gases within the body expand. The effects of reduced cabin air pressure are usually well tolerated by healthy passengers.

Oxygen and hypoxia

          Cabin air contains ample oxygen for healthy passengers and crew. However, because cabin air pressure is relatively low, the amount of oxygen carried in the blood is reduced compared with that at sea level. Passengers with certain medical conditions, particularly heart and lung diseases and blood disorders such as anaemia (in particular sickle-cell anaemia), may not tolerate this reduced oxygen level (hypoxia) very well. Some of these passengers are able to travel safely if arrangements are made with the airline for the provision of an additional oxygen supply during flight. However, because regulations and practices differ from country to country and between airlines, it is strongly recommended that these travellers, especially those wishing to carry their own oxygen, contact the airline early in their travel plans. An additional charge is often levied on passengers who require supplemental oxygen to be provided by the airline.

Gas expansion

           As the aircraft climbs in altitude after take-off, the decreasing cabin air pressure causes gases to expand. Similarly, as the aircraft descends in altitude before landing, the increasing pressure in the cabin causes gases to contract. These changes may have effects where air is trapped in the body.

          Passengers often experience a “popping” sensation in the ears caused by air escaping from the middle ear and the sinuses during the aircraft’s climb. This is not usually considered a problem. As the aircraft descends in altitude prior to landing, air must flow back into the middle ear and sinuses in order to equalize pressure. If this does not happen, the ears or sinuses may feel as if they are blocked and pain can result. Swallowing, chewing or yawning (“clearing the ears”) will usually relieve any discomfort. As soon as it is recognized that the problem will not resolve itself using these methods, a short forceful expiration against a pinched nose and closed mouth (Valsalva manoeuvre) should be tried and will usually help. For infants, feeding or giving a pacifier (dummy) to stimulate swallowing may reduce the symptoms.

          Individuals with ear, nose and sinus infections should avoid flying because pain and injury may result from the inability to equalize pressure differences. If travel cannot be avoided, the use of decongestant nasal drops shortly before the flight and again before descent may be helpful.

          As the aircraft climbs, expansion of gas in the abdomen can cause discomfort, although this is usually mild.

         Some forms of surgery (e.g. abdominal surgery) and other medical treatments or tests (e.g. treatment for a detached retina) may introduce air or other gases into a body cavity. Travellers who have recently undergone such procedures should ask a travel medicine physician or their treating physician how long they should wait before undertaking air travel.

Picture Credit : Google

          An instrument called a barometer is used to measure air pressure. A mercury barometer consists of a glass tube standing in an open dish of mercury. The air pressure pushes against the mercury and forces it up the tube. The level of the mercury is recorded against a scale. Mercury barometers are clumsy, and mercury is poisonous, so aneroid barometers are more commonly used. A sealed metal box inside the barometer is connected to the pointer on the clock-like face. The vacuum inside the metal box means that an increase in pressure will squash it; a drop in pressure will make it expand. These changes make the pointer move around the dial.

          Even though we can’t see air, it is real and has pressure. The pressure of the atmosphere changes. It is higher at sea level, and lessens as you go higher up in the atmosphere. Some weather systems have slightly higher pressure than others – you may have heard of High pressure and Low Pressure weather system.

          Let's look at how atmospheric pressure is measured. For a long time, atmospheric pressure has been measured by a mercury barometer. The first was invented in 1643 by one of Galileo’s assistants. A mercurial barometer has a section of mercury exposed to the atmosphere. The atmosphere pushes downward on the mercury (see image). If there is an increase in pressure, it forces the mercury to rise inside the glass tube and a higher measurement is shown. If atmospheric pressure lessens, downward force on the mercury lessens and the height of the mercury inside the tube lowers. A lower measurement would be shown. This type of instrument can be used in a lab or a weather station, but is not easy to move! Measurements from a mercury barometer are usually made in inches of Mercury (in Hg).

          An aneroid barometer can be used in place of a mercury barometer. It is easier to move and is often easier to read. This instrument contains sealed wafers that shrink or spread out depending on changes of atmospheric pressure. If atmospheric pressure is higher, the wafers will be squished together. If atmospheric pressure lessens, it allows the wafers to grow bigger. The changes in the wafers move a mechanical arm that shows higher or lower air pressure (see image).

