Why is the dewpoint never higher than the temperature of the air?

Question

Please explain.

Thank you!

Answer 1

The dewpoint is the temperature the air must be cooled to for saturation to occur.  If you are already at saturation, the relative humidity will be 100% (it will probably be very foggy) and you'll already be at the dewpoint.  If you the humidity is less than 100%, you'll need to cool the air to get it closer to saturation, if you raise the temperature the relative humidity will fall and you'll be farther away from the dewpoint.  That is why the dewpoint is never warmer than the air temperature. You could make an argument that if you're in a foggy situation the dewpoint might be higher than the air temperature, since you could remain at saturation even at a slightly higher temperature since you would start evaporating fog droplets, but I think that might be impractical to determine.

Note that g_pecault's answer about how you measure dewpoint is slightly wrong.  If you have a thermometer with a wet wick what you will measure will be the wet bulb temperature, not the dewpoint.  The wet bulb temperature is always greater than or equal to the dewpoint (they are only equal at saturation).

Answer 2

Prior answers are good, so I'll try and give you an idea of how Dew Point is measured. You have a thermometer that tells you the Temperature. Then if you take a wick and put it over the buld on the thermometer and stick that in a dish of water when the wick is wet water evaporates off it cooling the thermometer giving you the DEW POINT. When the air is dry, the water evaporates rapidly cooling the thermometer more (LOW DEW POINT) . When the air is very humid, little water evaporates so the DEW POINT is very close to the Temperature. When no water evaporates off the wick over the thermometers bulb, the dew point and the Temperature will be the same. The air is sad to be at 100% Humidity then. No more water can be evaporated into the air.  Both the wet bulb and the dry thermometer will be at the same Temp. The only way to get the wet bulb Thermometer to read a higher temp. than the dry Thermometer is to put it into the sun, but Temp. is alway (as well as Dew Point) in the shade. A Hygrometer reads the Humidity directly, but tables can be used to convert Dew Point into humidity.  Temps below dew point will create fog. That's why boaters and pilots watch the dew point. If the temp is 80 now and the sun is setting, and the Dew Point is 78, they know that a heavy fog is going to set in soon. As soon as the temp. hits 78.

Answer 3

When the relative humidity of air in a room is 100% the air is holding all the humidity it can and any excess humidity will condense.  The room temperature will be at the dew point (the point where dew can form).  If a room has say 50% relative humidity and is cooled down (without adding or removing humidity) condensation will occur when the dew point is reached and will continue as the temperature and dew point continue to fall together.

Answer 4

The dew point is a measure of moisture in the air. When the dew point and the air temperature is the same, then the humidity is 100%. Thus if its 68 degrees and the dew point is 68 then its 100% humidity, and its is very likely raining . (probably heavily)  If not raining, dew would form. Obviously you can't have more than 100% humidity, so the dew point can never be higher than the temp.

Usually precipitation will start falling before the dew point reaches the temp., especially in the summer.

The dew point is actually a better measure of moisture in the air than relative humidity %,  because it is "relative" on the temperature. Warm air holds more moisture than cold air.

So, 40 degrees with 35% humidity is very dry air, while 95 degrees with 35% humidity is very moist and humid air.

Generally when dew points reach the 60's it starts to feel humid and uncomfortable. Dew points in the 70's become oppressive, esp into the mid 70's+.  Dew points rarely reach 80+ because thunderstorms would likely break out before it can get that high, but it has happened,  and it would feel absolutely horrible, and be dangerous combined with high temps.

Answer 5

The only thing that the dew point means is the temperature that the air is saturated. So if the dewpoint is 70, that means that if the temp is 70 degrees, then the air is saturated. If the air is saturated, then that means that the atmosphere can no longer hold any more water.  The temp only needs to be 70 for the air to be saturated, not 71.  I hope i helped.