Term
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Definition
The movement of water between and within the atmosphere and Earth. Water vapor is constantly being added to and removed from the atmosphere. Atmospheric residence time for water vapor is only 10 days.
1. Evaporation of water from Earth's surface into the atmosphere. Can occur directly from the oceans or from water bodies on land surfaces. Can also occur indirectly through plants via transpiration. Water vapor that goes into the air eventually becomes water droplets or icy crystals in the form of clouds or fog.
*Shortest route cycle can take is from the ocean to the atmosphere and back to the ocean. More complicated over land. There, some precip. may not reach the surface directly and may instead fall onto vegetation, accumulating as a coat of water or ice. The water that undergoes this process, called interception, might trickle down the plant to the surface or evaporate back into the air.
*Rainfall that does reach the land surface, either directly or after interception, might then flow above the surface into rivers, which then transport the water into a lake or ocean.
*Infiltration - liquid water at the ground penetrates into the ground. Such water is pulled downward by gravity and can collect in the pores of underlying soil or rock as groundwater. Groundwater usually seeps into rivers for eventual transport towards the ocean, where the cycle continues. |
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Term
| Evaporation and Condensation |
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Definition
Evaporation - the process by which molecules break free of the liquid volume. At the early stages of evaporation, the low water vapor content prevents much condensation from occuring, and the rate of evaporation exceeds that of condensation. Leads to increase of the amount of water vapor present. With this increase of water vapor, the condensation rate likewise increases. Eventually, the amount of water vapor above the surface is enough for the rates of condensation and evaporation to become equal. A constant amount of water vapor exists in the volume above the water due to offsetting gains by both processes. The equilibrium state is known as saturation.
Condensation - the opposite process, where water vapor molecules randomly collide with the water surface and bond with adajacent molecules. |
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Term
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Definition
| The state of saturation described can occur whether or not air (or other gases) exists in the container. the water vapor is not "held" by the air. Water vapor is a gas. Thus, it does not need to be "held" by air by any more than oxygen and nitrogen of the atmosphere need to be "held" by water vapor. When the air is saturated there is simply an equilibrium between evaporation and condensation; the dry air plays no role in achieving this state. |
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Term
| Sublimation and Deposition |
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Definition
Sublimation is the change of phase directly from ice to water vapor, without passing into the liquid phase.
Deposition - the reverse process (from water vapor to ice). |
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Term
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Definition
| Humidity refers to the amount of water vapor in the air. Can be expressed in many ways; in terms of density of the water vapor, the percentage of the amount of water vapor that can actually exist, or several other methods. There is no single "correct" measure, but each has its own advantages and disadvantages. All have one thing in common: they apply exclusively to water vapor, and not to liquid droplets or ice crystals suspended in or falling through the air. |
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Term
| Humidity Measures: Vapor Pressure |
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Definition
| Vapor pressure is the part of the total atmospheric pressure due to water vapor. Vapor pressure is measured in millibars by U.S. and kilopascals by Canada. The vapor pressure of a volume of air depends on both the temperature and the density of water vapor molecules. If the air temp is high, water vapor molecules move more rapidly and exert a greater pressure. A greater concentration of water vapor molecules means that a greater amount of mass is available to exert pressure. Vapor pressure closely follows changes in the density or abundance of water molecules. Becaus there is a maximum amount of water vapor that can exist, there is a corresponding maximum vapor pressure, called the saturation vapor pressure. The SVP does not represent the current amount of moisture, rather it is an expression of the maximum that CAN exist. The SVP depends on one thing: temperature. Higher temps = higher SVPs. The increase in SVP with temp is not linear. At low temps there is only a modest increase in SVP but at high temps SVP rapidly grows. At temps encountered at Earth's surface the SVP doubles afor every 10 degrees C increase in temp. |
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Term
| Humidity Measures: Absolute Humidity |
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Definition
| Absolute humidity is the density of water vapor, expressed as the number of grams of water vapor contained in a cubic meter of air. Because absolute humidity represents the amount of moisture contained in a volume of air, its value changes whenever air expands or contracts. If an air parcel expands, its absolute humidity will fall, even though no water vapor was removed from the parcel of air. Because absolute humidity suffers from this drawback, it is not widely used. |
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Term
| Humidity Measures: Specific Humidity |
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Definition
Specific humidity is a useful index for representing atmospheric moisture. Specific humidity expresses the mass of water vapor existing in a given mass of air. Does not change as air expands and contracts. It is not temperature dependent either. Specific humidity is a good indicator for comparing water vapor in the air at different locations whose air temperatures might be different from each other.
