Hi Snap,
Unlike Oil stains, Acrylic latex stain will tolerate humid conditions. The surface should not be wet when you apply the stain. If you have had driving rains that saturated the siding then hold off until the siding is dry. Dew can also be a factor, do not paint or stain over dew, which can contain acids that can affect the finish.
Environmental damp conditions should not adversly affect the stain.
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