Dew, fog or clouds form when the invisible water vapour in the air condenses into liquid water. For this to happen, the parcel of air must be saturated (Relative Humidity = 100%) - which means that the air parcel is unable to hold all the water it contains in vapour form, so it starts to condense.

• There are two ways by which saturation is reached:

(a) By increasing the water content in the air. I.e. through evaporation, to a point where the air can hold no more.
(b) By cooling the air so that it reaches its dew point - this is the temperature at which condensation occurs, and is unable to "hold" any more water. The dew point is always lower than (or equal to) the air temperature => If the air temperature cools to the dew point, or if the dew point rises to equal the air temperature, then water vapour condenses and dew, fog or clouds begin to form. At this point where the dew point temperature equals the air temperature, the relative humidity is 100%.

---

"temperature and dew point are nearly equal." => for condensation to occur, temperature must equal dew point.
"relative humidity reaches 98%." => condensation occurs when the air is saturated; RH = 100%.
"water vapour is present." => this is not enough for condensation to occur, the air must be saturated.