          Either a mercury barometer or an aneroid barometer can be set up to make constant measurements of atmospheric pressure. Then it is called a barograph (see image). The barograph may constantly record pressure on paper or foil wrapped around a drum that makes one turn per day, per week, or per month. Nowadays, many mechanical weather instruments have been replaced by electronic instruments that record atmospheric pressure onto a computer.

          Atmospheric pressure can be recorded and reported in many different units. This can get a little confusing! As mentioned, a mercury barometer makes measurements in inches of Mercury (in Hg). Pounds per square inch (abbreviated as p.s.i.) is common in the English system of units, and the pascal (abbreviated Pa) is the standard in the Metric (SI) system. Since the pressure exerted by Earth's atmosphere is of great importance, pressure is sometimes expressed in terms of "atmospheres" (abbreviated atm). In weather, the bar and millibar (mb) describe pressure. You'll often hear millibar used by meteorologists when describing low or high pressure weather systems.

Picture Credit : Google

 

 

          An area of high pressure is created where the air is cold. The cold air sinks, pushing down and creating high pressure. This causes the air molecules to be squashed together, creating heat. As the air warms up, it tends to bring warm and pleasant weather.

Since surface air pressure is a measure of the weight of the atmosphere above any location, a high pressure area represents a region where there is somewhat more atmosphere overlying it. High pressure areas are usually caused by air masses being cooled, either from below (for instance, the subtropical high pressure zones that form over relatively cool ocean waters to the west of Califormia, Africa, and South America), or from above as infrared cooling of winter air masses over land exceeds the warming of those airmasses by sunlight. As the airmass cools, it shrinks, allowing air from the surroundings to fill in above it, thus increasing thte total mass of atmosphere above the surface, which then results in higher surface barometric pressures. The pressure difference between the high pressure area and its lower-pressure surroundings causes a wind to develop flowing from higher to lower pressure. But because of the rotation of the Earth, the wind is deflected to the right (in the Northern Hemisphere) which then causes the wind to flow in a clockwise direction around the high pressure zone.

          In an anticyclone, air masses drop extensively. At the same time, the air warms itself up, so that no condensation and consequently no cloud formation can take place. Near to the ground, the air flows out of the anticyclone in the direction of depression - it diverges. Hence, there is no formation of fronts in altitude. During the subsidence of the air masses, an inversion forms. That is where the clouds are dissolved.

An anticyclone is builded quiet slowly. The forces of circulation in the subtropic areas lead to stable anticyclones.

          Because of the differences in the origin or development, the anticyclones are divided into three categories:

          A cold anticyclone originates if air cools off, for example, in winter above a cool land mass (e.g., Central Asian high). Then the air has a bigger density and exerts a higher pressure on the base. In the middle latitudes, it can also originate in the form of flat wedges in the back of cyclones as a ridge of high pressure.

          A dynamic anticyclone is generated by the Rossby-waves (Polar front, Jet Stream). The dynamic Azores anticyclone exerts, on this occasion, a big influence on the weather of Central Europe.

         A high anticyclone is an anticyclone which appears at big heights and is thus shown in high weather maps. It is always connected with a ground low-pressure area, because with the warming of the surfaces, the vertical pressure gradient is lowered and reflects itself the relative atmospheric pressure reduction on the ground with increasing height in a pressure relatively higher to the horizontal surroundings. Hence, one can derive the other way around a height low-pressure area also from a ground anticyclone (also thermal anticyclone).

Picture Credit : Google

 

 

 

 

         AN AREA of warm air can create low pressure because warm air rises, reducing the level of air pressure. If the warm air evaporates water on the surface, clouds may form, producing the rain and bad weather associated with low pressure.

