The picture on the left is an illustration of the jet stream on the northern hemisphere during summer. In the tutorial it states, the temperature contrast between cold and warm air masses are smaller, so the jet stream is not that strong only travelling at 50-70 mph. The picture to the right is an illustration of the jet stream on the northern hemisphere during winter. In the tutorial it states the temperature contrast between cold and warm air masses are greater, so the jet stream is very strong travelling at 90-160 mph.
