Small cracking within and between hardwood flooring planks are normal and should be expected during certain seasons and will vary depending up specific geographical locations. When the air is dryer, wood flooring will release some of its moisture into the atmosphere and naturally shrink. Cracking or gapping between the planks will appear and may be slight or somewhat noticeable.

When the outside air is filled with more moisture, such as in spring, the boards will absorb some of the moisture and swell back to their original size. This is normal for all hardwood flooring and will happen as long as the wood floor is installed. Engineered hardwoods will display less of these characteristics.

Woods that are naturally darker or carry a darker stain will help cover up some of these cracks. Further, purchasing hardwood flooring that has added texturing such as hand scraping, chiseling or brushing also will help to blend the cracks and spacing.

If a homeowner does not want cracking or gapping to occur, humidity control devices should be placed within the home to achieve nearly constant humidity levels. If putting a humidifier or dehumidifier in the home is not an option, homeowners can increase the humidity levels during drier months by not turning on the bathroom fan when showering, opening up a dishwasher when the final rinsing is finished or hang clothing to dry indoors. A large pot of water can be placed on a conventional stove top on one of its lowest settings as long as it is carefully monitored for safety purposes. Homeowners may find placing a cast iron pan full of water on top of a level heated wood stove may also serve the same purpose. 

Source: http://woodmonsters.com