icing/freezing
If the temperature is below freezing and you are in clear air, there is no icing threat. For icing to be a threat, there has to be visible moisture in the atmosphere in the form of cloud, mist, FZFG FZRA FZDZ etc or SLD. If these conditions exist, then there is an icing threat and the severity of that threat is based on a number of different factors. Also saturated air temperature will determine what sort of ice will be encountered. In aviation terms, there are really two types that you will hear mentioned. These being Rime ice which will occur at colder temperatures and Clear ice which will occur closer to the freezing level. Mixed ice is a combination of the two and you will also see this in icing forecasts or pilot reports. Ice will not normally build or accrete on an aircraft outside the SAT range of +2 through -20c but the only physically limiting factor is -40c.
Hope this helps,