Since surface air pressure is a measure of the weight of the atmosphere above any location, a low pressure area represents a region where there is somewhat less atmosphere overlying it. Low pressure areas form when atmospheric circulations of air up and down remove a small amount of atmosphere from a region. This usually happens along the boundary between warm and cold air masses by air flows "trying" to reduce that temperature contrast. The air flows that develop around the low pressure system then help to accomplish that reduction of contrast in temperature, with the colder air flowing under the warmer air mass, and the warmer air flowing over the colder air mass. "Thermal lows" occur when an air mass warms, either from being over a warm land or ocean surface. For instance, a very weak thermal low forms over islands heated by the sun, which then causes a sea breeze to form with oceanic air flowing toward the island. Similarly, very cold winter air flowing over the Great Lakes produces localized low pressure over the relatively warmer lake waters. Low pressure can be enhanced by the air column over it being warmed by condensation of water vapor in large rain or snow systems. The warming causes the air layer to expand upward and outward, removing some of the air from the column, and thus reducing the surface air pressure. The most extreme example of this is the intense low pressure that forms in the eye of a hurricane, where latent heat release from rain formation causes warming of the air column within the eye. It the most intense hurricanes and typhoons, over 10% of the atmosphere can be removed from the eye of the storm through this process. But outside of the tropics, as mentioned above, low pressure centers are usually associated with extratropical cyclone systems, along with their fronts and precipitation systems.

Interesting facts:

The lowest air pressure in the world occurs in intense tropical cyclones where condensation of water vapor to form clouds and rain releases heat that warms the air column in the eye of the storm. The lowest pressure ever recorded was in the eye of Typhoon Tip, in the tropical western Pacific Ocean, on October 12, 1979: 25.69 inches of mercury (870 millibars). Since average sea level pressure is 29.92 inches (1013.23 millibars), this record pressure was about 14% lower than normal, indicating that 14% of the atmosphere's mass had been removed from the column of air.

 

Picture Credit : Google

          Air pressure is created by the effect of gravity pulling the atmosphere towards the Earth. It can vary according to temperature, causing different amounts of pressure in different parts of the world. It also changes according to altitude —pressure is greater at sea level because there is more air pushing down than there is at higher altitudes.

          The air around you has weight, and it presses against everything it touches. That pressure is called atmospheric pressure, or air pressure. It is the force exerted on a surface by the air above it as gravity pulls it to Earth.

          Atmospheric pressure is commonly measured with a barometer. In a barometer, a column of mercury in a glass tube rises or falls as the weight of the atmosphere changes. Meteorologists describe the atmospheric pressure by how high the mercury rises.

          An atmosphere (atm) is a unit of measurement equal to the average air pressure at sea level at a temperature of 15 degrees Celsius (59 degrees Fahrenheit). One atmosphere is 1,013 millibars, or 760 millimeters (29.92 inches) of mercury.

          As the pressure decreases, the amount of oxygen available to breathe also decreases. At very high altitudes, atmospheric pressure and available oxygen get so low that people can become sick and even die.

          Mountain climbers use bottled oxygen when they ascend very high peaks. They also take time to get used to the altitude because quickly moving from higher pressure to lower pressure can cause decompression sickness.

          Decompression sickness, also called "the bends", is also a problem for scuba divers who come to the surface too quickly.

          Aircraft create artificial pressure in the cabin so passengers remain comfortable while flying.

          Atmospheric pressure is an indicator of weather. When a low-pressure system moves into an area, it usually leads to cloudiness, wind, and precipitation. High-pressure systems usually lead to fair, calm weather.

Picture Credit : Google

          The layer directly above the troposphere is called the stratosphere. The stratosphere is warmer than the upper part of the troposphere and this warm, relatively heavy air acts like a lid, trapping clouds in the troposphere. Going up through the layers, the air gets thinner and thinner — only in the lower parts of the troposphere is there enough air to breathe normally.

          The stratosphere is a layer of Earth's atmosphere. It is the second layer of the atmosphere as you go upward. The troposphere, the lowest layer, is right below the stratosphere. The next higher layer above the stratosphere is the mesosphere.

          The bottom of the stratosphere is around 10 km (6.2 miles or about 33,000 feet) above the ground at middle latitudes. The top of the stratosphere occurs at an altitude of 50 km (31 miles). The height of the bottom of the stratosphere varies with latitude and with the seasons. The lower boundary of the stratosphere can be as high as 20 km (12 miles or 65,000 feet) near the equator and as low as 7 km (4 miles or 23,000 feet) at the poles in winter. The lower boundary of the stratosphere is called the tropopause; the upper boundary is called the stratopause.

