›Rainfall can be classified into three main types based on origin: convectional, orographic (or relief), and cyclonic (or frontal). (ch10-water-in-the-atmosphere.md, ch14-climate.md)
›**Convectional Rain:**
›Caused by air being heated, becoming light, and rising in convection currents. (ch10-water-in-the-atmosphere.md)
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Rainfall can be classified into three main types based on its origin: convectional, orographic (or relief), and cyclonic (or frontal). Convectional rainfall occurs when the air is heated, becomes light, and rises in convection currents. As this air ascends, it expands and loses heat, leading to condensation and the formation of cumulonimbus clouds with a great vertical extent. This process results in torrential, but short-lived, rainfall often accompanied by thunder and lightning, typically reaching its maximum in the afternoon. It is commonly observed in summer or the hotter parts of the day, particularly in equatorial regions and the interior parts of continents, including temperate interiors, especially in the Northern Hemisphere. These intense downpours may not be entirely useful for agriculture due to rapid runoff.
Orographic rainfall, also known as relief rain, happens when a moist air mass is forced to ascend a mountain barrier. The air mass is compelled to rise, causing it to expand, cool down due to decreasing atmospheric pressure, and condense its moisture, especially on the windward slopes where prevailing moisture-laden winds originate from the sea. This results in significant rainfall on the windward slopes of the mountain. After crossing the mountain, these winds descend on the leeward side. As they descend, the air is compressed and warmed due to increased pressure and temperature, causing relative humidity to drop, resulting in evaporation and little or no precipitation. Consequently, the leeward slopes remain dry and rainless, creating a "rain-shadow area."
All key facts
›Rainfall can be classified into three main types based on origin: convectional, orographic (or relief), and cyclonic (or frontal). (ch10-water-in-the-atmosphere.md, ch14-climate.md)
›**Convectional Rain:**
›Caused by air being heated, becoming light, and rising in convection currents. (ch10-water-in-the-atmosphere.md)
›Caused by air being heated *by conduction*, becoming lighter as it expands, and rising in convection currents after a prolonged period of intense heating. (ch14-climate.md)
›Rising air expands, loses heat, leading to condensation and cumulus cloud formation. (ch10-water-in-the-atmosphere.md)
›Rising air expands and cools, leading to condensation and the formation of *cumulonimbus clouds with a great vertical extent*. (ch14-climate.md)
›Characterized by heavy rainfall with thunder and lightning, but it does not last long. (ch10-water-in-the-atmosphere.md)
›It typically reaches its maximum in the afternoon when the convectional system is well developed. (ch14-climate.md)
›Hot, rising air has great capacity for holding moisture, which is abundant in regions of high relative humidity, leading to torrential downpours when saturation point is reached. (ch14-climate.md)
›Summer showers in temperate regions are also heavy with occasional thunderstorms. (ch14-climate.md)
›Intense convectional downpours may not be entirely useful for agriculture as the rain often drains off immediately without sinking into the soil. (ch14-climate.md)
›Common in summer or the hotter part of the day. (ch10-water-in-the-atmosphere.md)
›Very common in equatorial regions and interior parts of continents, especially in the northern hemisphere, and in temperate interiors during summer. (ch10-water-in-the-atmosphere.md, ch14-climate.md)
›**Orographic Rain (Relief Rain):**
›Occurs when a saturated air mass is forced to ascend a mountain. (ch10-water-in-the-atmosphere.md)
›Orographic rain is best developed on the windward slopes of mountains where prevailing moisture-laden winds come from the sea. (ch14-climate.md)
›As air rises, it expands, temperature falls, and moisture condenses. (ch10-water-in-the-atmosphere.md)
›As air is compelled to rise, it expands and cools due to a decrease in atmospheric pressure, eventually becoming completely saturated (100% relative humidity) leading to condensation and cloud formation. (ch14-climate.md)
›Windward slopes of the mountain receive greater rainfall. (ch10-water-in-the-atmosphere.md)
›Winds descending on the leeward side increase in temperature and moisture-holding capacity. (ch10-water-in-the-atmosphere.md)
›On the leeward slope, descending air is compressed and warmed due to increased pressure and temperature, causing relative humidity to drop, resulting in evaporation and little or no precipitation. (ch14-climate.md)
›Leeward slopes remain rainless and dry, forming a "rain-shadow area." (ch10-water-in-the-atmosphere.md)
›Examples of regions experiencing significant relief rain include the north-east of Peninsular Malaysia, western New Zealand, western Scotland and Wales, and the Assam Hills of India and Bangladesh. (ch14-climate.md)
›Examples of rain-shadow areas include the Canterbury Plain of South Island, New Zealand, and the western slopes of the Northern and Central Andes. (ch14-climate.md)
Evaporation and Condensation
›The amount of water vapour in the atmosphere is added through evaporation and withdrawn through condensation. (ch10-water-in-the-atmosphere.md)
›Evaporation is the process by which water is transformed from a liquid to a gaseous state. (ch10-water-in-the-atmosphere.md)
›Heat is the main cause for evaporation. (ch10-water-in-the-atmosphere.md)
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Evaporation and condensation are fundamental processes that regulate the amount of water vapour in the atmosphere, facilitating a continuous exchange of water between the atmosphere, oceans, and continents. Evaporation is the process by which water changes from a liquid state to a gaseous state (water vapour). This transformation is primarily driven by heat. The specific amount of heat energy required to convert a unit mass of a liquid into vapour without changing its temperature is known as the latent heat of vaporisation. Evaporation is increased by higher temperatures, lower moisture content in the air (which allows for greater absorption and retention), and increased air movement that replaces saturated air layers with unsaturated ones.
