Category Weather & Climate

          Humidity is highest in tropical areas, where the climate is warm. A continuous cycle of water movement exists, where water evaporates from the sea into the air and falls again in heavy rainfall. The conditions are ideal for plants and other forms of life — the plants themselves add more moisture to the atmosphere.

          It’s often been suggested (or, at least, hoped) that no matter how much the climate warms, that humans might be able to adapt. But there’s a hard upper limit on that: If the “wet-bulb temperature” — measured by wrapping the bulb of a thermometer in a wet cloth and taking the temperature of the air — exceeds 95 degrees Fahrenheit (or, 35ºC), humans are pretty much toast. It might seem like an odd way to measure the upper limit of human survivability, but that’s the simplest way to measure how much a human body could theoretically cool itself, assuming a perfectly healthy body.

          “You rapidly approach a situation where it’s thermodynamically impossible to keep your body cool,” Radley Horton, an associate research professor at Columbia University’s Earth Institute and the Lamont-Doherty Earth Observatory, a co-author of the study, told VICE News.

          As humidity increases, so does the wet-bulb temperature. Because we never hit a wet-bulb temperature of 95 degrees Fahrenheit in today’s climate, it’s hard to say what the societal effects would be. But wet-bulb temperatures between 84 degrees and 88 degrees Fahrenheit (29–31ºC) have been responsible for tens of thousands of deaths around the world. A wet-bulb temperature of 86 degrees Fahrenheit (30ºC) was recorded during a heat wave in 2015 in the southeastern coastal Indian state of Andhra Pradesh that killed at least 2,500 people.

          The risks are starkest for places that already see high heat and humidity, like the Persian Gulf and the Tropics. And some of the regions most at risk of these spikes are the most densely populated — places like Northeast India, East China, West Africa, and the Southeastern U.S. Many of those places don’t currently have good access to medical care, and the population flight from these regions could make it even harder to help the most vulnerable.

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          Dew is condensed water vapour, which forms when air comes into contact with a cold surface. It will form on a clear, still night, but it is especially noticeable after a night of fog, when there is a lot of water vapour in the air close to the ground. Dew will appear as water droplets on any cold surface.

          As the exposed surface cools by radiating its heat, atmospheric moisture condenses at a rate greater than that at which it can evaporate, resulting in the formation of water droplets.

          When temperatures are low enough, dew takes the form of ice; this form is called frost.

          Because dew is related to the temperature of surfaces, in late summer it forms most easily on surfaces that are not warmed by conducted heat from deep ground, such as grass, leaves, railings, car roofs, and bridges.

          Dew should not be confused with guttation, which is the process by which plants release excess water from the tips of their leaves.

          Water vapour will condense into droplets depending on the temperature. The temperature at which droplets form is called the dew point. When surface temperature drops, eventually reaching the dew point, atmospheric water vapor condenses to form small droplets on the surface. This process distinguishes dew from those hydrometeors (meteorological occurrences of water), which form directly in air that has cooled to its dew point (typically around condensation nuclei), such as fog or clouds. The thermodynamic principles of formation, however, are the same. Dew is usually formed at night.

          Adequate cooling of the surface typically takes place when it loses more energy by infrared radiation than it receives as solar radiation from the sun, which is especially the case on clear nights. Poor thermal conductivity restricts the replacement of such losses from deeper ground layers, which are typically warmer at night. Preferred objects of dew formation are thus poor conducting or well isolated from the ground, and non-metallic, while shiny metal coated surfaces are poor infrared radiators. Preferred weather conditions include the absence of clouds and little water vapor in the higher atmosphere to minimize greenhouse effects and sufficient humidity of the air near the ground. Typical dew nights are classically considered calm, because the wind transports (nocturnally) warmer air from higher levels to the cold surface. However, if the atmosphere is the major source of moisture (this type is called dewfall), a certain amount of ventilation is needed to replace the vapor that is already condensed. The highest optimum wind speeds could be found on arid islands. If the wet soil beneath is the major source of vapor, however (this type of dew formation is called distillation), wind always seems adverse.

          The processes of dew formation do not restrict its occurrence to the night and the outdoors. They are also working when eyeglasses get steamy in a warm, wet room or in industrial processes. However, the term condensation is preferred in these cases.

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          Frost can sometimes create beautiful patterns on the inside of windows. The delicate shapes, called fern frost because of their resemblance to fern plant, form when water vapour condenses into tiny droplets on the window pane. Ice crystals form, making water freeze on to the sharp points of the ice crystals, creating a chain reaction that creates the patterns.

