Lecture 5: Stability (Quiz 1)

Geography 103 Weather Lecture 5: Stability (Quiz 1) 1. Terminology (1). Convection: rising air current (vertical motion). (2). Advection: wind or horizontal movement of air current. (3). Turbulence: irregular air motion. (4). Adiabatic process No energy or mass exchange between an air parcel and its surrounding air when the air parcel rises or subsides (sinks). (5). Environmental air or ambient air: Air surrounding an air parcel. (6). Air parcel: any given volume of air (can be any size) 2. Lapse rates (1). Environmental lapse rate or existing lapse rate or ambient lapse rate: A. The observed vertical change of air temperature with increasing altitude. B. Determined from radiosonde observation. (2). Adiabatic lapse rate (air parcel’s lapse rate) A theoretical lapse rate determined by the first law of thermodynamics (conservation of energy). A. Dry (unsaturated) adiabatic lapse rate (A). Unsaturated air parcel. (B). -1 oC/100 m (-10 oC/1000 m) or -5.4 oF/1000 ft. Minus sign means air temperature decreases with increasing altitude. B. Wet (saturated) adiabatic lapse rate (A). Saturated air parcel within clouds or fogs. (B). Varies according to moisture content (C). About -0.6 oC/100 m (-6 oC/1000 m) or - 3.2 oF/1000 ft in the lower atmosphere. Release of latent heat during condensation adds heat to the saturated air parcel as it rises. Hence, wet adiabatic lapse rate is smaller than dry adiabatic lapse. 3. Stability The tendency of the resistance of an air parcel against the vertical displacement exerted by an external force or forces (orographic uplifting, solar heating, and weather systems) (1). Stable: A. The air parcel tends to resist against the vertical displacement when uplifted by an external force or forces. B. The air parcel’s temperature is colder than its surrounding air at the same height. C. Small environmental lapse rate [< Dry adiabatic lapse rate, or < 1 oC/100 m (or 10 oC/1000 m), ignore the minus sign]. D. Inversion and/or isothermal (air temperature increases or remains constant with altitude). Environmental or ambient lapse rate. E. An air parcel tends to sink if uplifted from the ground surface. Air parcel’s temperature decreases more than (small environmental or ambient lapse rate) than its environmental (ambient) air and hence is colder than its environmental when uplifted to a given height. Cold air parcel tends to sink, resisting against uplifting. (2). Unstable A. The air parcel tends to exert no resistance against the vertical displacement when uplifted. B. The air parcel’s temperature is warmer than its surrounding air at the same height. C. Large environmental lapse rate (> Dry adiabatic lapse rate, or > 1 oC/100 m or 10 oC/1000 m). D. An air parcel tends to continually rise if uplifted from the ground surface. Air parcel’s temperature decreases less than (large environmental or ambient lapse rate) than its environmental (ambient) air and hence is warmer than its environmental when uplifted to a given height. Warm air parcel continues to rise (no resistance against uplifting). (3). Conditional stability A. Stable for unsaturated air parcel (environmental lapse rate < 10 oC/1000 m). B. unstable for saturated air parcel (environmental lapse rate > 6 oC/1000 m). C. Environmental lapse rate is between dry and wet adiabatic lapse Rates (Environmental lapse rate is between 10 oC/1000 m and 6 oC/1000 m). (4). Neutral stability A. Saturated air parcel: Environmental lapse rate equals wet adiabatic lapse rate. B. Unsaturated air parcel: Environmental lapse rate equals dry adiabatic lapse rate. * Since dry (unsaturated air parcel) and wet (saturated air parcel in clouds or fogs) adiabatic lapse rates are constant (does not change), stability is determined by the environmental or ambient lapse rate only. We assume the wet adiabatic lapse rate in the lower atmosphere is constant since it does not vary much! 4. Weather (1). Stable: stratus clouds, fog, smog, haze, drizzle-type rain (if there is rain, light rains or small raindrops lasting for a long time) (2). Unstable air: cumulus, no fog and less smoggy or hazy, shower rain (if any, heavy rain in short-time). **Please keep in mind, the sky may be clear with either stable or unstable air. It requires other conditions to form smog, haze, fog, cloud and precipitation.