Conversely, condensation is the process where water vapour transforms back into a liquid or solid state. This process occurs due to the loss of heat. When moist air cools to a point where its capacity to hold water vapour is exceeded, the excess vapour condenses. In free air, condensation often occurs around very small particles called hygroscopic condensation nuclei, such as dust, smoke, and salt particles from the ocean, which readily absorb water. Condensation can also happen when moist air encounters a colder object or when the temperature drops close to the dew point. The extent of condensation is influenced by the degree of cooling, the relative humidity of the air, the volume of air, temperature, pressure, and overall humidity. The most favorable condition for condensation is a decrease in air temperature. After condensation, the moisture in the atmosphere can take various forms, including dew, frost, fog, and clouds, which are classified based on temperature and location.
All key facts
›The amount of water vapour in the atmosphere is added through evaporation and withdrawn through condensation. (ch10-water-in-the-atmosphere.md)
›Evaporation is the process by which water is transformed from a liquid to a gaseous state. (ch10-water-in-the-atmosphere.md)
›Heat is the main cause for evaporation. (ch10-water-in-the-atmosphere.md)
›Latent heat of vaporisation is the amount of heat energy required to convert a unit mass of a liquid into vapour without a change in temperature. (ch10-water-in-the-atmosphere.md)
›Increase in temperature increases the water absorption and retention capacity of air, thus increasing evaporation. (ch10-water-in-the-atmosphere.md)
›Low moisture content in air increases its potential to absorb and retain moisture, leading to greater evaporation. (ch10-water-in-the-atmosphere.md)
›Movement of air replaces saturated layers with unsaturated ones, so greater air movement leads to greater evaporation. (ch10-water-in-the-atmosphere.md)
›Condensation is the transformation of water vapour into water. (ch10-water-in-the-atmosphere.md)
›Condensation is caused by the loss of heat. (ch10-water-in-the-atmosphere.md)
›When moist air is cooled to its saturation point, excess water vapour condenses into liquid form. (ch10-water-in-the-atmosphere.md)
›In free air, condensation occurs around hygroscopic condensation nuclei, such as particles of dust, smoke, and salt from the ocean. (ch10-water-in-the-atmosphere.md)
›**Haze** is caused by smoke and dust particles in industrial areas, or by unequal refraction of light in air of different densities in the lower atmosphere. (ch13-weather.md, p. 126)
›Haze is associated with a reduction of visibility in regions of **lower humidity**, specifically less than 75 per cent. (ch13-weather.md, p. 126)
›Haze is present when visibility is less than **2 km (1.25 miles)**. (ch13-weather.md, p. 126)
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Atmospheric visibility phenomena like haze, mist, and fog are elements that significantly affect visibility in the lower atmosphere. Haze is characterized by reduced visibility in regions of lower humidity (less than 75%), typically caused by smoke, dust particles, or unequal light refraction in air of varying densities. Mist, conversely, forms when water vapour condenses into small droplets floating in the air, creating a ground-level cloud, and occurs in wet air with relative humidity exceeding 75%. Fog is a denser form of ground-level cloud, formed by water condensing on dust and other particles, and leads to even lower visibility than mist. Specialized types of fog include smog (thick, smoky fog in industrial areas), hill fog (found on hills, common in mornings), radiation or land fog (resulting from land surface cooling by radiation on clear nights), and sea fog (formed over the sea when cooling occurs or warm and cold ocean currents meet, or cold ocean currents meet hot desert air). These phenomena differ in their causes, humidity conditions, and the extent to which they reduce visibility.
