What is the basis for the delay between the atrial and ventricular contractions in a normal heart?

The main responsible for the delay among the atrial and ventricular contractions is the action potential conduction velocity of the AV node cells. If the atria and ventricles contract simultaneously, atria cannot give any help in filling the ventricles, which is needed, especially in those with stiff ventricles as in the elderly. Hence atria have to contract first and then the ventricles.

answer: ice crystals in air

the water from the water body or another source of water gets evaporated as water vapor as an effect of heat on water. these water vapor rises in the upper layers of the earth atmosphere under high atmospheric pressure. the atmospheric pressure of these water molecules starts to decrease due to the effect of cold air. the invisible water vapor condenses into visible water droplets and ice crystals.these ice crystals and water droplets being less denser than the surrounding air floats on air as clouds.