Here's why:

* Solar blankets rely on sunlight: They work by absorbing solar radiation and converting it into heat.

* Clouds block sunlight: When clouds are present, they block the sun's rays from reaching the solar blanket.

* No sunlight, no heat: Without sunlight, the solar blanket cannot generate heat.

However, there are some nuances:

* Some light penetration: Thin, wispy clouds might allow some sunlight to reach the blanket, but it would be insufficient to generate significant heat.

* Passive heating: Even on cloudy days, solar blankets can still passively absorb some heat from the surrounding environment, but this is minimal compared to their performance on sunny days.

In conclusion: While solar blankets can provide some passive heating on cloudy days, they are not designed to function effectively without direct sunlight.