The following are rules of thumb for forecasting the weather.
|Will it be cloudy or clear?||On the 700 mB chart, the 70 percent relative humidity line usually encloses areas that are likely to have clouds.|
|Will it rain?||On the 700 mB forecast chart, the 90 percent relative humidity line often encloses areas where precipitation is likely. If upward velocities are present, the chance of measurable precipitation is enhanced.|
|Will it rain or snow?||On the 850 mB forecast chart, snow is likely north of the -5° isotherm, whereas rain is likely south of this line.|
|Will the surface low intensify?||For the storm to intensify, an area of upper-level divergence must be over the cyclonic storm. On a 500 mB forecast chart that shows vorticity, look for a vorticity maximum. To the east of an area of positive vorticity we usually find upper-level divergence, upward air motions, and cyclonic storm development.|