All key facts
›**Haze** is caused by smoke and dust particles in industrial areas, or by unequal refraction of light in air of different densities in the lower atmosphere. (ch13-weather.md, p. 126)
›Haze is associated with a reduction of visibility in regions of **lower humidity**, specifically less than 75 per cent. (ch13-weather.md, p. 126)
›Haze is present when visibility is less than **2 km (1.25 miles)**. (ch13-weather.md, p. 126)
›**Mist** forms when the condensation of water vapour in the air causes small droplets of water to float about, forming clouds at ground level. (ch13-weather.md, p. 127)
›Mist reduces visibility to about **1000 metres or 1,100 yards**. (ch13-weather.md, p. 127)
›Unlike haze, mist occurs in **wet air**, where the relative humidity is over 75 per cent. (ch13-weather.md, p. 127)
›**Ordinary fog** is caused by water condensing on dust and other particles, such as smoke from houses and factories. (ch13-weather.md, p. 127)
›Fog occurs only in the **lower strata of the atmosphere** as a dense 'ground cloud'. (ch13-weather.md, p. 127)
›Visibility in fog is **even less than 1000 metres**. (ch13-weather.md, p. 127)
›**Smog** is a very thick smoky fog formed in industrial areas like the Black Country and northern England. (ch13-weather.md, p. 127)
›In smog, visibility may be reduced to **200 metres (220 yards)** or even less. (ch13-weather.md, p. 127)
›**Hill fogs** occur on hills, are most common in the morning, even in the tropics, and disperse when the sun rises. (ch13-weather.md, p. 127)
Clouds and Cloud Classification
›**Definition:** A cloud is a mass of minute water droplets or tiny crystals of ice formed by the condensation of water vapour in free air at considerable elevations.
›**Formation:** Clouds are formed at some height over the surface of the Earth by the condensation of water vapour.
›**Formation Process (Detailed):** Air rises and is cooled by expansion; after the dew-point is reached, cooling leads to the condensation of water vapour into tiny droplets (less than 0.01 mm / 0.0005 inches in radius) that remain suspended. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
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A cloud is defined as a significant mass of minute water droplets or tiny ice crystals, which forms due to the condensation of water vapour in free air at considerable heights above the Earth's surface. When air rises, it is cooled by expansion, and after the dew-point has been reached, this cooling leads to the condensation of water vapour. These tiny droplets, too small to fall as rain or snow (less than 0.01 mm in radius), remain suspended and float as clouds. The formation of clouds at elevated positions leads to their varied shapes. Clouds are primarily classified into four basic types based on their height, expanse, density, and transparency or opaqueness: cirrus, cumulus, stratus, and nimbus. The classification of clouds is also based on a combination of their form, height, and appearance.
Cirrus clouds are high-altitude formations, characterized by their thin, detached, and feathery white appearance. Cumulus clouds resemble cotton wool, typically found at medium altitudes, appearing in scattered patches with a flat base. Stratus clouds are layered, covering vast areas of the sky, forming either from heat loss or the mixing of air masses with differing temperatures. Nimbus clouds are dark gray or black, dense, opaque, and can form at middle levels or very close to the ground, appearing as shapeless masses of thick vapour. These four basic types can combine to form more specific cloud classifications, such as high clouds (cirrostratus, cirrocumulus), middle clouds (altostratus, altocumulus), low clouds (stratocumulus, nimbostratus), and clouds with extensive vertical development (cumulus, cumulonimbus).
All key facts
›**Definition:** A cloud is a mass of minute water droplets or tiny crystals of ice formed by the condensation of water vapour in free air at considerable elevations.
›**Formation:** Clouds are formed at some height over the surface of the Earth by the condensation of water vapour.
›**Formation Process (Detailed):** Air rises and is cooled by expansion; after the dew-point is reached, cooling leads to the condensation of water vapour into tiny droplets (less than 0.01 mm / 0.0005 inches in radius) that remain suspended. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
›**Classification Basis:** Clouds are grouped into four types based on their height, expanse, density, and transparency or opaqueness.
›**Classification Basis (Detailed):** The classification of clouds is based on a combination of form, height, and appearance. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
›**Four Basic Types:** The primary cloud types are cirrus, cumulus, stratus, and nimbus.
