Cold air is harder to ride in. On my in bound commute there is a gentle downslope about a km long and road is smooth. During warm weather (above 18 c) - there is a road marker at the top, if I am going 18 kph when I pass it- then if i coast to the bottom - i usually am going 35 or 36 kph. In colder weather (near 0 c), same location - barely top 30 kph at bottom. You can really feel the air density affect when temp goes below 5 c.