          Ozone, an unusual type of oxygen molecule that is relatively abundant in the stratosphere, heats this layer as it absorbs energy from incoming ultraviolet radiation from the Sun. Temperatures rise as one moves upward through the stratosphere. This is exactly the opposite of the behavior in the troposphere in which we live, where temperatures drop with increasing altitude. Because of this temperature stratification, there is little convection and mixing in the stratosphere, so the layers of air there are quite stable. Commercial jet aircraft fly in the lower stratosphere to avoid the turbulence which is common in the troposphere below.

          The stratosphere is very dry; air there contains little water vapor. Because of this, few clouds are found in this layer; almost all clouds occur in the lower, more humid troposphere. Polar stratospheric clouds (PSCs) are the exception. PSCs appear in the lower stratosphere near the poles in winter. They are found at altitudes of 15 to 25 km (9.3 to 15.5 miles) and form only when temperatures at those heights dip below -78° C. They appear to help cause the formation of the infamous holes in the ozone layer by "encouraging" certain chemical reactions that destroy ozone. PSCs are also called nacreous clouds.

          Air is roughly a thousand times thinner at the top of the stratosphere than it is at sea level. Because of this, jet aircraft and weather balloons reach their maximum operational altitudes within the stratosphere.

          Due to the lack of vertical convection in the stratosphere, materials that get into the stratosphere can stay there for long times. Such is the case for the ozone-destroying chemicals called CFCs (chlorofluorocarbons). Large volcanic eruptions and major meteorite impacts can fling aerosol particles up into the stratosphere where they may linger for months or years, sometimes altering Earth's global climate. Rocket launches inject exhaust gases into the stratosphere, producing uncertain consequences.

          Various types of waves and tides in the atmosphere influence the stratosphere. Some of these waves and tides carry energy from the troposphere upward into the stratosphere; others convey energy from the stratosphere up into the mesosphere. The waves and tides influence the flows of air in the stratosphere and can also cause regional heating of this layer of the atmosphere.

          A rare type of electrical discharge, somewhat akin to lightning, occurs in the stratosphere. These "blue jets" appear above thunderstorms, and extend from the bottom of the stratosphere up to altitudes of 40 or 50 km (25 to 31 miles).

Picture Credit : Google

          The Troposphere is sometimes called the weather layer. Here, the air is constantly moving as it is heated and cooled in a process known as convection. Clouds form as water in the atmosphere evaporates and then condenses. This movement of air, heat and water creates the world’s weather systems.

          The height of the troposphere varies between different areas of the Earth. At the Equator, for example, it stretches to about 20km (12 miles) above the surface. At the poles, the layer reaches a height of about 10km (6 miles).

          The troposphere is the lowest layer of Earth's atmosphere. The troposphere extends from Earth's surface up to a height of 7 to 20 km (4 to 12 miles, or 23,000 to 65,000 feet) above sea level. Most of the mass (about 75-80%) of the atmosphere is in the troposphere, and almost all weather occurs within this layer. Air is warmest at the bottom of the troposphere near ground level. As one rises through the troposphere the temperature decreases. Air pressure and the density of the air also decrease with altitude. The layer immediately above the troposphere is called the stratosphere.

         Nearly all of the water vapor and aerosol particles in the atmosphere are in the troposphere. Because of this, most clouds are found in this lowest layer as well.

          The troposphere is heated from below; sunlight warms the ground or ocean, which in turn radiates the heat into the air immediately above it. Temperature drops off at a rate of about 6.5° C per km (about 3.6° F per thousand feet) of increased altitude within the troposphere. This is why mountaintops are much cooler than lower elevations nearby. Since warm 'parcels' of air are less dense than colder air, warmer air is buoyant and tends to rise up from Earth's surface towards the top of the troposphere. If you've ever watched cumulonimbus thunderstorm clouds form and grow on a hot summer day, you've seen this rising air in action. The air in the troposphere is 'well mixed' because it is constantly churning and 'turning over' as warm air at the surface rises and colder, denser air at altitude descends to take its place. This is not the case for all layers in the atmosphere. At the top of the troposphere the temperature drops to a chilly -55° C (-64° F)!

