CH 6 7 8 9 10 16 2012 Thunderstorms contain air that is rapidly rising Stability ALONE determines location and intensity of convective storms A non atmospheric example of stability Air parcel A distinct blob of air that we will imagine we can identify as it moves through the atmosphere Doesn t exchange energy with its environment Care about it s temperature and if it is saturated unsaturated Air Parcels and Stability Stable if the parcel is displaced vertically it will return to its original position Unstable if the parcel is displaced horizontally it will accelerate 2 Neutral the parcel will stay in its new vertical location What happens to the size of an air parcel as it moves vertically in the atmosphere An air parcel will expand as it cools and rises Vertical motion causes changes of temperature because of expansion and compression What happens to the temperature of an air parcel as it expands rises or compresses sink The air parcel cools as it expands rises Air parcel warms as it compresses sinks Adiabatic a process in which an air parcel does not mix with its environment or exchange energy with its environment Lapse rate the rate as which temperature decreases lapses with increasing altitude Dry Adiabatic Lapse Rate that rate at which an unsaturated air parcel will cool if it rises or warm as it sinks Applies to an air parcel with a relative humidity of less than 100 Dry adiabatic lapse rate 10 deg How might this process be different for saturated air 3 Presence of moisture Latent heat condensation releases heat into the environment Will moist air cool faster or slower then dry air o Slower because there are two competing effects cooling form rising and heating from condensation The Moist Adiabatic Lapse rate the rate as which a saturated air parcel will cool if it rises moist adiabatic lapse rate 6 deg C km Applies to an air parcel with a relative humidity of 100 Saturated air follows the moist adiabatic lapse rate as it rises The air parcel will cool as it rises Water vapor will condense as the parcel rises a cloud forms As water vapor condenses latent heat is released The latent heat what is released will offset some of the cooling that occurred Environmental Lapse Rate is the lapse rate of the environment around a parcel Environment the atmosphere outside of an air parcel Environmental lapse rate the rate as which the environment s temperature decreases with increasing altitude o Use the environmental lapse rate to determine the temperature of the environment as you move up or down in the atmosphere o Use the dry or moist adiabatic lapse rate to determine the temperature of a parcel as it moves up and down Inversion layer a layer of the atmosphere where the environmental temperature increases with increasing altitude To determine the stability we need to compare the temperature of its environment If an air parcel is warmer then its environment it will rise If an air parcel is colder then its environment it will sink 4 For these examples an air parcel starts at the surface and has the same temperature as the environment at the surface The air parcel is then lifted 1 km Conditional instability is often critical for storm development If a parcel is unsaturated it will be stable If a parcel is saturated it will be unstable The parcel always rises at either the DALR or the MALR The environmental lapse rate will vary sometimes over the course of a day sometimes within one profile 5 SUMMARY Stability changes when we look at different levels of the atmosphere As a parcel rises does it dew point temp change No it is a absolute measure Remember what happens to an unsaturated air parcel as it is lifted size temperature relative humidity What happens when the relative humidity of the air parcel increases to 100 6 Lifting condensation level the level where condensation first occurs as an air parcel is lifted where the relative humidity of the air parcel becomes 100 Level of free convection the level where an air parcel first becomes buoyant warmer than its environment Diurnal variation Air near surface warms during the day and cools at night decreasing stability in the afternoon increasing stability at night Convection is not the only lifting mechanism Frontal Lifting Lifting along a sea breeze front Lifting by mountains Local Convergence Four Forces Effect Atmospheric Motion Pressure Gradients The rate at which pressure changes with distance Pressure Gradient Force the force applied on an air parcel due to pressure difference Lower molecular density Lower air pressure How to calculate a pressure gradient Calculate pressure difference o Miami Beach Homestead 1000 mb 925 mb 75 mb Calculate distance between locations o Homestead Miami Beach 42 km Divide o PG 75 mb 42 km 1 8 mb km On a surface weather map the PGF is always perpendicular to the isobars and points from high to low pressure The PGF is large when isobars are closely spaced and small when the isobars are spaced far apart 7 The PGF increases as the pressure gradient becomes larger isobars are close together The wind speed will increase as the horizontal pressure gradient and horizontal PGF increases Gravitational Force balances the vertical pressure gradient force Frictional Force a drag force acting in a direction opposite the motion of the air Caused by a mixing of air parcels turbulence moving at different speeds o Mechanical Turbulence occurs when air encounters obstructions on the surface It is larger over a rough surface o Thermal Turbulence occurs when heated air rises and mixes with the air above most common during the day o Shear Turbulence occurs when the wind speed changes rapidly ove a short distance The frictional force is generally confined to the atmospheric boundary layer Turbulence and thus the frictional force tends to be most important near the surface of the earth and rapidly decrease as you move away from the surface Friction layer boundary layer the atmospheric layer in which friction is an important force 8 The Coriolis Force 1 The Coriolis force causes objects to turn to the right of their direction of motion in the Northern hemisphere left in the Southern hemisphere 2 The Coriolis force can affect the direction of motion of an object but cannot change the speed of the object 3 The Coriolis force is strongest for fast moving objects and zero for objects that are not moving 4 The Coriolis force is zero at the equator and increases to a maximum at the poles 5 The Coriolis force is most important for