Question

why is more concentrated yellowish urine excreted in summers

Answer 1

Dark yellow urine indicates concentrated urine and the excretion of waste products in a smaller quantity of water. During summer we sweat more in order to maintain our body temperature. So more water is lost from our body.

Answer 2

In$\textbf{ Summers }$ water consumption is increased and as compared with it excretory amount of urine is less.

Most of the water actually evaporated via sweat in summer.

It keeps our body cool and hence concentrated yellowish urine is excreted.