The following are rules of thumb for forecasting the weather.

Question Answer
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.