What causes the northern and southern halves of the planet to experience winter and summer at opposite times?

The angle of the Earth on its axis causing the sun to beam on the Northern and Southern Hemispheres differently. 

Is there a picture of the diagram?

Humid continental climate is found in the interior of continents between the 30 and 60 degrees latitude, but usually above the 40 degree line.   40 degrees north of the equator means less direct sunlight and therefore less warmth.   this climate is mainly found in the northern hemisphere since there are no major landmasses in the southern hemisphere.  if you live in northern indiana, you know all about humid continental because that is the climate type you have!