Glowclouds are free-floating radiation fields lying close to the ground. Determine the Radiation intensity level with a d10 as normal. Glowclouds of strength 3 or less are invisible to the naked eye. Intensity 4 through 7 glowclouds are dim and can only be seen at night or possibly during overcast daytime conditions. More potent glowclouds can be seen even in the noonday sun.
Glowclouds move with the winds, using a roll of 1d6 on the chart below:
DAILY GLOWCLOUD MOVEMENT
1-3 Glowcloud floats to random adjacent hex
4-5 Glowcloud remains in present wilderness hex
6 Glowcloud dissipates
Glowclouds are generally not so large as to encompass an entire overland wilderness hex. If you enter a hex containing a glowcloud the basic chance of stumbling upon it is only 1 in 6. In open areas the visible ones can be easily avoided unless a high wind is blowing in a unfavorable direction. In rough terrain roll to see if the party is surprised. If they are not surprised they can avoid contact with the glowcloud provided a saving throw versus radiation is successful. Surprise indicates at least one round of exposure, with additional rounds of exposure if the winds are particularly uncooperative.
Dissipated glowclouds lose 2d6 radiation levels. Any remaining radiation becomes a permanent feature of the map and should be noted in the wilderness key.
There are several ways to get glowclouds into your game. If you have something like a Monthly Events chart you can make an entry like “New glowcloud appears on random map edge”. Alternatively, you could place glowclouds on your wilderness wandering monster charts. Another idea would be to place a source of glowclouds on your map, some ancient object that leaks radiation. Periodically it spits out a new glowcloud onto the map, maybe a 1 in 6 chance each month. The region immediately surrounding the source would undoubtedly contain a fair amount of background radiation.