NZ Weather Forum

As I drove across the Waimakarrir bridge today and noticed that there was lower cloud drifting across on the southern side of the bridge into the Christchurch city but back my way in North Canterbury it was fine I thought to myself why is weather different on one side of the bridge .
It has happend many other days as well especially in NE onflow conditions such as today.
And also why do for instance Thunderstorms develope south of the Rakia mostly on the southern stretch and not the northern side.
Is it because Christchurch city is heated by the buildings or is it the Port hills .
Something to think about I guess.
Cheers
JASON TIPPET.

Fog forms preferentially over the Waimak in winter due to the water being warmer, warming the air but also keeping it at 100% RH, so when that air advects over surrounding land then condensation takes place and hence fog. I think the cloud is related as the broad river bed is a source of both moisture and heating off the stones (for those out of Chch the Waimak is a very broad braided river bed).

Rivers also channel winds along them (lower surface roughness) so areas along the banks of Canterbury's rivers are often the windiest in NWers.

As for TS, I dont think rivers have anything to do with their formation, its just that the inland topography on the south side of the Rakaia is more favourable (slope angle etc) than on the north side.