If you're in a weak signal area, you probably need a 48-element aerial
http://www.maplin.co.uk/Search.aspx?criteria=p t18u&source=15&SD=Y
In some areas, a 60-element aerial is necessary:
http://www.maplin.co.uk/Module.aspx?TabID=1&DO Y=10m6&ModuleNo=906&criteria=
Also, if the guy who installed your 'new digital aerial' didn't both to replace the coaxial cabling, he shouldn't be doing the job. Good quality coax is essential to good Freeview reception.
Chris
PS: To answer your actual question:
Yes, Freeview reception can be affected by the weather. Both 'path obstruction' (e.g. by rain or snow) and high pressure can adversely affect reception. Additionally, if your aerial has been badly installed, reception can be affected by water ingress into the connections and by the movements of the aerial in high winds. On top of that, you need to remember that all radio & TV reception can be affected by 'sunspots' (which follow an 11-year cycle) and solar flares, on the surface of the sun.