Time of picking.
So it's "pick your herbs in the morning on a sunny day, after the dew has evaporated."
That's what it says in most any herb book that includes picking instructions.
Oo-kay. But what if it's a rainy dreary wet summer, like the one we had last year? I don't think the plants had time to really dry off at all between May and October. There were no lengthier sunny spots. Or if they were, they disappeared into the wholesale dreariness. The whole summer was wet, cold, wet, gray, wet, depressing, wet, and wet. (... it's been a few months. Parts of this description might be just a teensy bit exaggerated.)
Last summer, weeds took over a lot of my herb gardens because they could: whenever I ventured into the garden to pull some of them up the weather got wet and shivery again, and I went back inside rather faster than usual.
Of course, with all that dampness, the fungal diseases hit record-fast, too, so the usual rainy picking routine ("oh, calendula, I'll pick that next week") didn't work, because the calendula caught blight. As did meadowsweet, and willowherb, and a few others, all far too soon. Bleh.
In summers like that you can't pick your herbs on a sunny morning after the dew has evaporated. No, in summers like that you pick your herbs in pouring rain, glad to at least pick something, happy that there's no thunderstorms right now, and when you come in you dry yourself off, pour a cuppa hot chocolate, put your pickings between spread-out bedsheets (if you picked lots) or between towels (if you picked less), and gently pat things dry.
Once the plants are no longer dripping wet you continue the drying as usual: hang up long-stemmed plants in bundles in an airy shady spot, spread picked single flowers, leaves, seedpods and the like on towels or bedsheets on a thick layer of newspapers, or pop things into one or the other dehydrator.
That dehydrator is good to have if the humid conditions continue, because herbs won't dry properly in high humidity, so you'll have to finish the drying with an hour or two of blown heat, before jamming things into tight jars. Else they'll grow mold in next to no time, nevermind that you dried them for the usual 7-10 days.
After patting things gently you can also wait until your pickings are as dry as they would have been if picked on a sunny morning after the dew has evaporated. Then you can go on to make fresh herb tinctures, vinegars, teas, oils, and so on.
(About oils: I prefer bone-dry herb when making herbal oils, but for some oils you need fresh herb - for instance for the infused oils of St. John's wort, chickweed, or jewelweed.)
Roots, well, rain is no trouble when you're picking roots. You're going to wash them anyway (see "Cleaning roots") before you process them, so a little extra water is of no consequence. It's more uncomfortable for you, and the dirt you're digging up will stick to everything from your boots on up, but, shrug, that's what rainy summers are like. There'll be sunnier summers. Some of those summers will be so sunny and sizzling and hot that you'll yearn for the record rainy dreary gray one.