The simple explanation is in two parts In non tropical or subtropical climate regions where moisture content is moderate to low, in the warmer months , temps tend to rise rapidly . Warm air holds more moisture than cooler air . So let's say the high temp is 90% and the peak dew point is 55° Thats relatively dry air.. Once the sun begins to approach the horizon and ultimately sets, the air begins to cool rapidly. And while the relative humidity increases, the actual moisture content stays relatively the same. In this case the dew point of 55° Under certain conditions, the ambient temperature will cool to the dew point.. So the low temp will be in the mid 50s In winter, there tends to be more cloud cover, and if in a region where precipitation is at its highest, moisture levels in the lower atmosphere tend to remain high.. So let's say on a given day the high temp is 35° And let's say there is either a snow pack or the ground is moist. Moisture must evaporate. And does so into the atmosohere closest to the ground.. This actually drives the dew point higher.. And because the air temp can not fall below the dew point, this limits the range between today's high and low air temp.