From Jon Harder, NG0E's APRS-IS derived VHF propagation maps. This one
is a good indicator of Tropospheric DX openings, but depends on a vigorous
APRS network for best function.

From Unisys, a chart of the atmospheric K index. This index is a measure of stability and moisture in the air, and is a predictor of thunderstorm activity. It is useful for VHF and UHF DXers due to its ability to show areas of stable air and inverted lapse rates (temperature inversions). Best conditions along a path are when the K index is well below 20, to its lowest values around -5. When the values are above 25, think not of radio, but lightning safety and a dry place to watch the storms!!

From Unisys, here is a chart depicting the atmospheric Lifted Index. It is a measure of stability, and is best for tropospheric propagation when its value is greater than 2. This means that the air, if lifted from sea level to 18,0000 ft, would not be bouyant enough (warm enough) to continue bubbling up on its own. Positive values of the lifted index indicate stable air, and possibly temperature inversions if the values are well above 2. If you see values below -2, forget about radio, and expect thunderstorms. Thunderstorms and tropo DX don't mix.