Dewpoint for Dummies

When I studied weather to be a pilot dew point had to be understood. I recently googled 'dew point' to see what was out there on the topic and I did not see anything that explained it at a 'for dummies' level  so here goes...

To understand dew point there are a couple of concepts that need to be understood first.

The first thing is that hot air can hold more water vapor than cold air. So if its 90 degrees, that air will hold more water vapor than air that is at 50 degrees. So you can think of the current air temperature analogous to the size of a glass that can hold water.  This large glass here will represent a 90 degree day and it can hold more water than this smaller glass that represents a 50 degree day.

90 degree day can hold more water

Now lets look at relative humidity. Relative humidity is a percentage of how much water is in the air (glass) relative to the size of the glass (air temperature). So lets fill the 90 degree day glass halfway. Its 50% humidity here. Now lets fill the 50 degree day glass halfway. Its also 50% humidity here.  But look at how much water is in each day (glass). There's a lot more water in the 90 degree day than in the 50 degree day.  So even though the relative humidity is the same at 50% on both days, it will be a lot muggier on the hotter day because there is a lot more water vapor in the air for you to slog thru. 

Both days are 50% humidity but the hot day holds more water

So what we know so far is that 

1. Temperatures determines how much water the air can hold (the size of the glass). 
2. Relative humidity tells you how full the glass is percentage wise. But it does not tell you how muggy it will be because it doesn't tell you how much water vapor is in the air. On both days the humidity was 50% but in the 90 degree day there was a lot of water vapor and in the 40 degree day there was much less.

So if relative humidity does not tell you how much water vapor is in the air what will? That is what the dew point is, it tells you how much water vapor is currently in the air.  The dew point tells you how much you have to lower the current temp to make the relative humidity 100%.  Using our glass analogy, the dew point tell you how much water is in the glass at any temperature (or glass size) and it determines how much you have to lower the temperature (shrink the size of the glass) until the glass gets full. Lets visualize this.

Lets fill the 90 degree day glass up to 40%. So its 90 degrees, and you have 40% humidity so its 40% full. Now lets pour this water into the 50 degree day glass. Now its 100% humidity because its full. So given this amount of water on the 90 degree day, the dew point is 50 degrees because that is when the air gets to 100% humidity.  So here's the key point, the dew point tells you the temp you have to cool down to in order to be at 100% humidity and the amount of water vapor in the air determines what it will be (higher if a lot of water vapor, lower if little water vapor).

On this 90 degree day, the dew point is 50 degrees because that is when the humidity becomes 100%

Now lets see some examples.

Lets say you have three glasses that are full to the top

1. A large glass representing a 90 degree day
2. A medium glass representing a 50 degree day
3. A small glass representing a 35 degree day

Since they are full to capacity, the humidity is 100% in all three, and by definition the dew point is the same as the current temperature in each day because the humidity is 100%.  And it will be foggy and rainy.

Lets add more humidity (water) to all three. What will happen? 

• The water will overflow on all three meaning it will rain. Since the air is already 100% saturated (glass full), it can't hold any more water and it rains. The humidity stays at 100%, it can never be greater because extra humidity turns into rain which is no longer held by the air (glass).

Now lets say a super cold front passes and the temperature drops to 50 degrees on the 90 degree 100% humidity day. What will happen?

•  It will rain a lot. That would be like pouring the full 90 degree day glass into an empty 50 degree day glass. The water will overflow and it will rain. The end result is that the temperature will be 50 degrees, the humidity will still be 100% because the 50 degree day glass is full and thus dew point is also 50 degrees. But note the amount of water in the air is much less than before because you have a full but smaller 50 degree day glass.

Now lets say for some reason on the 50 degree 100% humidity day it gets a lot hotter to 90 degrees because the sun comes out. What will happen?

• It will get drier. No rain or fog. That would be like pouring the full 50 degree day glass into an empty 90 degree day glass. Now the 90 degree day glass is half full so the humidity is at 50% and the dew point is still 50 degrees. The amount of water is the same as before but since the 90 degree day glass is half full, it can hold a lot more water so there is no threat of rain or fog.

Takeaways:

1. Temperature tells you how much water vapor the air can hold (glass size)
2. Humidity tells you the percentage of water vapor in the air irregardless of glass size (temperature).
3. Dew Point tells you how much water vapor is currently in the air (glass).
4. A high dew point means that there is lots of water vapor in the air. A dew point of 90 degrees means it will be 100% humid at 90 degrees and that will hold a lot of water!
5. A low dew point means there is little water in the air. A dew point of 40 degrees means it will be 100% humid at 40 degrees and cold air will not hold much water
6. If the temperature and the dew point are the same, then the glass is full, the humidity is at 100% and it will probably rain or fog.
7. The dew point can never be higher than the temperature because if you put more water in the glass (add moisture to a 100% humidity day), it just overflows (rains) and the temperature and dew point stay the same.  Or if you lower the temperature on a 100% humidity day, then its like pouring a full large glass into a smaller glass and it will overflow (rain) and the dew point will drop with the temperature because there is now less water vapor in the air.
8. If the dew point is much lower than the temperature, then it will be dry, low humidity and little chance of rain.
9. Typically what happens in a day is that given a certain dew point (amount of water vapor), lets say 50 degree dew point, in the morning it might be 55 degrees so we are near the dew point and there will be fog, then as the sun comes out and the temperature rises, the fog goes away because it gets absorbed by the air because the hotter air can hold much more water vapor.
10. On a very high humidity day, the temperature and the dew point are very close together. But if its a cold day, there will be little water vapor in the air whereas on a hot day, there will be a lot of water vapor in the air. So dew point is a better way to determine how muggy a day is because it tells you how much water vapor is in the air. Which explains why in the winter there can be humid days but they are not muggy.
11. On a low humidity day, the dew point is much lower than the temperature.
12. The colder it is, the lower the dew point must be (since it can't be higher than the temperature). 
13. The hotter it is, the higher the dew point might be. It will be high if there is a lot of water vapor in the air. It will be low if there is little water vapor in the air.