Because there is a maximum amount of water vapor that can exist at a particular temperature, there is a maximum specific humidity, called saturation specific humidity. This property is directly analogous to the SVP and increases in the nonlinear manner. |
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Term
| Humidity Measures: Mixing Ratio |
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Definition
| The mixing ratio is similar to the specific humidity. It is a measure of the mass of water vapor relative to the mass of the other gases in the atmosphere. The mixing ratio and specific humidity will always have nearly equal values. The maximum possible mixing ratio is called the saturation mixing ratio. |
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Term
| Humidity Measures: Relative Humidity |
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Definition
| The most familiar measure of water vapor content is relative humidity, which relates the amount of water vapor in the air to the maximum possible at the current temperature. The relative humidity is not uniquely determined by the amount of water vapor present. Because more water vapor can exist in warm air than in cold air, the relative humidity depends on both the actual moisture content and the air temperature. If the temperature of the air increases, more water vapor can exist and the ratio of the amount of water vapor in the air relative to the saturation decreases. Thus the relative humidity declines even if the moisture content is unchanged. Because of its dependence on temperature , the relative humidity will change throughout the course of the day even if the amount of moisture in the air is unchanged. Relative humidity is highest in the morning, and declines later because the saturation specific humidity increases. The influence of temperature on relative humidity creates another problem: it confounds direct comparisons of moisture contents at different places with unequal temperatures. |
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Term
| Humidity Measures: Dew Point |
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Definition
| A useful moisture index that is free of the temperature relationship is the dew point temperature, or simply the dew point, the temperature at which saturation occurs. It is dependent exclusively on the amount of water vapor present. The dew point is a valuable indicator of the moisture content; when the dew point is high, abundant water vapor is in the air. Moreover, when combined with air temperature, it is an indicator of the relative humidity. When the dew point is much lower than the air temperature, the relative humidity is very low. When the dew point is nearly equal to the air temperature, the relative humidity is very high. When the air temperature and the dew point are equal, the air is saturated and the relative humidity is 100 percent. The dew point does not change simply because air temperature changes. If one location has a higher dew point than another, it will also have a greater amount of water vapor in the air, assuming the same air pressure. Can sometimes serve as a predictor of overnight cooling. The dew point is always equal to or less than the air temperature, under no circumstances does it ever exceed the temperature. When the temperature at which saturation occurs is below 0 degrees C, we use the term frost point. |
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Term
| Methods of Achieving Saturation |
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Definition
| Air can become saturated by any one of three general processes: adding water vapor to the air, mixing cold air with warm, moist air, and lowering the temperature to the dew point. In the natural environment, the evaporation of water from falling raindrops can raise the dew point in the air beneath the cloud from which the rain falls. If enough vapor is added to the air to saturate it, a precipitation fog forms beneath the cloud. Steam fog - a naturally occuring phenomenon. Atmospheric colling, the third method, is the most common process for cloud formation. |
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Term
| The Effects of Curvature and Solution |
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Definition
| Effect of Curvature: Water droplets exist in nature not as tiny cubes with flat sides, but rather as microscopically small spheres with considerable curvature. Cloud droplets exhibit a lot of curvature. Curvature has an effect on evaporation from cloud droplet surfaces and therefore on the vapor pressure necessary for saturation. Effects arising from surface tension lead to differences in the saturation point. For curved water surfaces, the evaporation rate is greater. The enhanced rate of evaporation requires that condensation also be increased for the two to remain in balance. Thus, a highly curved droplet of pure water at any given temperature has a higher saturation vapor pressure than indicated. Highly curved droplets of pure water require relative humidities in excess of 100 percent to keep them from evaporating away. For very small droplets relative humidities in excess of 110% are needed to achieve an equilibrium between evaporation and condensation. Those droplets require a "supersaturation" of 10%. The degree of supersaturation necessary to maintain a droplet rapidly decreases with increasing droplet size. |
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Term
| Effect of Curvature: Homogenous Nucleation, Hygroscopic, Heterogenous Nucleation, and Condensation Nuclei |
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Definition
| If the atmosphere were devoid of an aerosols, condensation would occur only by HN, in which droplets form by the chance collision and bonding of water vapor molecules under supersaturated conditions. Such droplets would necessarily have only a small number of molecules and a high degree of curvature, and therefore would only exist at high levels of supersaturation. This process seldom occurs if ever occurs, because certain hygroscopic (water-attracting) aerosols in the atmosphere assist in the formation of droplets at relative humidities far below those necessary for HN. The formation of water droplets onto hygroscopic particles is called heterogenous nucleation, and the particles onto which the droplets form are called condensation nuclei. These particles enhance condensation through two processes. |
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