Condensation: Dew, Fog, and Clouds

Dew: the water droplets formed by condensation of water vapor on a relatively cold surface of an object. It forms when the temperature of an object drops below the dew point temperature.

Dew point temperature: the temperature to which moist air is cooled at constant pressure and at constant water vapor to reach saturation with respect to water. It is not directly measured, rather it is obtained from measurements of the dry-bulb temperature and the wet-bulb depression.

The dew point temperature differs from the wet-bulb temperature, which is determined by inducing evaporative cooling. The wet-bulb temperature is higher than dew point temperature except at saturation where both temperatures are the same as the ambient temperature.

The dew point temperature is a measure of humidity, and therefore, it is an comfort index. It becomes a factor for human health when the dew point temperature exceeds 20C (68F). The four categories of apparent temperature ranges in heat index correspond to the following dew point temperature ranges: i) 80 or higher ii) 76 - 80F, iii) 72 - 76F, iv) 68 - 72F.

Table D-1, page #A-8 (Ahrens)

Figure 4.14, page #102 (Ahrens)

Figure 4.15, page #103 (Ahrens)

Frost: the ice crystals formed by deposition of water vapor on a relatively cold surface of an object. It forms when the temperature of an object drops below the frost point temperature.

Frost point temperature: the temperature to which moist air is cooled at constant pressure and at constant water vapor to reach saturation with respect to ice. It differs from the dew point temperature as the air temperature is at or below 0 C (32 F).

Both dew and frost occurs as a result of nocturnal radiative cooling. The length of the night plays an important role on the degree of radiative cooling and therefore, on the possibility of formation of dew or frost. They most likely form on blades of grass, branches of trees etc. during a clear and calm night following a rather warm clear day typically late spring or early autumn.

Frost is a very important factor for the growing agricultural products. Several methods have been developed to protect crops from freezing: locating on hillslopes, spraying a fine water mist, covering with plastic hot caps, circulating he warm air aloft down the ground with a motor-driven fan.

Fog: a suspended tiny water droplets or ice crystals in an air layer next to the Earth's surface that reduces the visibility to 1000 m (3250 ft.) or lower. For aviation purposes, the criterion for fog is 10 km (6.2 mi.) or less. If the visibility is higher than those limits, the suspension is called mist.

Radiation fog: a ground-level cloud induced by nocturnal radiative cooling. It forms when radiative cooling of a ground surface cools the overlying air below its dew point. It is more common on a clear night with light winds over marshy areas or soil saturated by a recent heavy rainfall or rapid snowmelt.

Radiation fog is usually a few meter deep and typically lasts for only a few hours after sunrise. The duration and the thickness of the radiation fog depends on the local conditions as well as the solar heating. The fog is thick and persistent in low-lying areas such as river valleys, valley fog, and in the presence of weak rays of sun during late autumn and early winter.

Upslope fog: a ground-level cloud induced by expansional cooling. It forms when moist air flows upon windward side of hillsides or mountain slopes. It can persist for many days over an extensive area.

Advection fog: a ground-level cloud induced by advective cooling. It forms when relatively warm air is chilled to saturation by over-running a sufficiently cool surface.

Advection fog is usually thicker (tens of meters deep) and lasts longer than the radiation fog. It is more common for the following three conditions: i) when mild, humid air flows over relatively cold, snow-covered ground in early spring, if the ground is extremely cold, ice crystals form, ice fog. ii) when warm, humid air flows over relatively cool ocean and lake water in late spring and early summer, coastal fog iii) when an advected warm, humid air is subjected to nocturnal radiative cooling, advection-radiation fog.

Evaporation (Steam) fog: a ground-level cloud induced by mixing. It occurs when cold, dry air comes in contact with the relatively warm water. Over the ocean in polar regions where the air is extremely cold during winter, stream fog is referred to as arctic sea smoke. Stream fog also develops on a cold day over a heated outdoor swimming pool or over a warm lake in early autumn.

