G304 – Physical Meteorology and Climatology Chapter Atmospheric moisture By Vu Thanh Hang, Department of Meteorology, HUS 5.1 The hydrologic cycle • The total amount of precipitation for the entire globe is relatively constant at about 104cm per year • The movement of water between and within the atmosphere and Earth is reffered to as the hydrological cycle • Atmospheric residence time for water vapor is only 10 days or so • The hydrological cycle is a continuous series of processes that occur simultaneously, has no real end and beginning 5.1 The hydrologic cycle (cont.) 5.2 Water vapor and liquid water • The process whereby molecules break free of liquid water is known as evaporation • The opposite process is condensation, wherein water vapor molecules collide with the water surface and bond with adjacent molecules • 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 5.2 Water vapor and liquid water (cont.) 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) 5.2 Water vapor and liquid water (cont.) • 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 (mb) • 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 5.2 Water vapor and liquid water (cont.) 5.2 Water vapor and liquid water (cont.) • Absolute humidity is the density of water vapor, expressed as the number of grams of water vapor contained in a cubic meter of air (g/m3) • Specific humidity expresses the mass of water vapor existing in a given mass of air (g/kg) q = mv/(mv + md) • 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 (g/kg) r = mv/md • The maximum possible mixing ratio is called the saturation mixing ratio 5.2 Water vapor and liquid water (cont.) • 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 Æ relative humidity depends on both the actual moisture content and the air temperature • Air temperature increases Æ more water vapor can exist Æ the ratio of the amount of water vapor in the air relative to saturation decreases 5.2 Water vapor and liquid water (cont.) 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 grams per kilogram, the relative humidity is 60 percent In (b), the specific humidity is still 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 5.6 Cooling the air to the dew or frost point (cont.) 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) 5.6 Cooling the air to the dew or frost point (cont.) • 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) Æ not a constant value 5.6 Cooling the air to the dew or frost point (cont.) 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 °C drop in temperature from °C to °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 5.6 Cooling the air to the dew or frost point (cont.) 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 °C at the 200-m level 5.7 Forms of condensation • Dew is liquid condensation on a surface that occurs during the early morning after a clear, windless night • At night the loss of longwave radiation can cause the surface to cool diabatically Æ air in contact with cold surface cools by conduction Æ temperature decreases to the dew point Æ condensation 5.7 Forms of condensation (cont.) • The formation of frost is similar to that of dew, except that saturation occurs when the temperature is below °C depositing small ice crystals 5.7 Forms of condensation (cont.) • Frozen dew begins when saturation forms liquid dew at temperatures slightly above °C • When further cooling brings its temperature below the freezing point, the liquid solidifies into a thin, continuous layer of ice 5.7 Forms of condensation (cont.) • Fog is a cloud whose base is at or near ground level • Fog can form by the lowering of the air temperature to the dew point, an increase in the water vapor content, or the mixing of cold air with warm, moist air • Precipitation fog forms from the evaporation of falling raindrops 5.7 Forms of condensation (cont.) • Steam fog occurs when cold, dry air mixes with warm, moist air above a water surface 5.7 Forms of condensation (cont.) • Radiation fog develops when the nighttime loss of longwave radiation causes cooling to the dew point 5.7 Forms of condensation (cont.) • Advection fog forms when relatively warm, moist air moves horizontally over a cooler surface 5.7 Forms of condensation (cont.) • Upslope fog is formed by adiabatic cooling as air flows upward along a sloping surface, expanding and cooling 5.8 Formation and dissipation of cloud droplets • Clouds are usually the result of the adiabatic cooling associated with rising air parcels • The dew point decreases as the air rises, at the rate of about 0.2°C/100m Æ the dew point lapse rate • If the air temperature and dew point start out at 18°C and 10°C, respectively, an ascent of 1000m is necessary to cause saturation 5.8 Formation and dissipation of cloud droplets (cont.) • Raising an air parcel above the LCL leads to the formation of small cloud droplets • But at about 50m or so above LCL, all the condensation nuclei in the air will have attracted water, further uplift leads only to the growth of existing water droplets Æ no new droplets form as the air continues to rise Æ the existing droplets grow larger • The processes that lead to the formation of a cloud not continue forever Æ lifting will cease, no further condensation Æ cloud development ends 5.8 Formation and dissipation of cloud droplets (cont.) • Consider a rising air parcel begins to subside Æ warms at the SALR Æ evaporation of droplets • The evaporation continues descended back to LCL until the parcel has • Below the LCL, air parcel wamrs at the DALR Æ all droplets will have evaporated • At the initial level, the air will have its original temperature and dew point • Æ reversible processes [...]... saturated adiabatic lapse rate (SALR), which is about 0 .5 °C/100 m (3.3 °F/1000 ft) Æ not a constant value 5. 6 Cooling the air to the dew or frost point (cont.) 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... energy (a decrease in temperature), and work performed on the gas (compression) leads to warming 5. 6 Cooling the air to the dew or frost point (cont.) 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) 5. 6 Cooling the air to the dew or frost point (cont.) • If a parcel of air rises high enough and cools... humidity 5. 5 High humidities and human discomfort • The effect of humidity and high temperatures can be expressed in a heat index (the apparent temperature) • 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... raindrops 5. 7 Forms of condensation (cont.) • Steam fog occurs when cold, dry air mixes with warm, moist air above a water surface 5. 7 Forms of condensation (cont.) • Radiation fog develops when the nighttime loss of longwave radiation causes cooling to the dew point 5. 7 Forms of condensation (cont.) • Advection fog forms when relatively warm, moist air moves horizontally over a cooler surface 5. 7 Forms... expansion, as well as a greater saturated adiabatic lapse rate 5. 6 Cooling the air to the dew or frost point (cont.) 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... muscle cramps or heat exhaustion are likely for high-risk people • Apparent temperatures above 54 °C are considered extremely dangerous, and heat stroke is likely for at-risk people 5. 5 High humidities and human discomfort (cont.) 5. 6 Cooling the air to the dew or frost point • A diabatic process is one in which energy is added to or removed from a system • The direction of heat transfer is in accordance... equal to each other When the temperature at which saturation would occur is below 0 °C, we use the term frost point 5. 3 Distribution of water vapor • Water vapor gets into the atmosphere either from local evaporation or from the horizontal transport of moisture from other locations 5. 4 Measuring humidity • The simplest and most widely used instrument for measuring humidity is the sling psychrometer,... temperature of 8 °C at the 200-m level 5. 7 Forms of condensation • Dew is liquid condensation on a surface that occurs during the early morning after a clear, windless night • At night the loss of longwave radiation can cause the surface to cool diabatically Æ air in contact with cold surface cools by conduction Æ temperature decreases to the dew point Æ condensation 5. 7 Forms of condensation (cont.) •... occurs when the temperature is below 0 °C depositing small ice crystals 5. 7 Forms of condensation (cont.) • 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 5. 7 Forms of condensation (cont.) • Fog is a cloud whose base is at or near... point and relative humidity 5. 4 Measuring humidity (cont.) Å Sling psychrometer Hygrothermograph Æ 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 5. 4 Measuring humidity (cont.)