Warmer weather is welcomed by many who were driven indoors by extreme cold and snow earlier this month, but not all results of the warmup are as welcome as others. It has made it easier to remove mountains of snow and rock-hard ice from streets and sidewalks, but it is a trade-off. Warmer air flowing over snow cover forms fog that not only limits visibility, but can also freeze on surfaces still recovering from last week’s sub-zero temperatures, creating black ice in spots, and the Wisconsin DOT’s 511 webpage showed slippery conditions statewide as of early Wednesday morning.