          Window frost (also called fern frost or ice flowers) forms when a glass pane is exposed to very cold air on the outside and warmer, moderately moist air on the inside. If the pane is not a good insulator (for example, if it is a single pane window), water vapour condenses on the glass forming frost patterns. With very low temperatures outside, frost can appear on the bottom of the window even with double pane energy efficient windows because the air convection between two panes of glass ensures that the bottom part of the glazing unit is colder than the top part. On unheated motor vehicles the frost will usually form on the outside surface of the glass first. The glass surface influences the shape of crystals, so imperfections, scratches, or dust can modify the way ice nucleates. The patterns in window frost form a fractal with a fractal dimension greater than one but less than two. This is a consequence of the nucleation process being constrained to unfold in two dimensions, unlike a snowflake which is shaped by a similar process but forms in three dimensions and has a fractal dimension greater than two.

          If the indoor air is very humid, rather than moderately so, water will first condense in small droplets and then freeze into clear ice.

          Fern frost can form on windowpanes when the air outside is very cold and the air inside is moist. The outside air temperature on the winter’s day when this photo was snapped was 15 degrees F (-9 C) and the inside air temperature was 66 F (19 C). Crystal formation is affected by surface features of the glass, like dust and dirt particles, which serve as nucleation points for crystalline growth. Towels had been left to dry near the old and weathered bathroom window shown here  provided just the right level of moisture conducive to frost formation. If there had been considerably more moisture in the air, for instance after a steamy shower, or if the windowpane had not been extremely cold, the water vapor would have merely formed ice or water droplets on the glass. The very low temperature of the glass allowed the moisture to go directly from a gaseous state to a solid state— in the form of frost.

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        The air absorbs water from oceans, rivers, lakes and also from trees and plants. Humidity describes the amount of water vapour that the air contains. The warmer the weather, the more moisture the air can hold. The air can reach a point of saturation, where it is no longer able to absorb any more water — this is 100% humidity. In such conditions, water vapour condenses to form mist, clouds and rain.

        Humidity is the concentration of water vapour present in air. Water vapour, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. The amount of water vapour needed to achieve saturation increases as the temperature increases. As the temperature of a parcel of air decreases it will eventually reach the saturation point without adding or losing water mass. The amount of water vapour contained within a parcel of air can vary significantly. For example, a parcel of air near saturation may contain 28 grams of water per cubic metre of air at 30 °C, but only 8 grams of water per cubic metre of air at 8 °C.

        Three primary measurements of humidity are widely employed: absolute, relative and specific. Absolute humidity describes the water content of air and is expressed in either grams per cubic metre or grams per kilogram. Relative humidity, expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity given the same temperature. Specific humidity is the ratio of water vapor mass to total moist air parcel mass.

        Humidity plays an important role for surface life. For animal life dependent on perspiration (sweating) to regulate internal body temperature, high humidity impairs heat exchange efficiency by reducing the rate of moisture evaporation from skin surfaces. This effect can be calculated using a heat index table, also known as a humidex.

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          It is possible for a type of icicle to  form upside down. This happens in small, shallow pools of water — ornamental bird baths, for example. When the water freezes, it expands and forms a dome of ice in the centre. A crack in the dome will allow water out, which then freezes. As this happens over time, an “ice spike” will form.

          An ice spike is an ice formation, often in the shape of an inverted icicle that projects upwards from the surface of a body of frozen water. Ice spikes created by natural processes on the surface of small bodies of frozen water have been reported for many decades, although their occurrence is quite rare. A mechanism for their formation, now known as the Bally–Dorsey model, was proposed in the early 20th century but this was not tested in the laboratory for many years. In recent years a number of photographs of natural ice spikes have appeared on the Internet as well as methods of producing them artificially by freezing distilled water in domestic refrigerators or freezers. This has allowed a small number of scientists to test the hypothesis in a laboratory setting and, although the experiments appear to confirm the validity of the Bally–Dorsey model, they have raised further questions about how natural ice spikes form, and more work remains to be done before the phenomenon is fully understood. Natural ice spikes can grow into shapes other than a classic spike shape, and have been variously reported as ice candles, ice towers or ice vases as there is no standard nomenclature for these other forms. One particularly unusual form takes the shape of an inverted pyramid.

          Although natural ice spikes are usually measured in inches or centimeters, a report that appeared in the Harbor Creek Historical Society Newsletter by Canadian Gene Heuser, who hiked across frozen Lake Erie in 1963, spoke of “small pinholes in the ice through which the water below was periodically forced under pressure to spout up into the air and freeze” producing five-foot-high (1.5 m) “frozen spurts that looked to him like telephone poles standing straight up all over the lake”.

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