A weather phenomenon known as the föhn (with an identical effect as the chinook wind)
can occur at all times of the year and is characterised by a
unexpectedly warm wind, bringing air of very low relative humidity to
the north of the Alps during rainfall periods on the southern face of
the Alps. This works both ways across the alps but is more efficient if
blowing from the south due to the steeper step for oncoming wind from
the south. Valleys running south to north trigger the best effect. The
driest conditions persist in all inner alpine valleys that receive less
rain because arriving clouds loose a lot of their content while
crossing the mountains before reaching these areas. Large alpine areas
such as Graubünden remain drier than pre-alpine areas and as in the
main valley of the Valais wine grapes are grown there.
The wettest conditions persist in the high Alps and in the Ticino canton which has much sun yet heavy bursts of rain from time to time.
Precipitation in Switzerland tends to be spread moderately throughout the year with a
peak in summer. Autumn is the driest season, winter receives less
precipitation than summer, yet the weather patterns in Switzerland are
not in a stable climate system and can be variable from year to year
with no strict and predictable periods.
Alps cause many climatic variations throughout Switzerland. In the
higher Alpine regions temperatures tend to be low, while the lower land
of the northern area has higher temperatures and warm summers. Overall
the hottest months are July and August, though these are also the
busiest. Those looking to ski should visit Switzerland between December
and April, when snow is good in the Alps. The months of May and
September are great times to visit for those looking for sunny and mild
days, but without the throngs of tourists.