Why urine is slightly thicker in summer than winter?

Because you perspire less. Both processes, urination and perspiration, excrete water from the body. You may also notice little or no urination, even when drinking plentiful water, during periods of vigorous work or exercise to the point of heavy perspiration. Also, heated indoors areas tend to be low in humidity, which can influence a person to take in more liquids to offset the dryness. More liquids in means more water out.