When coming in to land can be difficult to work out the cross wind component rapidly. There are 2 quick methods I know to be able to do this and when you understand them choose the one that suits you best.
The first one is known as the clock code and with it you assume that any wind that is more than 60 degrees off the runway heading is a full strength cross wind. So if landing on say runway 27 which is 270 degrees from North, then if the wind direction is less than 210 or more than 330 degrees, whatever the strength is it is regarded as full cross wind. So if the wind is say 200 at 15 knots then it is a 15 knots cross wind.
Now to work out how much of a cross wind there is between these 60 degrees either side of the runway heading you imagine that the number of degrees off the heading are the numbers of minutes round a clock face. Then imagine how far round the clock face that is, and that proportion round the clock face is the proportion of the wind strength.
So if the wind is 20 degrees off the heading, say for example 290 at 30 knots, then 20 minutes is one third of the way round the clock face, so the cross wind component is one third of 30 knots which is 10 knots.
If the runway is 03 which is 030 degrees, and the wind is 070 at 20 knots, this is 40 degrees off the runway heading, and 40 minutes round the clock face is nearly nearly three quarters of the way round the clock face so the cross wind is three quarters of 20 or 15 knots.
As wind constantly varies in strength and direction, then you do not need to be highly accurate with your calculation. If the wind is roughly 30 degrees off, it is half strength so roughly half the wind strength is the cross wind component. 45 degrees off is 3/4 of the strength of the wind and 60 degrees or more full strength.
Another easy way to work out cross wind and head wind component is using this simple mathematical formula.
For calculating cross wind. If the wind it 30 degrees off the nose it is.5 the wind strength, 45 degrees off.7 the wind strength, 60 degrees off it is.9 the wind strength, and if 90 degrees of then obviously it is full strength. This also applies on cross country flights, and for working out the cross wind component when coming in to land.
If for example when coming in to land the wind is 60 degrees off the runway heading it is.9 times the wind strength, so using simple arithmetic on a 20 knot wind just multiply 9 by 2 which is 18 knots. For a wind of 30 knots and 45 degrees off the runway heading the calculation is 3 X 7 which is 21 cross wind component. If like me you learned your multiplication tables as a child, this is easy.
If you reverse the formula, you can use it to work out head wind or tail wind component as well. So if the wind is directly towards you, it is full strength, if 30 degrees off it is .9 of full strength, 45 degrees off .7 of full strength, and 60 degrees off it will be half strength.
If it is 90 degrees off then there is no head or tail wind component. However bear in mind that any strong wind will be affecting the aircraft by drifting and turning into wind will in effect mean that you have to fly a longer track than a straight line so it will always slow you down. The stronger the wind the more time your journey will take.
If the wind is coming from behind you, then the same proportions can be applied to work out the tail wind component, so if it is 30 degrees off your tail, it is.9 of the strength of the wind and so on.
For working out a diversion, you can apply this percentage to your airspeed to get the ground speed, and then to work out drift interpolate the following formula as well. The formula is that at 120 knots airspeed, half the cross wind component is drift. But most light aircraft fly a bit slower than this, and so the drift is greater. So if you are flying at 90 knots then your drift would be 25% more than half the drift as 90 knots is 75% of 120 knots, so you add the difference to the drift.
This method can be used to quickly calculate heading and ground speed if a diversion is necessary, or if you want to check your calculations after using a computer to plan your journey.