Of the remaining notes, only some will be covered in lecture. You still need to read them and the relevant pages of the book. You need the info here for Homework, and several questions from Test #3 will come from this material!

Fog

• Clouds result when air rises and cools adiabatically to saturation
• Fog can form by air cooling to saturation as well, but it can also form by addition of water vapor to cause saturation
  

Fogs Formed by Cooling

• Fogs formed by cooling:
• Radiation Fog
• Advection Fog
• Upslope Fog

Radiation Fog

• Radiation fog forms from radiational cooling of the ground and adjacent air
• Occurs at night and forms best with clear skies and high RH
• Clear skies allow ground and lowest air to cool quickly to dew point to form the fog

 Radiation Fog

• Calm wind causes patchiness and very shallow layer of fog
• Light wind (2-3 mph) creates enough turbulence to increase depth of fog without dispersing it
• Any stronger winds will disperse the fog too much

Radiation Fog

• The foggy air is relatively cold and dense
• So it sinks into valleys and is thickest there
• Radiation fog dissipates 1-3 hours after sunrise as sun heats ground, ground warms air near surface, fog evaporates from bottom up—giving the impression it lifted

Radiation Fog--Fig. 5-9

Advection Fog

• Advection fog: caused by advection of warm, moist air over a cold surface
• Cold surface cools the warm, moist air to saturation, and the fog forms
• Need stronger winds than with radiation fog—6 to 18 mph, so cooling can spread over larger depth

Advection Fog

• Generally thicker and more persistent than radiation fog
• Examples are the West Coast in summer and autumn
• Warm, moist Pacific Ocean air advects over cool California Current waters, creating the fog
• Then onshore westerly winds push the fog onto land

Advection Fog—Fig. 5-10

Advection Fog

• Midwest can get advection fog too
• In winter, warm, moist Gulf of Mexico air can be advected here and move over cold, snowy ground
• This often creates very dense fog with dangerously low visibilities

Upslope Fog

• Upslope Fog: created when relatively humid air moves gradually up a sloping plain or moves up steep slopes of mountains
• Air cools adiabatically as it goes up the slope, and when saturation is reached, the fog forms
• Can be quite extensive in areal coverage (like over several Great Plains states)

Evaporation Fogs

• Evaporation Fogs:
• Steam Fog
• Frontal (Precipitation) Fog

Steam Fog—Fig. 5-11

• Steam Fog: when cool air moves over warm water, enough water may evaporate into the cool air to saturate it immediately above the water
• Common over lakes and rivers on cold, clear mornings in autumn, when lakes aren’t frozen yet, but air temp is relatively cold
• Usually shallow because as it rises, water droplets evaporate as they mix with unsaturated air above

Frontal/Precip Fog

• Frontal/Precipitation fog: frontal wedging—warm air being lifted over cooler air creates clouds and precip
• As rain falls, some of it evaporates in the cooler air before reaching the ground
• The evaporation adds water vapor to the cooler air—perhaps enough to reach saturation and form the fog

Dense Fog--Fig. 5-12

Types of Precip--Table 5-4

Rain

• Rain: drops of water that fall to the ground from a cloud and have a diameter of at least 0.5 mm
• Drizzle: fine, uniform drops of water that fall to the ground but have diameters less than 0.5 mm
• Mist: the absolute smallest water drops that can reach the ground without evaporating--we barely notice mist
• Virga: drops the size of raindrops that evaporate before reaching the ground--air beneath cloud was too dry

Snow

• Snow: precip in the form of ice crystals or aggregates of ice crystals
• At low temps, well below freezing, there is not much moisture in the air, so this makes very light, fluffy, powdery snow
• Saturated air at warmer temps near freezing  has more moisture and creates larger snowflakes--the heavy wet snow that is conducive to snowball construction

Snow—Fig. 5-18 (All snowflakes have six sides)

Measuring Snow, pg. 151-152

• Question: What depth of snow needs to be melted to yield one inch of liquid water?
• Answer: It depends on the moisture content of the air that produced the snow
• Light/powdery yields less liquid than heavy/wet

Measuring Snow, pg. 151-152

• The general rule of thumb is 10 inches of snow melt down to 1 inch of liquid water
• Light/powdery snow may have a greater ratio like 15:1, or up to 30:1!
• Heavy/wet snow may have a smaller ratio like 8:1, or down to 4:1!

Sleet and Glaze

• Sleet: small particles of ice that form under certain wintertime conditions
• For sleet to form there has to be a temperature inversion such that the air near the surface is below freezing but then rises to above freezing in the inversion before usually falling back below freezing above the inversion

Sleet and Glaze

• Under this temp profile, precip initially falls as snow, melts to rain in the inversion, and then freezes into sleet near the ground, so it forms while still in the air

Sleet and Glaze

• When freezing rain/glaze forms, the depth of sub-freezing temps near the ground is smaller than for sleet--not enough time for the rain to freeze on its way down
• Thus, it hits the ground as supercooled rain and then freezes on contact with sub-freezing objects (i.e. roads, trees, power lines…everything), forming glaze

Sleet and Glaze—Figs. 5-16 & 5-19 

Hail

• Hail: hard rounded pellets or irregular lumps of ice
• Different from sleet in that it forms up in the clouds and is never falling below the cloud base as rain
• Hail is frozen from the start and gets bigger as cloud droplets collide with it and freeze onto its surface

Hail

• Hail can be as small as a pea and as large as a grapefruit
• Hail technically becomes severe-sized (a severe t-storm warning is issued) if it is one inch in diameter (size of a quarter) or larger

Largest Hailstone--8" diameter: Vivian, SD, July 23, 2010

Cedar Falls Hailstorm--April 06, 2010

Hail

• Hail forms most commonly in cumulonimbus clouds, because:
• To form hail, you need tall, thick clouds that have ice crystals, an abundance of supercooled water drops, and strong updrafts
• The stronger the updraft, the larger the hail that can be supported
• When the hail gets too large (too heavy), it falls to the ground

Hail—Fig. 5-21 (a)

Hail

• Once a downdraft develops in the thunderstorm, a hail shaft forms as a part of the downdraft
• Most of the hail falls out of the storm in the hail shaft