Chapter 5 Atmospheric Moisture The process whereby molecules

Chapter 5 Atmospheric Moisture

The process whereby molecules break free of liquid water is known as evaporation. The opposite process is condensation, wherein water vapor molecules become a liquid. The change of phase directly from ice to water vapor, without passing into the liquid phase, is called sublimation. The reverse process (from water vapor to ice) is called deposition.

Consider a hypothetical jar containing pure water with a flat surface and an overlying volume that initially contains no water vapor (a). As evaporation begins, water vapor starts to accumulate above the surface of the liquid. With increasing water vapor content, the condensation rate likewise increases (b). Eventually, the amount of water vapor above the surface is enough for the rates of condensation and evaporation to become equal. The resulting equilibrium state is called saturation (c).

Humidity refers to the amount of water vapor in the air. The part of the total atmospheric pressure due to water vapor is referred to as the vapor pressure. The vapor pressure of a volume of air depends on both the temperature and the density of water vapor molecules. The saturation vapor pressure is an expression of the maximum water vapor that can exist. The saturation vapor pressure depends only on temperature.

Absolute humidity is the density of water vapor, expressed as the number of grams of water vapor contained in a cubic meter of air. Specific humidity expresses the mass of water vapor existing in a given mass of air. Saturation specific humidity is the maximum specific humidity that can exist and is directly analogous to the saturation vapor pressure. The mixing ratio is a measure of the mass of water vapor relative to the mass of the other gases of the atmosphere. The maximum possible mixing ratio is called the saturation mixing ratio.

Relative humidity, RH, relates the amount of water vapor in the air to the maximum possible at the current temperature. RH = (specific humidity/saturation specific humidity) X 100% More water vapor can exist in warm air than in cold air, so relative humidity depends on both the actual moisture content and the air temperature. If the air temperature increases, more water vapor can exist, and the ratio of the amount of water vapor in the air relative to saturation decreases.

In (a), the temperature of 14°C has a saturation specific humidity of 10 grams of water vapor per kilogram of air. If the actual specific humidity is 6 grams per kilogram, the relative humidity is 60 percent. In (b), the specific humidity is still 6 grams per kilogram, but the higher temperature results in a greater saturation specific humidity. The relative humidity is less than in (a), even though the density of water vapor is the same.

The dew point is the temperature to which the air must be cooled to become saturated and is an expression of water vapor content. In (a), the temperature exceeds the dew point and the air is unsaturated. When the air temperature is lowered so that the saturation specific humidity is the same as the actual specific humidity (b), the air temperature and dew point are equal. Further cooling (c) leads to an equal reduction in the air temperature and dew point so that they remain equal to each other. When the temperature at which saturation would occur is below 0 °C, we use the term frost point.

In homogeneous nucleation, droplets form by the chance collision and bonding of water vapor molecules under supersaturated conditions. The formation of water droplets onto hygroscopic (water-attracting) particles is called heterogeneous nucleation, and the particles onto which the droplets form are called condensation nuclei. When condensation occurs, the condensation nuclei dissolve into the water to form a solution.

If saturation occurs at temperatures between 0 °C and -4 °C, the surplus water vapor invariably condenses to form supercooled water (temperatures below the melting point of ice existing in a liquid state). The formation of ice crystals at temperatures near 0 °C requires ice nuclei, which are rare in the atmosphere because they must have a six-sided structure.

The simplest and most widely used instrument for measuring humidity is the sling psychrometer, which has two thermometers called the wet bulb and dry bulb. The difference between the two temperatures, the wet bulb depression, depends on the moisture content of the air and can be used to determine dew point and relative humidity.

The value corresponding to the row for the dry bulb temperature and the column for the wet bulb depression yields the dew point temperature.

The value corresponding to the row for the dry bulb temperature and the column for the wet bulb depression yields the relative humidity.

Aspirated psychrometers are equipped with fans that circulate air across the bulbs of the two thermometers. The hair hygrometer uses human hair that expands and contracts in response to the relative humidity. A hygrothermograph is a hygrometer coupled with a bimetallic strip and rotating drum to give a continuous record of temperature and humidity.

The effect of humidity and high temperatures can be expressed in a heat index. The apparent temperatures caused by the combination of heat and humidity provide important guidelines for people. At values between 41 °C to 54 °C muscle cramps or heat exhaustion are likely for high-risk people. Apparent temperatures above 54 °C (129 °F) are considered extremely dangerous, and heat stroke is likely for at-risk people.

A diabatic process is one in which energy is added to or removed from a system, such as air that is warmed by conduction when in contact with a warm surface or air that passes over a cool surface and loses energy by conduction. The direction of heat transfer is in accordance with the second law of thermodynamics, which dictates that energy moves from regions of higher to lower temperatures.

Processes in which temperature changes but no heat is added to or removed from a substance are said to be adiabatic. The rate at which a rising parcel of unsaturated air cools, called the dry adiabatic lapse rate (DALR), is very nearly 1. 0 °C/100 m (5. 5 °F/1000 ft).

If a parcel of air rises high enough and cools sufficiently, expansion lowers its temperature to the dew or frost point, and condensation or deposition commences. The altitude at which this occurs is known as the lifting condensation level (LCL). The rate at which saturated air cools is the saturated adiabatic lapse rate (SALR), which is about 0. 5 °C/100 m (3. 3 °F/1000 ft).

Unlike the DALR, the SALR is not a constant value. If saturated air cools from 30 °C to 25 °C (a 5° decrease), the specific humidity decreases from 27. 7 grams of water vapor per kilogram of air to 20. 4. A 5 °C drop in temperature from 5 °C to 0 °C lowers the specific humidity only 1. 7 grams for each kilogram of air. This brings about less warming to offset the cooling by expansion, as well as a greater saturated adiabatic lapse rate.

The environmental lapse rate (ELR), applies to the vertical change in temperature through still air. A balloon rising through air with an ELR of 0. 5 °C/100 m passes through air whose temperature decreases from 10 °C at the surface, to 9. 5 °C at 100 m, and 9. 0 °C at 200 m. The air within the balloon cools at the dry adiabatic lapse rate of 1. 0 °C/100 m, faster than the ELR, and therefore attains a temperature of 8 °C at the 200 -m level.

Dew is liquid condensation on a surface that occurs during the early morning after a clear, windless night. The formation of frost is similar to that of dew, except that saturation occurs when the temperature is below 0 °C depositing small ice crystals. Frozen dew begins when saturation forms liquid dew at temperatures slightly above 0 °C. When further cooling brings its temperature below the freezing point, the liquid solidifies into a thin, continuous layer of ice.

Fog is a cloud whose base is at or near ground level. Radiation fog (ground fog) develops when the nighttime loss of longwave radiation causes cooling to the dew point. Advection fog forms when relatively warm, moist air moves horizontally over a cooler surface. Upslope fog is formed by adiabatic cooling as air flows upward along a sloping surface, expanding and cooling. Precipitation fog forms from the evaporation of falling raindrops. Steam fog occurs when cold, dry air mixes with warm, moist air above a water surface.

The different types of fog commonly found throughout North America.

The dew point decreases as the air rises, at the rate of about 0. 2 °C/100 m (1. 1 °F/1000 ft). This decrease is called the dew point lapse rate. As unsaturated air is lifted, its temperature therefore approaches the dew point by 0. 8 °C for every 100 m of ascent (i. e. , 1. 0 °C minus 0. 2 °C). Thus, if the air temperature and dew point start out at 18 °C and 10 °C, respectively, an ascent of 1000 m is necessary to cause saturation.

The next chapter examines cloud development and forms.