1 Sarah Rose Book Notes Chapter 6 Storms contain rapidly rising air known as convection Stability determines the location and intensity of convection storms An Air Parcel is a distinct blob of air we will imagine we can identify as it moves through the atmosphere Stable if the parcel is displaced vertically it will return to its original position Neutral if the parcel displaced vertically it will remain in its new position Unstable if the parcel is displaced vertically it will accelerate away upward or downward form its original position Adiabatic Process is one in which a parcel of air does not mix with its environment or exchange heat energy with its environment A balloon who s skin does not permit the exchange of mass energy Compression heats the air bicycle pump heats up when you compress air Decompressing air cools as it expands aerosol spray coming out of a can Air parcels rising through the atmosphere will expand as they rise because they encounter lower pressure Rising parcels cool because they must do work to expand The air parcel cools as it expands rises The air parcel warms as it compresses sinks Lapse Rate is the rate at which temperature decreases lapses with increasing altitude Dry Adiabatic Lapse Rate is 10 degrees C per Km 10 Km A rising air parcel will cool at a rate of 10degrees C per Km of ascent and a sinking air parcel will warm at the same rate What happens to a saturated air parcel as it rises 1 The air parcel will cool as it rises 2 Water vapor will condense as the parcel rises a cloud forms 3 As water vapor condenses latent heat is released 2 4 The latent heat that is released will offset some of the cooling that occurred as the air parcel rose the amount of latent heat released will vary but on average will offset about 4 deg C km of cooling Moist Adiabatic Lapse Rate is determine by considering the combination effects of expansion cooling and latent heating When clouds form water vapor condenses to liquid and latent heat warms the air in which the could is forming Approximate moist adiabatic lapse rate is 6 degrees C per Km The rate at which a saturated air parcel will cool if it rises or warm if it sinks applies to an air parcel with a relative humidity of 100 Use the dry adiabatic lapse rate or moist adiabatic lapse rate to determine the temperature of an air parcel as it rises or sinks in the atmosphere Environment is simply the atmosphere outside the parcel Environmental Lapse Rate rawinsondes calculate the rate at which the environment s temperature decreases with increasing altitude Rate is variable about 4 9 degrees C per Km Varies from location to location day to day and each layer of the atmosphere When the temperature of a layer of air increases with height it s called an inversion And inversion will always have a negative lapse rate An air parcel will rise or sink if its temperature is warmer or cooler then the environment s temperature 3 We determine the stability of the parcel by comparing the parcel s temperature after lifting to the temperature of the surrounding environments at the same altitude A is unsaturated the parcel 15 C is warmer then the environment 10 C it will continue to rise The parcel is unstable B is unsaturated Displace unsaturated air parcel 1 km it will cool at 10 C km The air parcel 15 C is colder then its environment 20 C so it will sink back to its original position It is stable C the temperature of the air parcel is much warmer then its environment and it will continue to rise unstable D the parcel will be cooler then the environment and the parcel will sink back to its original position stable Examples A and B show that a rising air parcel will have the same temperature as its environment at all levels only if the environmental lapse rate is exactly 10C km this is called neutral stability If a displaced parcel is saturated and unstable it is called conditionally unstable Most common changes in stability is the diurnal variation that occurs as air near the surface warms by day and cools at night resulting in decreased stability in the afternoons Thunderstorms consist of unstable air parcels rising though the atmosphere that is cooler then the parcel the atmosphere is normally conditionally unstable Condensation Level is the level where saturation first occurs cloud base 4 Level of Free Convection the levels at which an air parcel first become buoyant its temperature first exceeds the surrounding environments temperature Lifting Mechanisms Horizontal movement of relatively cold dense air to a region occupied by warmer less dense air Air parcels are lifted when a dome of cold air moves forward displacing the air ahead of it upward If the warm air is conditionally unstable instability may develop The leading edge of the cold air would be a cold front Lifting occurs whenever low level winds blow towards each other convergence Low pressure systems are characterized by an inflow of low level air arising from frictional deflection of the winds Air near the surface can be heated during the day by solar radiation and rises spontaneously Chapter 7 Four forces that control wind 1 Pressure Gradient Force PGF a The force applied on an air parcel due to pressure differences 2 Gravitational Force 3 Frictional Force 4 Coriolis Force Pressure gradient is the rate at which pressure changes with distance Pressure Gradient Force The direction of the PGF is always perpendicular to the isobars and points form high to low pressure The PGF is large when the isobars are closely spaced and small when they are spaced far apart The direction and strength of the PGF is indicated by the arrows longer arrows stronger force The wind speed will increase as the horizontal PGF and horizontal pressure gradient increase Gravitational Force the magnitude of the gravitational force acting on any air parcel can be considered constant throughout the troposphere Frictional Force the force of friction acts in the direction opposite the wind motion of air always acts to reduce the speed of the flow 5 Frictional drag is strongest near Earth s surface and decreases with height The atmospheric layer in which friction is an important force is call the boundary layer Mechanical Turbulence develops when air encounters obstructions associated with ground roughness Thermal Turbulence develops when the air near the Earth s surface is heated sufficiently during the day and becomes unstable and rises to higher altitude Shear Induced Turbulence occurs when wind speed