Moisture and Stability in the Atmosphere
Humidity can be measured as: HUMIDITY Absolute humidity the mass of water vapour in a volume of air (g/m 3.) Relative Humidity the proportion of the actual mass of water vapour in a given volume of air to the maximum amount that could be contained at that temperature; it can be expressed as a percentage. Dew point the temperature at which further cooling of air will lead to condensation. Condensation Level the altitude in the atmosphere at which condensation occurs. It is usually marked by the cloud base. Condensation nuclei microscopic particles of dust or salt crystals in the lower atmosphere on which water (or ice) can condense. They are hygroscopic (have an affinity for water). Without condensation nuclei, air will become super-saturated with water vapour. This can occur in the upper atmosphere above 6000m where there are few condensation nuclei. Latent heat The energy required to evaporate a particular volume of water. In a sense it remains hidden or latent until the quantity of water condenses when the same amount of energy is released.
The relationship between humidity and air temperature Air which is saturated at 4 O C (i.e. has a relative humidity of 100%) will have a relative humidity of 70% at 10 O C and 20% at 33 O C.
Condensation can take the form of: Cloud, mist or fog droplets. Cloud, mist or fog ice crystals. Dew. Hoar frost & Rime. Precipitation forms by either coalescence or the Bergeron- Feindeison mechanism (The growth of ice crystals at the expense of water droplets). Precipitation takes the form of Snow/sleet. Rain/ freezing rain. Hail.
CONDENSATION usually occurs when air is cooled. Cooling can be caused by: Radiation cooling (usually leading to dew, frost, mist and fog). Advection cooling (usually leading to mist or fog) Cooling also occurs when air is forced to rise and to expand as the air pressure declines with altitude. This is called adiabatic cooling. The clouds that lead to most rain, snow or hail are caused in this way and hence precipitation may be classified according to the cause of the mechanism of uplift: Orographic precipitation (forced over a relief feature). Frontal precipitation (warm air is forced to rise over cooler, denser air along a warm or cold front). Convectional precipitation (humid air is warmed by the land or sea surface and rises as it expands and becomes more buoyant).
LAPSE RATES The rate at which air temperature decreases with increasing altitude is caused a lapse rate. In tropical areas, the rate of decline may be very rapid in the first few hundred metres as the earth s surface is the chief heat source. Lapse rates vary considerably from place to place and over time. ENVIRONMENTAL LAPSE RATE (ELR) This is the actual temperature decrease with increasing altitude at a particular place and at a particular time. It varies considerably but averages about 0.65 O C per 100m rise. DRY ADIABATIC LAPSE RATE (DALR) - The atmosphere is never completely dry; but unsaturated air is described as dry if its moisture content does not condense. If a dry mass of air in contact with the earth s surface is forced to rise without mixing with the air around it (the environmental air), it is said to cool adiabatically. The cooling rate of this unsaturated (dry) air is about 1.0 O C per 100m. SATURATED ADIABATIC LAPSE RATE (SALR) Saturated air that is forced to rise cools more slowly than dry air. This is explained by the release of latent heat as the water vapour condenses into droplets and/or ice crystals. At first the SALR is about 0.5 O C per 100m, but as the rate of condensation decreases, the SALR will increase to about 0.75 O C per 100m when the air temperature is between 0 O C and -40 O C.
A Range of Stability Conditions shown on sketch tephigrams
A Tephigram showing Conditional Instability
An example of instability to an altitude of 5000m. Above this altitude, stable conditions prevail.
Unstable Air: a parcel of air that is lifted will continue to rise as it immediately becomes warmer than its surroundings. Stable Air: Stable air will only rise when it is forced to do so, for example over high ground, along the gradient of a front or as a result of convection.
Stability and Air Masses As air masses migrate away from their source regions, they modify the environmental lapse rate. For example, warm tropical maritime air arriving in Britain will be chilled, becomes more dense and increases stability. Condensation such as fog or stratus cloud may form. If cold polar maritime air moves across a warmer land surface it will warm from below, become more buoyant leading to increasing instability (often showers dominate the weather).
The Föhn or Chinook Wind 1. Why does the air not continue to rise on the lee side of the mountain? 2. What are the differences between the air at A and B? 3. What impact will the airstream have on the weather on the lee side of the mountain?