John Monteverdi
Assignments
Williams: Preface, Introduction, Chapter 1
Zebrowski, Preface, Chapter 1 (point of view, definition of "disaster")
Housekeeping
- received the syllabus
- reviewed the class website
Definition of Storm
We reviewed the handout on the various definitions of "storm."
Storm --Any disturbed state of the atmosphere, especially as affecting the earth's surface and strongly implying destructive or otherwise unpleasant weather.
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%.
We also discussed the ways in which meteorologists visualize weather data in order to characterize (describe generally) storms. The patterns of the same weather data are used as a basis for predicting storms.
Storms To Be Discussed In this Course
(Relate mentally to Zebrowski's notion of a 'natural disaster' in Chapter 1).
Tropical Storm (Cyclone) -- Generic term for a nonfrontal synoptic-scale cyclone orginating over tropical or subtropical waters with organized convection and definite cyclonic surface wind circulation. (Hurricane -- sustained wind >74 mph)
Severe Thunderstorms -- A Severe Thunderstorm is a thunderstorm that produces any one or more of the following:
Watches and Warnings
The National Weather Service advises the public about potentially damaging and/or hazardous weather (e.g., hurricanes, severe thunderstorms and/or tornadoes). The basis for such advisories is to prevent or minimize loss of life and destruction of property
The "Watch" is issued if conditions in a general area are expected to be favorable for the given hazardous weather to occur about 12 hours in advance. The "Warning" is issued if the given hazardous weather is occurring, has been observed and cover should be taken immediately
Visualizing Storms and Other Weather Phenomena
In order to make a simple description of storms, or to accomplish more complex things, such as characterizing the weather associated with them and predicting their motions, meteorologists need to be able "diagnose" the storm. In much the same way that medical doctor's diagnose medical conditions in a patient, meteorologists need to see how the storm appears in a number of different ways. This involves "looking" at the storm directly (weather data, weather maps and charts), or remotely (via satellite and radar information)( to be discussed separately).
Weather Symbols
Check the handout distributed in class for the weather symbols we will be using. The use of these symbols on weather charts allows us to characterize the weather map, and graphically portray storms and their effects.
Wind
Meteorologists denote the wind direction as the direction from which the wind is coming. For example, a southeast wind is a wind flowing from the southeast to the northwest.
Satellite Images
In order to view weather over the oceans we rely on two types of satellite images sent to us by weather satellites. These satellites are positioned above the equator at a constant distance from the Earth and orbit at the same rate of speed that the Earth rotates. Thus, they remain over the same location constantly.
The first of these two types of images is called the VISIBLE IMAGE because the satellites are capable of sensing visible radiation (light) from the sun which is reflected by clouds. In the day time clouds reflect visible solar radiation to space. At night time the sun's solar radiation is absent and visible imagery shows nothing.
The second type of image is called the INFRARED IMAGE and is an image of the radiation emitted by the clouds themselves. The droplets in clouds differ in temperature according to the altitude of the clouds. The satellite is able to detect these temperature differences by using an infared sensor. The information about the temperature of the clouds is sent to a ground based computer which creates an image which can, finally, be perceived by human eyes. These night time images can be colored (called "enhanced" or in black and white.
We know, from observation, that clouds move with the wind. When viewing either satellite image we can see where the clouds are or aren't and make estimates about the clouds thickness. Look at the satellite images on the class website. We see that low lying clouds (fog) stops at the coast. Coastal mountains stop coastal breezes from moving clouds Eastward. The exception to coastal mountains stopping cloud movements depends on two things. The first is the presence of gaps in the mountains. The Golden Gate is a example of a gap that allows low lying clouds through the coastal mountains. The second thing would be if clouds are forming at altitudes and being pushed by winds that are higher than the coastal mountains.
Use of such images has allowed meteorologists to learn, for example, a great deal about fog and its motion. California summer fog outlines the coast because the fog layer is more shallow than the tops of the mountains, which are generally on the order of 200 to 400 meters (600 ft to 1200 ft). Since the fog layer is less deep than the tops of the mountains, the fog will not generally go up a mountain range; it moves to the sides--mainly southward but sometimes northwards--or around mountains.
Clouds and Weather Radar
Clouds form when moist air is cooled to the dewpoint temperature. If this happens near the surface of the earth, the resulting cloud is called fog, or stratus (layer or layered). Air that rises also cools. Air can therefore be cooled to the dewpoint temp. as it rises. If the air continues to rise, a continuous stream of cloud particles is formed, resulting in a towering cloud form described as cumulus (accumulated).
Since towering cloud forms are very large, the chances of individual cloud particles colliding and growing is much greater than for stratus clouds. So rain clouds tend to be these towering clouds, while stratus clouds tend only to be associated with drizzle.
Rain is formed when water droplets that have merged are to large to be supported by the surrounding air currents.
Weather Radar can be used to show rainfall, since raindrops reflect the energy emitted by the radar back to the radar itself. Thus, meteorologists can visualize areas of precipitation at the time that they are occurring. Here is a loop of radar images, depicting rainfall around a tropical storm in the Gulf of Mexico.
We also looked at radar images of Hurricane Dennis. The radar shows the depth of the precipitation area (in hundreds of feet, e.g., "300" = top of precipitation echo is 30000 feet), the motion of the rain area (in what direction and in what speed) and the shading is proportional to the heaviness of the precipitation.
Latitude and Longitude
Latitude lines run East/West and measure distance North/South. These lines are concentric circles which are parallel to one another. They are sometimes called parallel lines. How do they measure anything? Seventeenth century cartographers devised a system, based on geometry, by which measurements can be made from the equator which bisects the Earth into equal Northern and Southern halves. The measurement of the equator is called zero degrees. The North pole then becomes "90 degrees North" and the South pole is called "90 degrees South." Imagine each hemisphere as a protractor that only measures from 0 to 90 degrees. Latitude lines are assigned and named according to the respective angle between 0 and 90 degrees and the hemisphere. For example, San Francisco is latitude 38.5 degrees North. This means San Francisco is North of the equator at the angle, between the equator and the North pole, 38.5 degrees.
Longitude lines run North/South and measure distance East/West. Lines of longitude form circles of equal size and also use geometric measures of degrees. Again there needs to be a line which will be called 0 degrees. We call it the Prime Meridian or the International Date Line which runs through both the North and South poles and Greenwich, England. Imagine looking down at the North pole of the Earth (imagine the Earth as a circle with the North pole is the center). The Prime Meridian bisects the Earth into equal West and East halves. Since both the North and South poles fall on the Prime Meridian the pole's location is called "0 degrees longitude." Both the East and West halves are measured from 0 degrees to 180 degrees with due East and West are called "90 degrees" respectively. For example, San Francisco's longitude is approximately "122 degrees West"
When we refer to the location of San Francisco or any point on the globe we refer to the location in terms of its latitude and longitude. When sharing the location with others latitude is always mentioned before longitude. For example, San Francisco is located on the Earth at latitude 38.5 degrees North and 122 degrees West or 38 N 123 W.
Condensation
Condensation is water vapor changing its form or state to liquid water. Cooling air can force the water vapor in the air to condense. The point at which this occurs is called the "Dew Point Temperature", or the temperature at which condensation will occur. If the temperature has been cooled to the dewpoint then the relative humidity is 100%. The greater the difference between the acutal temperature and the dewpoint temperature the lower the relative humidity.
