Housekeeping
- Credit/No credit option
- noted importance of Section Headers/Format/following instructions on assignments--i.e. format is
25% of paper grade.
Midterm
- will cover all information covered in class through Tuedsay, October 10.
- will be made up of short-answer, essay and map i.d. questions.
- designed to be completed in 50 minutes.
- concept-oriented.
- will cover: Means/Normals/Extremes, CA advection fog, Rules of Thumb and how to apply them to
weather maps.
Slides on Fog
- fog covering East Pacific Ocean.
- cumulus clouds over Mt. Hamilton.
- Space Shuttle picture, exhibitin more texture, it is much closer than satellite images. This picture
demonstrates fog microclimates, i.e. small areas of fogless beach which are very hard to predict.
- overhead picture of Marine layer.
- overhead pictures of stratus, demonstrate texture of fog.
- series of pictures demonstrating the added texture of early evening clouds because of the angle of the sun.
- anvil-top of a cumulonimbus.
- lightning, biggest danger from a thunderstorm, particularly cloud to ground lightning.
- alpine-glow off of clouds/high peaks which reflect sun from beyond the horizon.
Coriolis Effect
When you look at a weather map you would notice that the air really spirals out of surface highs. The reason for that has to do with the fact that earth is rotating. As the air moves from high to low pressure in a straight line (which you would notice from space), the earth turns out of its way. This creates the illusion that the air is spiraling (and that illusion is real to people on the earth's surface moving with the surface).
This Effect is called " The Coriolis effect "- See page 301 of the Reader. Coriolis force is entirely due to the Earth's rotation around its axis. Corilois force accounts for the fact that air actually does not move in a straight path, directly away from high pressure to low pressure.
We are unaware that the Earth is rotating around us. This rotation accounts for the earth's surface speed of 1035 mph at the Equator and over 700 mph at our latitude. This point moves faster because it must get back to the same place at the same time as the other points, which move at slower speeds.
If a person standing at the North Pole through a ball to a person at 40N, the ball certainly would move in a straight line (discounting friction and gravity). If it took the ball one hour to move to 40N, an observer in space would notice that the ball did that and moved in a straight line. However, in the one hour, the person waiting to catch the ball moves eastward 718 miles, out of the path of the ball. Since that person thinks he is standing stationary on the earth, he assumes that the ball was affected by a force that deflected the ball to its right (to the west). All frictionless moving objects are affected by Coriolis force. In the Northern Hemisphere, all such objects are deflected to the right of their path, while in the Southern Hemisphere, the deflection is to the left. This includes planes, ships, projectiles, as well as winds, and ocean currents.
Applications of Coriolis
Air moving around surface high pressure areas spirals clockwise and outward. Normally, air should move directly from high pressure to low pressure, but because of Coriolis, the motion is actually a spiral. We will talk more about that next lecture.
On summer afternoons, wind speeds increase because the onshore North American Thermal Low intensifies during the day (because of heating). The winds increase because of this and physically cause the ocean surface to move, initially, in the same direction (towards the southeast). This water is then deflected by Coriolis off shore and cold water wells up to replace this water traveling offshore.
So the waters of the Pacific along the California coast are cold for two reasons: 1. because of the drift from the polar regions; 2. upwelllling along the coast. Both of these factors account for the cold water along the coast that is so important to advection fog formation.
Coastal Upwelling:
During Summer, in the morning the continent heats up creating a Thermal Low
By 4:00 pm, NW winds have picked up and the coriolis effect causes surface waters to move off shore
Cold waters move up from the ocean floor causing upwelling
A Upwelling is important to fisheries, some fish die or leave the area when upwelling stops During El Nino, there is a decreased coastal upwelling from mid Summer through fall.
Weather Map Interpretation
The question was raised, "What characteristics would you expect to find from air that is moving towards a Low pressure system." To answer this we used an example of a Low pressure system located in the Northern United States; therefore, the different air masses moving into the Low are warm/moist air from the Gulf and cold/dry air from the North.
Weather Symbols
Check the handouts in the reader and the Appendix in Anthes for the weather symbols we will be using in this class.
Summer Fog Satellite Image
We looked at the satellite image of summer fog along the coast. When we draw stream lines we see that after the fog comes in through the Golden Gate itÏthen splits and goes both north and south. The reason it does this is because the winds areÏmoving toward the low pressure zones that are located both north and south of the Golden Gate. [See p. 257 of our Reader --a map of predominant surface wind flow patterns during the summer in California. The map is an indication of averages over the past 30 years and clearly illustrates the typical wind pattern associated with summer fog that we see in the Bay Area.]
Steps in Studying Surface Data
- locate highs/anti-cyclones, lows/cyclones, and label.
- look for relative humidity, windspeed/direction.
- determind local weather: fog, rain, snow, etc.
Holistic View of Southern Weather Pattern
- hurricanes need ocean temps. of at least 82 degrees.
- 80 degree isotherm extends Southwest from the tip of the baja penisula, except during el Nino, when the
isotherm extends from as far North as central California.
- cumulonimbus clouds get more intense as the weather gets more intense (hotter).
Examination of Weather Maps/Jetstream at 700, 500 and 300 mb levels
general:
- wind flows parallel to isobars.
- notice the distance between isobars, isobars in close proximity predict high winds.
700 mb:
- approx. 10,000 ft.
500 mb:
- approx. 18,000 ft.
- higher winds, similar pattern to the 700 mb map.
- notice progression, and expect even stronger winds at the 300 mb level.
300 mb:
- appox. 36,000 ft.
- very strong winds, again the pattern is similar to the 700 and 500 mb maps.
- the isobar lines are so packed, i.e. the winds are so strong, that we record every othe isobar for readability.
The jet stream is the current where the winds are moving the fastest between the isobars. It is also referred to as the "storm track" due to the cumulonimbus clouds that travel along its path at about 18,000 feet. Clouds move along the jet stream like a conveyor belt. The jet stream can be found all over the globe, though may be weaker in some places. It is also referred to as the Polar Jet Stream. There is one in the Northern Hemisphere and one in the Southern Hemisphere - there is even one on Mars! Jet streams exist anyplace with two cold poles that ate rotating.
500 millibars of air pressure at 18,000 feet in the atmosphere. The highest mountains in the country are about 14,000 feet. The winds on the mountains are much stronger than winds down below. Remember rule of thumb: what level is the weather map drawn at? Look at number 2 on the handout from today -air flows clockwise and parallel around high pressure isobars, and counterclockwise and parallel around the low pressure isobars.
