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Structure and Composition of the Atmosphere
The atmosphere, or the gaseous envelope surrounding Earth, is described here as a layer approximately 480 kilometres thick. The article divides it into seven layers. It states that the atmosphere contains approximately 21% oxygen, 78% nitrogen, and 1% other gases. An important point is that about half of the total atmosphere lies within roughly six kilometres of Earth's surface.
Layers of the Atmosphere
1) Troposphere. The article explains tropo as a Greek word meaning “change” or “variable,” while sphere means “sphere.” Temperature generally decreases with increasing altitude in this layer. The height of the troposphere varies with latitude: it is lower near the poles and higher near the equator. Its average height is given as about 12 kilometres.
2) Tropopause. The tropopause is presented as the region where the temperature change pauses. Its thickness is given as about two kilometres, with temperature remaining approximately constant through the layer.
3) Stratosphere. The stratosphere is described as a layered region that absorbs harmful ultraviolet radiation. Because ozone is abundant here, this region is also associated with the ozone layer. Unlike the troposphere, temperature increases with altitude.
4) Mesosphere. This layer lies near the middle of the seven atmospheric regions described in the article. Temperature again decreases with altitude, with very low temperatures possible.
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5) Thermosphere. In the thermosphere, temperature rises sharply with altitude; the source gives values reaching about 1,500°C.
6) Exosphere. In this outer region, Earth's gravitational influence on atmospheric molecules becomes very weak and particles can escape outward.
7) Ionosphere. The ionosphere is described as a very thick region in which atmospheric molecules become ionized. The source discusses estimates extending across a broad altitude range.
Types of Clouds
Low clouds extend from near the surface to roughly two kilometres. The article lists cumulus (CU), stratus (ST), stratocumulus (SC), and fog, which it describes as stratus cloud in contact with the ground.
Middle clouds are described as extending from about two to seven kilometres. The listed forms include altocumulus (AC), altostratus (AS), nimbostratus (NS), nimbocumulus (NC), and mixed nimbostratus-cumulus forms. These clouds are associated in the source with much rain, snow, and hail activity.
High clouds are described as occurring from roughly five to fourteen kilometres. Their water is largely in the form of ice crystals because of the low temperature. The source lists cirrus (CI), cirrocumulus, and cirrostratus.
Clouds with extensive vertical development (EVDC) can produce intense rain and hail. Their bases may begin only a few hundred metres above the surface, while vigorous vertical currents can carry their tops to much greater heights. Towering clouds (TC) and cumulonimbus clouds (CB) are listed. Cumulonimbus is described as one of the most active and hazardous cloud types, capable of torrential rain, hail, strong winds, and thunderstorms.
Scud clouds are fragmented clouds often seen after strong weather activity. The source discusses fractocumulus (FC) and fractostratus (FS) as clouds breaking apart or being driven away after precipitation.
Atmospheric Circulation
The primary cause of weather phenomena is the uneven heating of Earth's surface by the Sun. Solar radiation sets the atmosphere in motion. Uneven heating changes air density, produces circulation patterns, and causes changes in atmospheric pressure.
Meteorologists identify locations with different pressures and connect locations of equal pressure with lines called isobars. Widely spaced isobars indicate a weak pressure gradient, while closely spaced isobars indicate a strong gradient. Pressure patterns help identify highs, lows, ridges, troughs, and cols.
Air tends to move from colder, denser, higher-pressure areas toward warmer, less-dense, lower-pressure areas. Wind speed depends partly on the strength of the pressure gradient. High-pressure systems and ridges are generally associated with descending air, which tends to disperse clouds and is therefore often connected with good visibility, light winds, and relatively little cloud. In low-pressure systems and troughs, converging air is forced upward, encouraging cloud and precipitation and often producing poorer visibility and more unsettled weather.
A col can describe a relatively neutral region between two highs and two lows, or the intersection of ridges and troughs. The pressure-gradient force initiates air movement. Because Earth rotates, the Coriolis force deflects moving air; its influence is small near the equator and increases toward the poles. It also becomes more important as the speed of the moving air increases.
The pressure-gradient and Coriolis forces work together in determining wind. In the Northern Hemisphere, circulation around high pressure is clockwise and circulation around low pressure is counter-clockwise. As the forces approach balance, wind tends to flow more nearly parallel to the isobars.
Humidity
Weather depends strongly on the amount of moisture in the air. Water in the atmosphere occurs as solid, liquid, and gas. Water vapour is added to the atmosphere through evaporation and sublimation and is removed through condensation and deposition/sublimation processes.
Sublimation is described as a direct change between ice and water vapour without passing through the liquid state. Melting changes solid water to liquid, while freezing changes liquid water to solid. Every change of state involves an exchange of heat.
When water evaporates, heat is absorbed as latent heat of evaporation. When water vapour condenses, heat is released; this latent heat is important in cloud formation. As air temperature decreases, the amount of water vapour the air can contain decreases. Saturation occurs at the dew point, when relative humidity reaches 100%.
As air cools toward saturation, condensation or deposition makes previously invisible water vapour visible. Condensation forms clouds, fog, and dew. Clouds consist of tiny water droplets or ice crystals; a cloud formed at the surface is called fog. The source notes that when air temperature and dew point become very close, the air is approaching saturation.
When cloud droplets grow too large to remain suspended, they fall as rain. Liquid water below freezing is called supercooled water; it may freeze on contact with an aircraft or the ground. On cold nights, surfaces can cool below the dew point and collect dew. If the dew point is below freezing, frost may form.