In general, fog creates pollution, health, and transport problems. The foggiest place near sea level in the United States is Cape Disappointment, Washington, which has an average of 106 foggy days per year.

Figure 5.11, page #123 (Ahrens)

Cloud: a visible aggregate of tiny water droplets or ice crystals or a mixture of both suspended in the air. It forms when the condensation (or deposition) of water vapor meets supersaturation conditions (that is, relative humidity greater than 100%) within the air that is free of dust and other aerosols. Within the atmosphere, at most, only a slight supersaturation conditions are necessary for cloud development.

Curvature effect: At a given temperature, the saturation vapor pressure is higher in the air surrounding a spherical water droplet than in the air over a flat surface. As water surface increases its curvature, more water molecules evaporate. Water molecules that form a curved surface have fewer neighboring molecules and hence, they are weakly bound than the molecules over a flat surface.

The curvature effect increases with decreasing radius of a water droplet. Thus, water molecules are more readily escape small droplets than large droplets. The saturation vapor pressure and the degree of supersaturation needed for cloud development increases rapidly as the radius of the droplet decreases.

Nuclei: tiny solid and liquid particles of matter on which condensation or deposition of water vapor takes place. They are products of both human and natural activity such as forest fires, volcanic eruptions, soil erosions, saltwater spray and the discharge from domestic and industrial chimneys. There are typically about 10,000 nuclei per cm3 of air.

Hygroscopic nuclei: tiny particles of matter that have a special chemical composition for water molecules such that condensation may begin on these nuclei at relative humidity less than 100%. For example, magnesium chloride, a salt in sea spray, is a hygroscopic substance that can initiate condensation at relative humidity as low as 70%.

Hydrophobic nuclei:tiny particles (oil and gasoline) of matter that resist condensation even if the relative humidity is above 100%.

Cloud condensation nuclei (CCN): tiny particles that promote the formation of water droplets. CCN are active at temperatures both above and below freezing. If the temperature is below 0 C (32 F), supercooled water droplets will form.

Ice-forming nuclei (IN): tiny particles that promote the formation of ice crystals. They are much less abundant than CCN and become active only at temperatures well below freezing.

There are two types of ice nuclei: i) freezing nuclei, on which water droplet freezes. They are active only at temperatures below -9 C (16 F). ii) deposition nuclei, on which water vapor deposits directly to ice. They do not become fully active until temperatures drop below -20 C (-4 F).

Table 5.1, page #117 (Ahrens)

Haze:a later of dust or salt particles that reduce visibility through scattering sunlight. The dry haze particles are smaller than the wet haze particles. Dry haze is responsible for bluish sky color under dark background or yellowish color under light-color background. Wet haze results in gray or white color sky.

Classification of clouds: Clouds are given special Latin names and are classified by appearance and by altitude of occurrence. Latin names, which describe appearances and characteristics of clouds, are given by an English observer, Luke Howard (1803). Based on appearance, clouds are divided into three categories: i) cirrus clouds are fibrous, ii) stratus clouds are layered, and iii) cumulus clouds are heaped or puffy.

Convective clouds: are heaped or puffy in appearance with exhibiting significant vertical development that could exceed 30 m/sec (70 mi/hr) over a relatively narrow area.

Stratiform clouds: are layered in appearance with exhibiting weak vertical development such that it is typically less than 5 cm/sec (1 mi/hr) over a broad area.

On the basis of altitude, clouds are categorized into four classes: i) high clouds are found at altitudes above 7000 m (23,000 ft.), at temperatures below -25 C (-13 F), and are composed of almost exclusively ice crystals, ii) middle clouds are found at altitudes between 2000 and 7000 m (6600 and 23,000 ft.), at temperatures generally between 0 to -25 C (32 to -13 F), and are composed of supercooled water droplets, or a mixture of supercooled water droplets and ice crystals, iii) low clouds are found at altitudes below 2000 m (6600 ft.), at temperatures above -5 C (23 F), and are composed of mostly water droplets.

