I'm not sure about other production and gathering, but the bonus to the number of herbs collected in a homestead with the weather bonus is less than that of base herbs in fields during thunderstorms and medium-strong rain.
Doesn't aid my points, but it is still a reference should people be interested, and it provides other links.
http://wiki.mabinogiworld.com/view/WeatherVaries. Like you mentioned previously, the value of the thunderstorm, or w/e everyone's calling it, determines the boost to both production and gathering from base herbs. Bigger value, more output.
Think, in the field, it's like sunny=5, overcast=6, light rain=8, storm=9. Not certain on that, but i remember that i did harvest 9 base herbs from a patch before moving on in dunby, yesterday during a storm. Homestead is different, it's always a base value of 5 for each herb patch. Nearby prop can add +1, does not stack, to the herb patch. The weather can add +3 to the herb patch, ie only 2 weather patterns not 4, rain or sun.