In clouds with strong vertical currents and low temperatures, frozen droplets can collide and grow until the air currents can no longer support them, at which point they fall as hail. Snow can melt while passing through warmer layers below and reach the ground as rain.
Virga is precipitation that falls from a cloud but evaporates again in warmer or drier air before reaching the ground.
Atmospheric Stability
Stability is the atmosphere's resistance to vertical motion. The stability of an air mass determines whether it tends to rise or sink relative to the surrounding air. Stable air resists vertical motion; unstable air favours it.
Temperature and humidity together influence stability. The article describes warm, humid air as especially favourable to instability and uses tropical regions with frequent convective storms as an example. Cold, dry winter air in polar regions is given as an example of strongly stable conditions.
Rising air expands as it enters lower pressure and cools; descending air is compressed by increasing pressure and warms. A temperature change caused by expansion or compression without heat being added to or removed from the parcel is called an adiabatic temperature change. The decrease of temperature with increasing altitude is called the lapse rate.
The source gives different lapse rates for dry and moist air and explains that moist air generally cools more slowly than dry air. This difference can be used as an indicator of atmospheric stability. When warm, moist air rises, cumulus clouds can form at the altitude where air temperature and dew point meet. The article also notes that these temperature and dew-point changes can be used to estimate cloud level.
Sometimes temperature increases rather than decreases with height. This is a temperature inversion. An inversion can occur near the surface or aloft and can act as a lid that traps air and pollutants. Inversions are often associated with stable conditions and weak wind or turbulence. A frontal inversion can occur when colder air is trapped beneath warmer air.
Stable and unstable air have recognizable patterns of cloud, precipitation, visibility, turbulence, and icing. Stable air forced up a slope may produce stratus clouds, while unstable moist air lifted along a slope can produce clouds with strong vertical development.
Air Masses
A large body of air that covers a broad region and acquires the physical characteristics of that region is called an air mass. The article divides air masses broadly into tropical and polar categories, with continental forms tending to be drier and maritime forms tending to be more humid.
Heating an air mass from below reduces stability and encourages cumulus development. Cooling it from below increases stability and favours stratus. An air mass that gains water vapour while passing over seas and oceans becomes less stable; after prolonged precipitation removes moisture, stability may gradually increase and the weather may improve.
Fronts
When two air masses with different physical properties meet, the boundary between them is called a front. The source associates fronts with low-pressure systems and describes four important changes along fronts: temperature, dew point, pressure, and wind.
Cold front. Denser cold air advances along the surface and forces warmer, less-dense air upward. The article associates cold fronts with cumulus-type clouds, turbulence, precipitation, strong and gusty winds, and clearer air with better visibility after passage.
Fast-moving cold front. Highly unstable air and strong convective clouds may produce intermittent heavy rain or hail. The strongest activity may occur along a squall line, with thunderstorms and torrential rain.
Warm front. The source associates warm fronts with stratus clouds when the air is moist and stable, relatively little turbulence except in unstable air, precipitation ahead of the front, reduced visibility with haze or fog, and precipitation spread over a broad area. Stable warm moist air favours layered clouds, whereas unstable warm moist air favours convective clouds.
Stationary front. When the opposing forces of two air masses are nearly balanced, the boundary remains nearly stationary and may influence local conditions for several days.
Occluded front. An occlusion forms when a faster cold front catches a slower warm front. The article distinguishes cold-front occlusion and warm-front occlusion according to the relative temperatures of the air masses involved.
A frontal wave can develop as a disturbance along a slow-moving cold front or stationary front. If it intensifies, a low-pressure centre develops and portions of the boundary begin behaving as warm and cold fronts. Because the cold front normally moves faster, it can eventually catch the warm front and form an occlusion. Weather can change rapidly during these stages.
Turbulence
Sudden, irregular atmospheric motions are called turbulence. The article lists several types:
- Thermal or convection turbulence: produced by rising warm air, including moist and dry convective forms.
- Mechanical or surface-friction turbulence: produced when wind is disturbed by terrain or surface obstacles.
- Mountain turbulence: produced by airflow over mountainous terrain.
- Wake turbulence: produced behind a moving object such as an aircraft, vehicle, or train.
Turbulence intensity is classified as light, moderate, severe, or extreme. Conditions favourable for turbulence include unstable air, lifting mechanisms such as terrain or heated surfaces, and moisture.
The source then discusses very intense storm circulation, describing thunderstorms and the larger rotating storm systems known as hurricanes or typhoons. It compares a storm to an uncontrolled engine whose fuel is water vapour, whose starting mechanism is rising warm air, and whose energy is supplied by latent heat.
Jet Stream
A jet stream is described as a strong current of wind flowing along a roughly horizontal axis at speeds exceeding 90 km/h. The article portrays jet streams as elongated tunnels of high-speed wind encircling the globe. It gives approximate dimensions and wind speeds, notes that their positions shift poleward in warmer seasons and equatorward in winter, and states that their winds commonly reach roughly 185–280 km/h, sometimes considerably more.
Thunder, Lightning, and Saint Elmo's Fire
The article explains thunder and lightning in terms of electrical charge separation and discharge within storm clouds. A sudden electrical discharge produces the visible flash, while the associated atmospheric disturbance produces thunder.
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Several forms are listed:
- discharges occurring within a cloud;
- cloud-to-ground lightning, one of the most dangerous forms;
- discharges from a cloud toward charged air outside the cloud, described as a “bolt from the blue”;
- cloud-to-cloud lightning; and
- Saint Elmo's Fire, described in the source as an electrical luminous phenomenon that can create a brief glow around a cloud or object.