Tables 5.2, and 5.3, page #127 (Ahrens)

Figure 5.23, page #132 (Ahrens)

iv) clouds with vertical development: They are mainly convective clouds in which the cloud development is driven by solar heating. Therefore, convective clouds are formed middle to late morning and become more extensive by mid-afternoon, following the daily variation of insolution.

Cirrus clouds - high, thin wispy clouds up at jet stream level in the troposhere. They are almost always composed of ice crytals and are associated with fair weather.

Figure 5.12, page #128 (Ahrens)

Cirrostratus clouds - high, thin, sheet-like clouds, produce halos around the sun or moon, and weather is often 12/24 hours away.

Figure 5.14, page #128 (Ahrens)

Cirrocumulus clouds - high clouds, rounded white puffs.

Figure 5.13, page #128 (Ahrens)

Altocumulus clouds - puffy or wave-like clouds.

Figure 5.15, page #129 (Ahrens)

Altostratus clouds - grayish blue-gray thin layer covering entire sky uniformy, found ahead of storms, the sun can be seen through but no halo will be observed.

Figure 5.16, page #129 (Ahrens)

Stratus clouds - uniform grayish cloud covering entire sky. It is related to the fog.

Figure 5.19, page #130 (Ahrens)

Nimbostratus clouds - darker gray looking low clouds. They procude light/moderate rain over a large region.

Figure 5.17, page #130 (Ahrens)

Stratocumulus clouds - low, lumpy, puffy clouds in patches or rounded masses, visually appear larger than altocumulus.

Figure 5.18, page #130 (Ahrens)

Cumulus clouds - look like cotton balls, cauliflower in the sky. Cumulus humilis is slightly developed cumulus. Cumulus congestus is moderately developed cumulus cloud.

Figure 5.20, page #131 (Ahrens)

Figure 5.21, page #131 (Ahrens)

Cumulonimbus clouds - related to thunderstorms, can entend up to upper troposphere, can contain both ice and water, precipitation in forms of rain, hail, snow, produce lightning and severe weather, produce anvil cloud at the top of the storm.

Figure 5.22, page #132 (Ahrens)

Contrail: are formed by mixing hot and humid exhaust and dry and cold ambient air.

Figure 5.28, page 135 (Ahrens)

Nacreous clouds: colorful clouds that occur in the upper stratosphere. They are best viewed at high latitudes in winter. They may be composed of ice crystals and/or supercooled water droplets.

Figure 5.29, page #135 (Ahrens)

Noctilucent clouds: wavy, cirrus-like clouds that occur in the upper mesosphere. They are seen only at high latitudes during the twilight just after sunset or before sunrise. They may be composed of ice deposited on meteoric dust particles.

Figure 5.29, page #135 (Ahrens)

Ceiling: height of the lowest layer of clouds above the horizon that are either broken or overcast.

Ceiling (pilot) balloon: a small balloon filled by hydrogen or helium that rises at fairly constat and known speed. The time required to enter to the lowest cloud layer will be ceiling. A battery-operated light can be attached to the balloon for the night operation. Pilots can provide the direct information regarding cloud base.

Rotating beam ceilometer: a ground based projector that rotates vertically from horizon to horizon. As it rotates, it sends out a powerful light beam that moves along the cloud base. A light sensitive detector points upward and picks up the light from the cloud base. Knowing the projector angle and its distance from the detector, the cloud base is calculated.

Laser ceilometer: fixed laser beam ceilometer whose transmitter and receiver point straight up at the cloud base. The transmitter sends out short and intense pulses of infrared radiation that strikes the cloud base. A portion of this radiation is reflected back to the receiver. The time interval between the transmmision and its return determine the cloud base. It is used in ASOS. It cannot measure the cloud if the cloud is not above the station. It cannot measure the cloud base above 3,660 m (12,000 feet).