Assignment
Homework #2 due: Tuesday, October 31
Housekeeping
Reviewed the class website
Received Surface data plot for 17Z 22 OCT 00, Plot of 300 Height/Wind for ETA analysis for 12Z 22 OCT 00,Infrared satellite image for 2202Z OCT 00 NRL Monterey Code 7541, and Meteogram for KOAK from 0300Z 22 OCT 00 to 0300Z 23 OCT 00.
Factors that contribute to high fire danger in coastal California in the fall
à High winds: Gives more oxygen to flame. Causes flying embers and falling trees.
à Dry conditions (low relative humidity): Creates fuel for fire by drying out (removing moisture) from vegetation such as grass and trees.
à Hot: Contributes to drying.
à Dried out (normal 6-month period). Maximum in October.
From the Surface data plot for 17Z 22 OCT 00
Heavy rain in Texas, and water vapor is high near the Gulf of Mexico.
Moist air being drawn north.
Cold front through Baja California.
Winds are related pressure differences - high to low.
Pressure comparison between Winnemucca, NV and San Francisco, CA
| Winnemucca, Nevada | San Francisco, California |
| 1024.7 | 1013.7 |
* General High pressures mean: 1 minute average 30 miles per an hour.
Air coming from
region of higher elevation. It expands/decompresses as it rises.
Air comes from Reno, gets compressed because it descends.
From 300 height/wind plot 12Z 22 OCT 00
Vandenberg AFB -120 knot
Oakland - 80 knot
* Jet stream becomes more accurate on the 300 millibar chart.
From the Infrared satellite image 2202Z 22 OCT 00
California is totally clear, and ground is dark. Relative humidity is lower.
From the Meteogram for KOAK 0300Z 22 OCT 00 to 0300Z23 OCT 00
03000Z 22 OCT=2000 21 OCT PST
Hours are at the bottom of chart.
8 PM on Saturday Z is 7 AM Sunday PST.
The chart runs through 24-hour period.
Dew point temperature / relative temperature is about 20%.
When sun goes down, air pressure decreases.
VIS: Visibility per hour
Wind
Santa Anna winds: Foehn-type winds in the Southern California coastal sections.
Diablo Winds: Foehn-type winds in the Bay Area.
Yosemite: Mono winds
| Foehn-type Winds: Downslope winds that flow outward from a region of dynamically-induced surface high pressure located over a region of elevated topography. |
During warmer season the pressure is lower in the interior and higher along the coast.
We observed weather map for 16Z 24 OCT 00 (http://weather.unisys.com)
Radar shows rain. Yellow and orange color indicates heavy rain.
Looking at the map, wave cyclone drifted form Arizona into Texas.
Wind moves NE near DHT. DHT has a lot of rain but not continuously.
Red colored area à 3" rain per hour (red shows rain lasted at least one hour).
Green à moderate to heavy.
Yellow and orange à thunderstorm
We observed 5-day period of Navy weather map.
Jet stream can be thought out as storm track.
In middle latitude areas (like our area), wind blows from west to north.
About the Troposphere...
*The word Trop is Greek for heat.
*The Troposphere is charaterized by great overturning motions which occur because the atmosphere is heated from beneath. And therefore, it is also charaterized by the decrease of temp. with the increase of elevation.
*90% of all phenomena which teh average person calls weather occurs in the Troposphere.
Inversion
An inversion is a structure in which the usual temperature decrtease upwards in our atmospehric enviroment is reversed and temperature increases upward. An example of how this can occur takes place along the California coast in the summer. As air streams from the Pacific High to the California Thermal Low, it passes over the cold waters of the California current. As the air moves over the current and the upwelled waters along the coast, the air in the surface layer is cooled by conduction. Other layers of air above the surface are not in contact with the Pacific and are not cooled. Thus the air nearest the surface of the ocean is the coolest and we find the marine inversion in the sounding during the summer time.
Temperature and Pressure
This relates to pressure patterns because one of the two ways that Dr. Monteverdi will show us that surface pressures change is related to heating/cooling patterns on the earth's surface.
A certain class of surface low pressure systems (called thermal lows) is caused by higher temperatures with thermal high pressure associated with lower temperatures. During the summer season, continents warm more than the oceans to, which explains why it the North American Thermal Low is found on the continent in the warm season and the Pacific High is found offshore. On the other hand, during winter, the oceans are warmer than the continents, with high pressure over North America and Asia.
| For a given amount of sunshine (intensity or hours), land (rocks, soil, sand, ect.) heats up more (i.e. attains warmer temperature) than the ocean. For a given amount of darkness land cools downmore (i.e. attains colder temperatures) than the ocean. |
In the summer, wind speeds are of higher values later in the day because the temperature of the land mass is increasing. This creates a greater low since in the absence of other effects, heating a column of air results in the formation of a surface low pressure area (and, in the absence of other effects, cooling a column of air creates a surface high pressure area).
Formation of Surface Lows and Highs: Upper Tropospheric
Divergence
Divergence: the motions that result in neighboring air parcels at a given level "spreading" apart. The opposite is Convergence.
The reason for pressure changes at sealevel centers on how and why air parcels are removed laterally from the given air column. If there is a net removal or addition of air parcels into an air column pressure falls or rises at the boffom (sealevel).
The basic mechanism for this removal is called DIVERGENCE by meteorologists.
There are two general main ways in which such DIVERGENCE occurs. Hence, there are two ways in which pressure changes at sealevel can be initiated. The resulting pressure system is termed, either DYNAMIC or THERMAL depending on the initiating mechanism
Air currents spiraling inward at low levels of cyclones converge, ie. they move toward the center. At levels about 6 to 7 kilometers, horizontal convergence occurs over surface anticyclones while divergence occurs over surface cyclones.Divergence in the upper atmosphere tends be associated with falling surface pressures and surface convergence as surface air tends to accelerate into the surface low pressure area. Friction slows this inrush of air so that upper divergence is generally not balanced by lower convergence.
| RULE: Divergence aloft tends to be associated with surface low pressure area development (storminess if moisture is present in the air that is drawn upwards). |