Anything described with the word considerably happens substantially or in a major way. A considerably better job is a much better job.
A considerable amount of rain is a significant amount of rain: not just a little rain. So if something occurs considerably, it occurs to a great degree. A considerably bad day is a very bad day. A considerably wealthy person is an extremely wealthy person. A considerably dark sky is a dreary sky indeed. If you feel considerably worse than yesterday, you feel much worse. This word intensifies things. When you see considerably, you know something is being heightened.