Unstable Atmosphere: - Air forced upwards will continue to rise - As air rises, Temperature decreases and RH increases if RH=100% => condensation and cloud formation => latent heat released => possibility of storm formation Clicker Question Set Frequency to "BB" Which type of clouds are more likely to form in unstable conditions? (A) stratus clouds (layered) (B) cumulus clouds (vertically developed) (C) fog (cloud in contact with surface) 1. WHAT CAUSES ATMOSPHERE TO BECOME MORE/LESS UNSTABLE? 2. WHAT CAUSES AN AIR PARCEL TO INITIALLY RISE? 1. WHAT CAUSES ATMOSPHERE TO BECOME MORE/LESS UNSTABLE? ONE WAY IS THROUGH MIXING => Rising Air Cools Sinking Air Warms Height initial profile of environment air expands and cools compresses and warms Temperature Height Conditionally Unstable ΓM (6°C/km) ΓDRY (10°C/km) Stable Unstable Temperature 1. WHAT CAUSES ATMOSPHERE TO BECOME MORE/LESS UNSTABLE? ONE WAY IS THROUGH MIXING => Rising Air Cools Sinking Air Warms Height initial profile of environment air expands and cools compresses and warms Temperature 1. WHAT CAUSES ATMOSPHERE TO BECOME MORE/LESS UNSTABLE? ONE WAY IS THROUGH MIXING => Rising Air Cools Sinking Air Warms Height initial profile of environment air final profile of environment air expands and cools compresses and warms Temperature Another way the atmosphere can become more unstable: The large-scale lifting an entire layer of air - here a large-scale layer of air is much larger than a parcel - layer may be 100's of kilometers across - the lifting of a layer typically happens with a low pressure system (more later in course) Height 1000 m Temperature - Original layer (in blue) is very stable Height 1500 m dry adiabatic lapse rate (10C/km) 1000 m Temperature - Original layer (in blue) is very stable - As layer rises, it expands - As air rises => cools at dry adiabatic lapse rate (10°C/km) (assume no condensation) - Top of layer rises 500 meters more than bottom of layer => Top of layer cools more than bottom of layer - Result => environment lapse rate within layer more likely unstable This occurs when low pressure over area. Opposite happens with high pressure system. Low Pressure => Rising Air => Increasing INSTABILITY High Pressure => Sinking Air => Increasing STABILITY Height SPECIAL CASE: CONVECTIVE INSTABILITY 1500 m dry adiabatic lapse rate (10C/km) RH << 100% moist adiabatic lapse rate (6C/km) RH=100% 1000 m Temperature - Bottom of layer moist (RH=100%), top of layer dry (RH<<100%) - Bottom of layer cools at moist adiabatic lapse rate (6°C/km) - Top of layer cools at dry adiabatic lapse rate (10°C/km) => Now even more unstable than before!! CONVECTIVE INSTABILITY 2. WHAT CAUSES AN AIR PARCEL TO INITIALLY RISE? 2. WHAT CAUSES AN AIR PARCEL TO INITIALLY RISE? A. Surface heating leading to Convection 2. WHAT CAUSES AN AIR PARCEL TO INITIALLY RISE? A. Surface heating leading to Convection B. Topography (Mountains) = Orographic Uplift 2. WHAT CAUSES AN AIR PARCEL TO INITIALLY RISE? A. Surface heating leading to Convection B. Topography (Mountains) = Orographic Uplift C. Ascent due to Convergence 2. WHAT CAUSES AN AIR PARCEL TO INITIALLY RISE? A. Surface heating leading to Convection B. Topography (Mountains) = Orographic Uplift C. Ascent due to Convergence D. Uplift along Weather Fronts 2. WHAT CAUSES AN AIR PARCEL TO INITIALLY RISE? A. Surface heating leading to Convection B. Topography (Mountains) = Orographic Uplift C. Ascent due to Convergence D. Uplift along Weather Fronts Set Frequency to "AB" Clicker Question Wind Windward Side Mountain Leeward Side Which side of the mountain would generally receive more precipitation? (A) Windward (B) Leeward (C) Both sides would receive equal amounts Prevailing Wind Prevailing Wind Collision - Coalescence Process (Warm process) Clicker Question Set Frequency to "AB" In general, which cloud would you expect to produce larger rain drops through the collision-coalescence process? (A) cumulus cloud (B) stratus cloud (C) cirrus cloud