Ask the Weather Team

Alice asks: What is the difference between the humidity and the dew point?

Pete Bouchard says: This is a tough one to understand, and one that took me a while to grasp back in college.

Humidity is broken into two types: absolute humidity and relative humidity. Both are designated with percentages. Absolute humidity is the actual water vapor in a given volume of air. Sounds simple enough. Relative humidity, however, is the amount of water vapor in the air RELATIVE to the amount it can hold. This is where it gets complicated.

The key here is you have to know your airmasses. Cold air holds less water vapor than warm air. So let's say you have a glass of water. If you (hypothetically) threw that into a room of cold air, the air would saturate and a cloud could possibly form. But if you threw that into a warm room, the air's capacity is greater, and wouldn't saturate - no cloud. Admittedly, this is a poor way to designate how humid the air is, since you could have a relative humidity of say, 45% in both the dead of winter and the hottest day of summer.

Enter the dewpoint, which is more closely related to the absolute humidity. The higher the number, the more humid the air. The lower the number, the drier the air. The dewpoint also has other applications. Since the temperature NEVER goes below the dewpoint, it's sometimes used to gauge the low temperatures for the night. In its simplest form, the dewpoint is the temperature that the air must cool to reach saturation.

Hope that makes sense!

Judes asks: What effect will the Iceland volcano have on our weather?

Pete Bouchard says: I took my time before jumping at an answer to this question because until the volcano's eruption slowed, we weren't really sure how much ash was pumped into the atmosphere.

Here's the skinny from climate scientist Caspar Ammann of the National Center for Atmospheric Research in Boulder, Colo: "Right now, the magnitude and explosiveness of the Icelandic eruption is not enough to have significant climate effects."

Eyjafjallajökull's (say that five times fast!) ash doesn't contain much sulfur, which can generate sulfuric acid droplets that could linger in the upper atmosphere and have a cooling effect, Ammann says. Moreover, its explosive bursts haven't been able to hoist the ash much above 30,000 feet, where it would need to reside in the atmosphere for years to have a significant cooling effect. If the droplets don't get higher, like into the stratosphere, they can't reflect the sun's warmth away from Earth. Instead, they are washed out of the upper atmosphere by normal precipitation processes. According to Alan Robock, an environmental sciences professor at Rutgers University, the volcano has poured .003-.004 megatons of sulfur dioxide into the atmosphere. To put it in context, the 1991 Mt. Pinatubo eruption in the Philippines deposited 20 megatons of sulfur dioxide into the atmosphere, about 10,000 times as much! Pinatubo massive explosion also put that sulfur dioxide directly into the stratosphere and was in a better geographical position (the tropics) to disperse the ash throughout the globe. Volcanoes in the high latitudes typically sequester their ash and sulfur dioxide in the northern latitudes due to global circulation patterns. In the tropics, the global circulation is such that the ash and sulfur dioxide can readily be transported all over the world.

While there isn't much to be worried about in the short term, if Eyjafjallajökull continues to erupt, there may be greater implications. It could awaken a neighboring dormant volcano, Mt. Laki, which is more massive and HAS had a history of altering the climate in an earlier eruption dating back to the 1700s.