Dew is more likely to form if the sky is clear and in a similar folk rhyme:
When the dew is on the grass,
Rain will never come to pass;
When grass is dry at morning light,
Look for rain before the night.
The effect is linked to the absence or presence of clouds.
For dew to form, the grass or canoe must reach a temperature cold enough for saturation to occur, that is, the saturation point or dew point, at which air will no longer hold water vapor.
When water condenses to form dew on either grass or canoe, what you have is an object losing heat to the atmosphere faster than it is being replenished.
This is more likely when clouds are absent.
There is a constant exchange of energy in the form of heat radiation between the ground and the atmosphere.
When clouds are present, as is the case when rain is imminent, they radiate back some of the energy an object on the ground is losing. That object warms up a little, enough so that condensation does not occur.
At most times of the year, this general rule works rather nicely, and a wet canoe probably means that there are no clouds and that rain is not imminent.
The rule would not work for a period of twenty-four to thirty-six hours, but for twelve hours there is a loose correlation.
However, the higher the clouds are, the less impact they have on the surface temperature.
The rule is also less applicable in summer, when there are long periods of clear skies, especially overnight, but afternoon thunderstorms can form quickly, in a matter of an hour.
Then, morning dew on the ground might still mean there was enough moisture for afternoon showers to form, particularly at high elevations.
There are other exceptions. In very cold dry air, it is hard to get down to the temperature at which the air can no longer hold water.
