Some Helpful Definitions

true north = determined with reference to the earth's axis...true north is geographic north (defined ont he basis of the North Pole)

meteorological wind direction = the direction from which the wind is coming (e.g., west wind = a wind moving from west to east

latitude = system of coordinates that allows locating one self north or south relative to the equator and poles. Lines of latitude (parallels) run east west. 0^o is the Equator and 90^oN is the North Pole. San Francisco's latitude is roughly: ______________

longitude = system of coordinates that allows locating one self west or east relative to the Prime Meridian (runs through Greenwich, England). Lines of longitude (meridians) run north south and converge at the north and south poles. 0^o is the Prime Meridian and 180^o is the International Dateline. San Francisco's longitude is roughly: _________________

composition of the atmosphere --Our atmosphere is mostly nitrogen (78%), even though most people think that oxygen is the most dominant component (20%). Also of great importance to our atmosphere is water vapor (1%-4%). Obviously, the clouds that make up storms are made up of condensed water vapor. Carbon dioxide is also meteorologiclaly important (contributes to weather and/or climate patterns) and is present at much less than 1%.

onshore wind = wind moving from the ocean to the continent. During the summer in California, such winds are cool and moist (relative to conditions inland).

offshore wind = wind moving from the continent to the ocean. During the summer and early fall in California, such winds are hot/warm and dry.

severe thunderstorm = a thunderstorm characterized by hail at least 3/4" in diameter and/or sustained straight line winds of at least 57 mph and/or a tornado. (The thunderstorm is classified as severe if any one of these things occurs).

climatological "normal" -- defined as the average for the thirty years ending the last even decade. The current "normal" is for the period 1971-2000. (Note: does NOT mean the value that one SHOULD expect to occur.)