What are the reported values of each quantity?
- Current Conditions
- Cloud Cover, Type
- Visibility (miles)
- Temperature (degrees F or C)
- Pressure and Pressure Change (millibars)
- Dewpoint Temperature (degrees C or F)
- Wind Direction, Speed (knots)
- Precipitation (inches)
How are each of these quantities measured. What instruments are used?
[ See p. 18 and Appendix H in text for lists of weather symbols]
When plotting pressure data using station model notation, we save space by leaving off the last "9" or "10" as well as the decimal point (values of pressure and pressure change are reported to nearest tenth of a millibar).
Station measurement Value plotted on weather map 1023.4 mb 234 996.8 mb 968 Value on map Pressure measurement 105 1010.5 982 998.2
Remember: Pressure values are reported as equivalent sea level pressure! Why is this? How is this done?
High and Low Pressure Centers
The highest and lowest pressure values plotted on a weather map are labeled with an "H" or an "L", respectively. Isobars are also drawn to make it easier to pick out horizontal differences in surface pressure.
The term "gradient" refers to a change in a quantity in a particular direction. In this case we are looking at changes in surface pressure in the horizontal direction.
Lines marking the boundary between higher pressures and lower pressures are called isobars. Their name refers to the fact that they are essentially lines of constant pressure. Isobars illustrate the pressure gradient - when isobars are closely spaced, the gradient is large or "steep"; when isobars are relatively far apart, the gradient is weak. Since horizontal winds arise from differences in air pressure, the spacing of the isobars (and the strength of the gradient) can be used to indicate windy conditions.
Examination of a typical weather map (see Figure 1.14) shows how weather observations from many locations can be used to get the overall picture of conditions affecting a large area. Note the relationship between the locations of the high and low pressure centers, observed weather, clouds, and wind direction. In general, cloudy conditions and precipitation are associated with the low - clear conditions and fairly calm winds are associated with the high. Also note the counter-clockwise (or "cyclonic") wind pattern around the low, and the clockwise (or "anti-cyclonic") wind pattern around the high.
These patterns can be simply understood by considering three basic forces at work in the atmosphere:
- pressure gradient force - acts to move air from higher pressures to lower pressures
- Coriolis force - an effect arising from the Earth's rotation which acts to deflect large, slow-moving air masses to the right relative to the direction of motion in the Northern Hemisphere (in the Southern Hemisphere, the Coriolis force deflects air to the west).
- friction - collisions between air molecules and the surface (and with each other) act to slow molecules down; this effect is important at and near the surface
The cyclonic/anti-cyclonic flow around a surface low/high arises from the combination of the pressure gradient force and the Coriolis force (see handout for example). In the absence of friction, air would tend to flow parallel to the isobars drawn on a weather map. Friction causes air to flow slightly across isobars toward lower pressures (why is this?). Thus air tends to converge near the center of the low and diverge around the center of the high.
To understand what this convergence and divergence means, look at Figure 8.17 in the text. The principle of continuity illustrated there shows that convergence at the surface is balanced by rising air motions, while divergence at the surface requires sinking motion. As we'll find out later, rising motion is key to the formation of clouds and precipitation. Sinking motions inhibit cloud formation.
Fronts and the Frontal Cyclone Model
Refer to the class handout from Friday 9/3 for the definition of the different kinds of fronts drawn on a weather map, and how they are drawn.
To describe the postion of warm and cold fronts in the frontal cyclone model (Figures 1.18 and 1.19), remember which direction the surface winds blow around a low pressure center and also remember the basic temperature pattern - over the United States, for example, this temperature pattern consists of warmer air to the south and cooler air to the north, generally speaking.
Fronts are important because they provide a source of rising air motion, which can lead to cloud formation and precipitation. Cold air is more dense than warm air, so when they meet, the warm air is forced to rise. Note in particular how different kinds of precipitation are generated by cold fronts and warm fronts, respectivley. Precipitation associated with a cold front is usually more violent and short-lived; precipitation from a warm front is usually more steady and long lasting. Why is this?
The different stages of development in the life of a frontal cyclone are depicted in Figure 1.21. Note how the stationary and occluded fronts form. The discussion in the text is pretty good here, so I won't bother to repeat it. You should read pp 22-31 carefully, since I didn't cover all of it in class.