For the passengers out there, many planes can only have so much extra fuel onboard. The interesting thing is that while including one category (alternate fuel) to get to an extra airport which might have storms very close by (as today-we picked other alternates as back-ups), the actual contingency fuel (not including "reserve fuel: this is sacred) is almost always only calculated (for 15,000') at around 15-30 minutes! Taxi+block (enroute burn)+reserve+contingency+alternate (not always needed) = dispatch fuel.
We could use a small extra fuel tank on many jets for legs much over one hour, but if the Captain decides to carry a bit more fuel, i.e. just an extra 1,000 lbs. at the last minute, so to speak, mail, bags and maybe a few people would have to be bumped off of the plane. And very often, people are already boarding by the time we get to all of the flight paperwork and realize that we will burn a good bit extra deviating around large storms! It was nasty in central Florida this afternoon, but ok near Fort Lauderdale (FLL) when we arrived.