          The boundary between the top of the troposphere and the layer above it, the stratosphere, is called the tropopause. The height of the tropopause (and thus the top of the troposphere) varies with latitude, season, and between day and night. The troposphere is thickest in the tropics, where the top of the layer can be as high as about 20 km (12 miles or 65,000 feet) above sea level. At mid-latitudes, the typical height of the tropopause is around 11 km (7 miles or 36,000 feet), while near the poles it can dip down to as low as 7 km (4 miles or 23,000 feet). The jet stream, a fast-moving "river of air" that can zip along at speeds up to 400 km/hr (250 mph), is located just below the tropopause.

         Air gets 'thinner' with increasing altitude. That's why mountain climbers sometimes need bottled oxygen to breathe, and why it is so easy to get 'winded' while hiking in high mountains or even visiting someplace at elevation.

          The lowest part of the troposphere, right next to the surface of Earth, is called the "boundary layer". Differences in the surface texture (mountains, forests, flat water or ice) affect winds in the boundary layer.

Picture Credit : Google

          Rising temperatures cause sea levels to rise in two ways. A warmer sea is less dense, so its volume increases and the level rises as it expands. A warmer climate can also cause glaciers to melt into the sea, raising its level.

          The term sea-level rise generally designates the average long-term global rise of the ocean surface measured from the centre of the earth (or more precisely, from the earth reference ellipsoid), as derived from satellite observations. Relative sea-level rise refers to long-term average sea-level rise relative to the local land level, as derived from coastal tide gauges.

          Sea levels are highly variable over periods ranging from seconds to decades. Sea-level rise is the rising trend averaged over longer periods, which is observed at many coastal stations since a few centuries. It is almost certain that global warming due to human emissions of greenhouse gases is responsible for steepening this trend since at least a few decades. The most recent projections for future sea-level rise are presented in the Special IPCC Report on the Ocean and Cryosphere in a Changing Climate (2019). This report is an update of the previous IPCC AR5 report (2013), and includes newer insights in the response of the Greenland and Antarctic ice sheets to global warming. It also provides an estimation of the possible sea-level rise up to the year 2030, see Fig. 1. Two scenarios for greenhouse gas emissions are considered in this figure: (1) a "low" scenario, called RCP2.6, with strong reduction of global greenhouse gas emission, such that global warming will probably not exceed 2 oC; (2) a "high" scenario, called RCP8.5, in which no measures are taken to limit greenhouse gas emissions ('business as usual'). The high scenario can lead to a rise of up to 5 m of the global average sea level in 2300, but with great uncertainty.

          Several phenomena contribute to sea-level rise. On a global scale, sea-level rise is mainly due to an increase of the water mass and water volume of the oceans. This global sea-level rise (often termed Eustatic sea-level rise) has three components:

(1) thermal expansion of ocean waters related to decrease of the density (also referred to as thermo-steric component of sea-level rise, related to increasing temperature),

(2) water mass increase, which is mainly due to melting of mountain glaciers and decrease of the Greenland and Antarctic ice sheets, and

(3) decreasing storage of surface water and groundwater on land.

Other phenomena can substantially influence sea levels at regional scale, inducing either sea-level rise or sea-level fall. Most important are:

(4) vertical earth crust motions - in particular earth crust adjustment to melting of polar ice caps, the so-called isostatic rebound,

(5) land surface subsidence, related in particular to extraction of groundwater and oil/gas mining and compaction of soft deltaic soils,

(6) changes in the earth gravitational field, related in particular to decrease of the Greenland and Antarctic ice sheets,

(7) regional atmospheric pressure anomalies and changes in the strength and distribution of ocean currents, related in particular to ocean-atmosphere interaction, and

(8) regional sea-level change related to changes in seawater salinity.

          Due to these phenomena, sea-level rise is not uniform around the globe, but differs from place to place. Relative sea-level rise is the locally observed rise of the average sea level with respect to the land level. It is the sum of the components (1-8).

Picture Credit : Google