›**Cloud Cover Measurement:** The amount of cloud-cover in the sky is expressed in eights or oktas for meteorological purposes. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
›**Isonephs:** On weather maps, places with an equal degree of cloudiness are joined by lines known as isonephs, though these maps have little significance due to rapid cloud variation. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
›**Cirrus Clouds:**
›
›Condensation can also occur when moist air comes into contact with a colder object. (ch10-water-in-the-atmosphere.md)
›Condensation can take place when the temperature is close to the dew point. (ch10-water-in-the-atmosphere.md)
›Condensation depends upon the amount of cooling and the relative humidity of the air. (ch10-water-in-the-atmosphere.md)
›Condensation is influenced by the volume of air, temperature, pressure, and humidity. (ch10-water-in-the-atmosphere.md)
›The most favourable condition for condensation is the decrease in air temperature. (ch10-water-in-the-atmosphere.md)
›Condensation takes place when: (i) air temperature reduces to dew point with constant volume; (ii) both volume and temperature are reduced; (iii) moisture is added to air through evaporation. (ch10-water-in-the-atmosphere.md)
›The rate of evaporation, along with mean annual temperature, temperature during the growth season, and the frost-free period, contributes to the final yield of a crop in agriculture. (ch26-agriculture.md)
›**Radiation fog (or land fog)** results from the cooling of the land surface by radiation on hot days and clear, still nights in temperate lands, causing water vapour in the lower air layers to condense. (ch13-weather.md, p. 127)
›**Sea fog** is formed when a cooling surface is over the sea, or when a warm ocean current contacts a cold current (e.g., off Newfoundland), or when cold ocean currents contact hot desert air (e.g., off the Atacama Desert of Peru and Chile). (ch13-weather.md, p. 127)
›Sea fogs vary in depth and thickness, with some being shallow enough for ship masts to protrude above them. (ch13-weather.md, p. 127)
›Generally, fogs are **more common over seas than lands**, and most prevalent over coastal areas. (ch13-weather.md, p. 128)
›Dry interiors tend to experience haze or mist. (ch13-weather.md, p. 128)
›Dense fogs are more likely in the **high and middle latitudes** rather than in the tropics. (ch13-weather.md, p. 128)
›A **Stratus cloud** is described as a very low cloud, uniformly grey and thick, appearing like a low ceiling or highland fog. (ch13-weather.md, p. 124)
Formed at high altitudes (8,000 - 12,000m).
›Formed at high altitudes, mainly of feathery form at 6,100-12,200 metres (20,000-40,000 feet) above ground. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
›Are thin and detached clouds.
›Have a feathery appearance.
›Are always white in colour.
›Look fibrous and appear like wisps in the blue sky; often called 'mares' tails'. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
›Indicate fair weather and often give a brilliant sunset. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
›**Cumulus Clouds:**
›Look like cotton wool.
›Generally formed at a height of 4,000 - 7,000 m.
›Exist in patches and can be seen scattered.
›Have a flat base.
›**Stratus Clouds:**
›Are layered clouds, covering large portions of the sky.
›Generally formed either due to loss of heat or the mixing of air masses with different temperatures.
›**Nimbus Clouds:**
›Are black or dark gray.
›Form at middle levels or very near to the surface of the Earth.
›Are extremely dense and opaque to the rays of the sun.
›Sometimes appear so low that they seem to touch the ground.
›**Cirrocumulus (Cc):** Appear as white globular masses, forming ripples in a 'mackerel sky'. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
›**Cirrostratus (Cs):** Resemble a thin white sheet or veil, making the sky look milky and causing the sun or moon to shine through with a characteristic 'halo'. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
›**Altocumulus (Alt-Cu):** Woolly, bumpy clouds arranged in layers, appearing like waves in the blue sky, normally indicating fine weather. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
›**Altostratus (Alt-St):** Denser, greyish clouds with a 'watery' look, having a fibrous or striated structure through which the sun's rays shine faintly. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
›**Stratocumulus (St-Cu):** A rough, bumpy cloud with waves more pronounced than in altocumulus, showing great contrast between bright and shaded parts. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
›**Stratus (St):** A very low cloud, uniformly grey and thick, appearing like a low ceiling or highland fog; it brings dull weather with light drizzle and reduces aircraft visibility, posing a danger. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
›**Nimbostratus (Ni-St):** A dark, dull cloud, clearly layered, also known as a 'rain cloud', bringing continuous rain, snow, or sleet. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
›**Clouds with extensive vertical development:** cumulus, cumulonimbus. (with no definite height, generally 6,100-9,000 metres / 20,000-30,000 feet) — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
›**Cumulus (Cu):** A vertical cloud with a rounded top and horizontal base, typical of humid tropical regions, associated with up-rising convectional currents; its great white globular masses may look grey against the sun, but it is a 'fair weather cloud'. — GC Leong — Certificate Physical and Human Geography (3rd Ed), ch13-weather.md
›**Cumulonimbus (Cu-Ni):** An overgrown cumulus cloud extending for tremendous vertical height from a base of 600 metres (2,000 feet) to over 9,000 metres (30,000 feet); its black and white globular masses take fantastic shapes, with a cauliflower top often spreading out like an anvil, frequently seen in tropical afternoons; also referred to as a 'thunder-cloud' and