Assignments:
· Review writing assignment 2 guidelines for the November 9th class.· · Writing Assignment 2 is due the beginning of class November 16, 2000.·
Handouts:
· Received Santa Ana handout written by Rosenthal.· · Received Monteverdi,s document: "The Santa Ana Weather Type and Extreme Fire Hazard in the Oakland-Berkeley Hills.·
Background for Writing Assignment 2:
Foehn-type Winds are down sloped winds that flow outward from a region of dynamically induced surface high pressure located over a region of elevated topography. Foehn winds are located around the world and are given local names (see Reader). In Northern California Bay Area and the Southern California coastal section these winds are called Diablo and Santa Ana respectively. Diablo and Santa Ana winds flow northeasterly and are associated with warm, dry air. These winds are warm and dry because they originate from over the hot desserts of the Great Basin and due to the effects of the adiabatic rate as the air mass descends the western slopes of the Sierra Nevada. Santa Ana and Diablo winds are often strong due to the relative pressure gradient and funneling by major breaks in mountains. An upper air ridge and trough pattern over the West Coast is associated with Santa Ana,s and Diablo winds. This association was explained in a previous class discussion on Dynamic lows and highs.
Offshore Winds and Fire Hazard in California
The season for warm offshore winds is primarily late summer into the early and middle fall. However, the pattern that produces such flow often also occurs in late spring. Off shore winds in December tend to be associated with cold conditions in north-central California. These offshore winds tend to be very strong and associated with high fire hazard. Why?
First, our offshore winds are a local example of a general class of winds that occurs worldwide in simililar conditions. They are a specific example of "Foehn Winds" just as our tule fog is a specific example of Radiation Fog.
| Foehn Wind- (pronounced "fern "wind) Hot, dry wind that flows outward from a region of high pressure dynamically produced over a region of elevated topography. |
Local Names in California
Diablo Winds- (Foehn wind associated with our area)
Santa Ana Winds- (Foehn wind in Southern California)
Mono Winds- (Central Sierra Nevada)
Why are these winds associated with high fire hazard? Because they are hot, dry and strong and occur after six months of dryness.
Diablo Winds Are
Atmospheric Stability
Last, we review of environmental lapse rate which is 5.5 degrees F for every thousand feet (pages 309-313 in reader to deal with the lapse rate) and started to talked about why thunderstorms may be present after a Mid Latitude type storm. Using the diagram in the reader about the whaether or hot air balloon we learned how the air inside the balloon expands as it rises eventually to the point where the pressure is so great that it pops the balloon.
When an air parcel finds itself warmer than surrounding parcels of air, it will spontaneously rise. High dew point helps make air unstable. Dew point temperature maps cans show areas likely to be unstable.
Unstable State
If the air is colder aloft at 1000 ft than the ground level air- Air that is lifted is cooled but not as cool as the air around it, and it rises. Warm air wants to rise because it is warmer than the air around it at the same elevation. This unstable state causes clouds to form. Some ways to lift air--Fronts, divergence aloft, mountains. (See class handout)
The chief threat from a cummulonimbus cloud is cloud to ground lightning, flash flooding, and tornadoes. For example, the Texas Panhandle. It's anvil was at the ground to about 50,000 ft. It produced a tremendous amount of rain and hail. A lot of rainfall occurred in a short amount of time.
Water Vapor and Latent Heat and Instability
If there is a greater dew point there is more water vapor. Water vapor plays a great role in instability. If there is more water vapor there are more thunderstorms. Why?
Evaporation occurs when water changes, for every gram there is a tremendous amount of cooling. Evaporation chills/cools whatever it is on. It is associated with latent heat. Condensation does the opposite. Moisture condenses, heat lets out in the atmosphere. Thus, if moisture is condensing in a rising air parcel that is the same temperature as the surrounding air, latent heat makes the air parcel warmer and it becomes unstable by being made warmer than the surrounding air. Thus, meteorologists often look for regions of high surface dewpoint and assume that the air there is pretty unstable.
Thunderstorms:
If air is unstable it will rise. If air is very humid, little lifting is needed for the air to reach saturations and for cumulonimbus clouds to develop. Cumulonimbus are "cauliflower looking in form and visible where air is spontaneously rising. Cumulonimbus growth is limited to the tropopause.
"Nimbus" root in the word cumulonimbus means rain. The turbulent air in these clouds causes some water droplets to collide together forming larger water droplets; however, this mechanism is not a major means of precipitation. The lifecycle of ice crystals plays a greater role in precipitation. The Anvil section of the cumulonimbus cloud is above the freezing level and as water vapor rises into this area they become ice crystals. If the ice crystals fall below the freezing level, water will collect on the outsider side of the crystals. Should the turbulent air throw the ice crystals back above the freezing level the above process can repeat, until the updraft can no longer support the increased weight of the ice crystals. As the crystals fall they melt and rain falls from the cloud. Additionally, surrounding air is cooled as the ice melts. This cold heavy air outflows at the ground, which in turn, cools the land surface and in time stops the thunderstorm process. These outflows can travel 30 miles away and cause dust storms.
Lightning is natures attempt to equalize opposing charges. Lighting may forge between opposite charges within the cloud or between the cloud and the ground. Charge distribution within a thundercloud is the result of varying sized ice crystals sliding past each other and causing opposite charges. Updrafts separate these opposing charged ice crystals into different areas of the thundercloud.