Carterton lies in the lee of the Tararua Ranges and this has a significant effect on the weather as the the prevailing wing is W/NW.
The average wind speed for Carterton in January is around 7 meters per second and temperatures in to low to mid 25 degrees C.
There are various sources of weather forecasts especially in the age of the interweb some more accurate than others. Sometimes its best just to wet your finger and stick it out the window.
The Wellington Model Aeroplane Club has a weather station at Gladstone for the Vintage and Free Flight guys and gals (you may need to refresh the page to get the latest weather; press F5)