THE KEY: Watch the Weather, Look for Temperature Inversions.
Weather patterns over the earth's surface at heights from a few thousand feet to a mile or two at times can create boundaries between air masses of different temperature and humidity characteristics. Stratified warm air over cool moist air may be responsible for bending wave downward to the earth. Under stable weather conditions large air masses can retain their characteristics for hours or even days.
At sunrise the air aloft is warmed more rapidly then the air near the earth's surface. As the sun goes lower late in the day the upper air is kept warm while the ground cools. In fair calm weather look for some Troposphere propagation during sunrise and sunset that may improve line of sight communication as much as 20db.
Tropospheric effect can show up "any" time in any season. However late spring and early fall are likely time for Temperature inversion but not rule out a winter warming trend to create a strong and stable temperature inversion.
Tropospheric ducting is the condition when communications of greater than a few hundred miles occurs. Instead of the wave propagating between the temperature inversion and the ground, the radio wave is between two atmospheric layers thus creating a duct. Distances of 1000s of miles may be experienced, for example between California and Hawaii. The trick is that both stations must be in the atmospheric duct or capable or transmitting into it. This type of propagation is more likely to occur on frequencies above 144 MHz . . . more likely 432 MHz and 1296 MHz. Troposcatter can occur on 50 MHz and above. Check out http://www.qsl.net/oz1rh for details of this phenomenon.