Personally, I love rainy days.
They're slow and quiet, cozy, and make you want to curl up with a good book or movie for the day. Similarly, plants feel the same way in cloudy weather; lazy. If you've ever wondered why flowers, for example, will be covered in buds that won't open after on-again-off-again sunshine. It's possible they're "stuck," meaning the process of photosynthesis has slowed.
Inside or out, the process of photosynthesis allows the roots of a plant to act as the human heart does, delivering nutrients throughout the plant body. When it's overcast and rainy that process is interrupted. But why?
A Little bit of a science...
Photosynthesis is the process by which plants make their own food using sunlight. Long periods of no sun impact this process. When plants don't receive enough sunlight, they can't produce enough energy to transform and move nutrients; leading to a period of stunted growth.
Now, a little horticulture...
In extreme cases, the yellowing of leaves, and even death of the plant can occur when sunlight has been absent for too long. So, it is essential to understand how to identify when a plant is struggling in its environment.
Part of the reason that plants become stunted when the weather is cloudy is because the movement of coprophil within the plant body thickens, much like water transforming to ice. When this happens, the nutrients inside the plant literally can't transform.
So, if you notice your indoor plants aren't growing as they should, check their exposure to the light. In some cases, you can use an artificial sunlight to provide your plants with the necessary energy to reverse the stunting. In most cases, a simple shift into a sunny window may be all that is needed! In the case of outdoor plants, sometimes all you can do is wait until the sun returns.