Depending on which calendar one references, the battle took place on 17 September, by the Gregorian Calendar, introduced in 1582 and which we use today, or 6 September, by the less accurate and older Julian calendar (which was still in widespread use then). Regardless of the date, the day was sunny and with enough wind to blow dust. No record was made if the day was unusually hot or cold.
Letters from this period note that summer had been quite hot and dry: a drought. The Breitenfeld plain is covered with a thin layer of clayish soil atop moraine gravel. Prolonged dry weather reduced the top cover to a fine powder, easily picked up in a wind.
In addition, the drought shrank the volume of water flowing in streams and standing in marshes. Enough water, however, remained in the Lober Bach and its marsh to offer a hindrance to the advancing Swedish and Saxon forces.
The lay of the land was flat to very gently rolling land with two short exceptions: one protruberance named Gallows Hill and the other to the west of that, unnamed and wooded. Essentially flat - but not quite: there was a slight slope up from north to south. The low end of the slope was the Lober Bach, which had marshy areas to the east of Guntheritz. Much of the flat land was devoted to agriculture, although what was in crops, what was fallow, and what was pasture is not known today. What was in crops may have been harvested just prior to the battle; although a soldier could crouch down below the tops of ripening wheat, he could not hide in a harvested field.
Wind conditions figure significantly in battles prior to the late 19th century. Firing black powder produces smoke - a lot of greyish white smoke per firearm. The cumulative effects of hundreds, even thousands of muskets discharging plus supporting cannon fire masked the battlefield, so much so that soldiers on one side could barely make out their opposition. The direction of smoke-laden wind benefitted those from whom the wind was blowing and debilitated the those who got the wind in their faces - the side of the first could see better than the side of the second. This was the problem faced (literally) by the Swedes and Saxons with the first shots. But around the time the Imperial-Leaguists commenced their attack, the wind shifted enough that, at least for Horn's men in their new positions, the wind was in their favor. When their cannons fired and musketeers let off their volleys into the Imperial-Leaguist tercios, the curtain of smoke rolling onto those men must have added to their confusion and shock. Then add dust to the smoke.