Dew is the small droplets of water that appear on objects such as leaves or grass in the morning or evening due to processes of condensation.
How does dew form?
Dew forms when the temperature of a surface cools down to a temperature that is cooler than the dew point of the air next to it. When this happens water vapour will condense into droplets depending on the temperature. The temperature at which droplets form is called the dew point. When surface temperature drops, eventually reaching the dew point, atmospheric water vapour condenses to form small droplets on the surface.
Up to 0.5mm of dew can form at night in some climates. Whilst small, this amount is significant enough for dew to become an important source of moisture for some plants and animals in arid areas. Dew can be collected for human use from canopies erected above the surface which with the correct conditions can collect several litres of water.
Dew forms most easily on surfaces that do not conduct heat from the ground - grass and the rooftops of cars are some of the most frequently seen examples. It usually forms during the calm weather associated with high-pressure systems.
The temperature at which condensation occurs is called the Dew point and is dependent upon the humidity and pressure of the air. Calculating the exact value of the dew point is important when predicting frost or fog.
Dew point may be measured indirectly from wet-bulb and dry-bulb temperature readings with the aid of a humidity slide-rule or humidity tables, or directly with a 'dew-point hygrometer.
Whilst a relatively rare occurrence, the presence of dew can be cause a similar optical effect to a rainbow.
An example is in the picture below caused by drops of dew hanging on a spiders web reflecting and dispersing sunlight.