Downdraught comes in two flavours. Obvious downdraught is when there is something taller than the chimney terminal within 25 metres or so. What happens is that the wind pushes over the top of this obstacle and then drops down on the chimney pot, a bit like a waterfall. The less obvious downdraught is geological, and occurs when the lie of the land cause the wind to roll across the country like an enormous roll of carpet, and if that roll hits the terminal just so, the effect is exactly the same as the ‘waterfall’
This is what a cowl was designed to cure, and it will do that very nicely. HOWEVER, most cowls are unnecessarily complex.
Remember the waterfall. Your chimney is like a little man sitting under it, and all he needs to keep dry is an umbrella. To be honest, all that is needed is for someone to hold a dinner plate over his head. In Germany, you very often see that chimneys are covered at the top with a horizontal sheet of steel about 8 inches above the pot. This is a cowl stripped down to its bare essentials and it functions perfectly.
Here though we have a lot of problems with birds, so modern cowls are usually lined with a mesh that prevents birds getting in. Do remember, a cowl should be made of stainless steel (they do have terracotta coloured versions) and should be fitted to the outside of the pot. If you fit one that is made of plastic coated steel that is hooked onto the inside of the pot, then its life will be short indeed.