Cloud Formation  Ten Basic Types of Clouds (Genera): l High: Ci, Cs, Cc l Middle: As, Ac l Low: St, Ns, Sc l Clouds of Great Vertical Extent: Cu, Cb  Cumulus: heapedAlto: middle  Strato: layeredCirro: wisp  Nimbo: rain

 Clouds may then be further classified according to species and varieties terms.  Species: Castellanus, lenticularis, fibratus, etc.  Varieties: Undulatus, translucidus, opacus, etc.

Cloud Development  Cumuliform Clouds: May form from thermals (convectively unstable air warmed by the surface and rising (convection lifting)) or by vertical turbulence motion in statically stable air.  Cumulus, Cumulonimbus form by air warmed by the surface and which becomes unstable.  Altocumulus, Cirrocumulus form by small scale vertical turbulent motion and not from warmed surface air.

Cumulonimbus  Cirrocumulus  Cumulonimbus

 Stratiform Clouds: Flat layered appearance. l Form in statically stable environment where vertical motion is suppressed. l Must have an external lifting to cause the air to rise (forced lifting). Frontal lifting Orographic lifting Convergence lifting l Cirrus, altostratus, stratus, nimbostratus l Note: Cirrocumulus and Altocumulus should be classified as cumuliform (heaped), even though they form in a statically stable environment.

 Stratus  Nimbostratus

 Stratocumulus: May be caused by either thermal updrafts or turbulent eddies or both. l The vertical thickness of the cloud is less than it’s horizontal extent. l The turbulent eddies are relatively small and/or the thickness of unstable air above the cloud base is small.

Processes causing saturation  1. Cooling: Temperature cools to Dew Point l Adiabatic Cooling by lifting of air parcels. Forced lifting –Frontal –Orographic –Convergent Convective lifting (buoyancy) l Cooling by being in contact with cold ground. Advection fog l Cooling by radiation. Ground temperature cools by radiation. Radiation fog.

 2. Adding moisture: Dew Point increases to Temperature. l Evaporation of liquid water into unsaturated air. Forms precipitation or frontal fog. l Steam fog. Cold air moves over warm, humid surface (Cold air moves over warm lake or ocean).

 3. Mixing of two unsaturated parcels. l The temperature, vapor pressure, mixing ratio, specific humidity are the weighted averages of the original parcels.

 Example problem, pg. 149 l Why cannot table 5.1 be used to determine r and r s ?

Clouds and upslope fog  Air is forced upward or ground rises and air is forced to flow up over ground.  Rising air cools at dry adiabatic lapse rate until saturation (T = T d ). Base of the cloud, is the LCL.

Other Fogs  Advection Fog: Caused by advection of warm, moist air over cold surface. Cold surface cools the air to saturation. As air moves across the surface, it becomes colder by heat being conducted out of it into the ground, or water, surface. The potential temperature of the air depends on the time spent over the ground which is a function of the wind speed.

 z i = thinkness of layer   0 = initial potential temperature of air  C H = heat transfer coefficient (value between 2x10 -2 and 2x10 -3   sfc = potential temperature of surface.  Distance air must travel before fog begins forming:

 Radiation Fog: Infrared radiation at night from the ground surface lowers the temperature of the ground and results in lowering of the air in contact with the ground. If the temperature drops to, or below, the dew point temperature, fog forms.  The depth of the fog will be the height to which temperatures have dropped to the dew point.

 The time the radiation fog begins to form can be approximated by the empirical formula: (7.10) where, l a = 0.15m 1/4 s 1/4 l T RL =residual layer temperature (Temp. in statically neutral layer above near surface See pg. 151). The temperature of air unaffected by ground cooling. l M = wind speed in residual layer l F H = average surface kinematic heat flux. (pg. 52)

Residual Layer  Consider the structure of the Atmospheric Boundary Layer. Lowest (on the average) two km of the atmosphere.

 FA: Free atmosphere layer  EZ: Entrainment Zone  MI: Mixed Layer  z i : Mixed-layer depth  SL: Surface Layer  CI: Capping Inversion  RL: Residual Layer  SBL: Stable Boundary Layer  M = Wind Speed  r = Mixing ratio

Within Atmospheric Boundary Layer  During daytime: Statically-unstable mixed layer (ML). Separated from free atmosphere by the strongly stable entrainment zone (EZ) of intermittent turbulence. Mixed layer depth (z i ) is the distance from ground to the middle of the EZ.  During night: Statically stable boundary layer forms under a statically neutral residual layer. The residual layer contains the pollutants and moisture from the previous mixed layer, but is not turbulent.

 At night: Turbulence in the EZ ceases, leaving a non-turbulent separation layer called the capping inversion (CI), which is strongly statically stable.  Bottom 20 to 200 meters of ABL is the Surface Layer (SL). Here, frictional drag, heat conduction and evaporation from surface cause substantial changes in wind speed, temperature and humidity with height. Also known as the constant flux layer.

 The depth of the radiation fog layer can be determined by: (7.11)

Problems  N1 (a, c, e, g)(show thermo-diagram), N2 (a, c), N5(b, c, e), N7(a), N8(a,b)(use spreadsheet for plots), N9(a, b) (use spreadsheet for plots)  SHOW ALL EQUATIONS USED AND CALCULATIONS