Ideally, wind would always be perfectly aligned with runways. When runways are constructed, a consideration of the average wind direction for the area dictates the runway orientation. Los Angeles International, for instance, has east/west runways because of its proximity to the ocean and prevailing east/west winds. Dallas Fort Worth International, on the other hand, has mostly north/south runways because the wind in the area usually blows north/south. The problem is, fronts and other meteorological factors disrupt wind flow sometimes, and uncharacteristic crosswinds make pilots tighten their grip on the controls a bit.
The FAA requires airplanes to be satisfactorily controllable with no exceptional degree of pilot skill or alertness in 90° crosswinds up to a velocity equal to 0.2 Vso. For example, Vso, or the stalling speed in the landing configuration, for the Piper Archer I fly is 47 knots. That means the airplane can be landed by the average Joe pilot who isn't particularly skilled or alert in a direct 9.4 knot crosswind. A non-average Joe test pilot, on the other hand, who is highly skilled and alert, picks a windy day and lands the airplane with as much crosswind as he can handle and that number becomes the maximum demonstrated crosswind component for the airplane. Any airplane certificated after May 3, 1962 must have this number placarded in the cockpit where the pilot can see and be reminded that a test pilot could handle that amount of crosswind, but he probably can't. The maximum demonstrated crosswind component is NOT a limitation, and it can be exceeded. You can legally land with as much crosswind as you want. But if a test pilot could only handle say, seventeen knots, it's reasonable to consider that a limitation, or at least a very strong recommendation not to take on more than that (or even close to it).
When the wind is blowing across the runway and not along it and you're not certain you can land safely in the existing conditions, divert to an airport with a crosswind runway better aligned with the wind. Better yet, avoid that situation in the first place by giving a thorough check of the weather and forecasts before your flight. Wind isn't generally all that difficult to forecast, so if it looks like there's a possibility of encountering too much crosswind, delay the flight until the wind dies down or come back another day.
