|Photo by Wendy Miles|
Ah, the adiabatic lapse rate problem.
|Lifted from NASA.|
Humidity and temperature...
Our second step is to understand humidity. We generally define humidity (very basically) as water vapor in the atmosphere (2). The more water vapor you have in the atmosphere, the higher the humidity. Though there are several ways to measure humidity, here we're going to focus on the relative humidity. This is simply the amount of water vapor in the atmosphere relative to how much the atmosphere could potentially hold. Think of the atmosphere as a glass. A glass can be empty, or full, or partially full. So we think of the size of the glass as the amount of water vapor that the atmosphere could potentially hold, and the amount of water vapor can be thought of as how much water there is in the glass. Therefore the relative humidity is how "full" the glass is. Under normal circumstances humidity ranges from 0% to 100%. That's easy enough.
Now about that glass.....we can change its size. The way we do this is by changing the temperature. We have a simple relationship: the ability of air to hold water vapor depends on its temperature. The higher the temperature, the more water vapor the air can hold, the lower the temperature, the less. So if you increase the temperature the glass gets bigger, if you decrease it, the glass gets smaller.
So what happens if it keeps getting colder and colder? The "glass" will get smaller and smaller. Eventually it will get to the point where it can't hold the "water" anymore. You can probably imagine what happens at that point: the water spills out of the glass. A similar thing happens as the air gets colder. Eventually you reach the 100% humidity level, and some of the water vapor has to "spill" out. But in the case of air, the water vapor turns back into water. This is how clouds form (clouds are water, not water vapor). We'll have another post in the future about this topic. But for now you can see what happens: the trade winds bring in moisture-laden air, forcing it up over the mountains. As it rises it cools down, and as it cools it loses its capacity to hold water vapor, until eventually the air becomes saturated, meaning it is holding all the water vapor it can hold. It is at this point that clouds begin to form. If the air continues to rise water vapor continues to be converted back into water, and eventually there is so much water it will rain.
When the air reaches the top of Mt. Wai'ale'ale it starts coming back down the mountain, and as you might have guessed, it warms up as it moves down the mountain (3). This means that it can hold more water vapor (the "glass" is getting bigger), and so instead of converting water vapor into water, the reverse process happens: water is evaporated into water vapor. The clouds first start to evaporate; when all the clouds are gone any water on the ground starts to evaporate. One thing we assume here is that whenever there are clouds, the humidity is 100%. Clouds indicate that the air is saturated.
So What About the Temperature and Relative Humidity?
As the air moves up the mountain, the temperature decreases. That part we know from experience. The interesting thing is that it decreases at a constant rate. We call this rate the adiabatic lapse rate. When the relative humidity is less than 100%, this rate is about 5.5 degrees Fahrenheit per 1000 feet, or .55 degree per 100 feet. If the air travels 1000 feet up the slope, the temperature will decrease 10 degrees; if it travels 1000 feet down slope, it will decrease 10 degrees. This makes sense to us; this is why we sometimes find snow on the top of Mauna Kea.
The complex thing that throws everybody off (and it's not really that complex) is that the adiabatic lapse rate changes if the humidity is 100% (if the air is saturated). We call the first lapse rate (5.5 degrees/1000ft) the dry adiabatic lapse rate (DAR); when RH% is 100% we use the moist adiabatic lapse rate (MAR). The MAR is (in this example) 3 degrees Fahrenheit per 1000 meters. Why is it lower? The answer is simple, but you need to concentrate and think about it. If the RH is 100% and the temperature continues to decrease, your "glass" is getting smaller and so water vapor must be converted into water. But for every gram of water vapor that turns into water, a significant amount of heat is released into the atmosphere (the latent heat; this will be discussed in a future post). This heating slows down the cooling trend you have with increasing elevation, and that's why the MAR is less than the DAR. Now we have all we need to know to tackle the problem.
If we start in Hanalei Bay at 80 degrees and follow our air parcel up, the
|Does this guy look familiar?|
So as we go down the mountain the temperature increases at a rate of 3 degrees per 1000 feet (the MAR) until we get to 3000 feet, which is the cloud base. Remember that clouds mean 100% relative humidity, and so once you leave the clouds the relatively humidity is less than 100%. You may have noticed that cloud base on the windward side is 2000 feet, whereas it's 3000 feet on the leeward side. Why is this? Because there is less moisture available on the leeward side. There is less moisture available because it was raining on the top of Mt. Wai'ale'ale, and so some of that rain flowed down the mountain or infiltrated down into the soil, and so it is no longer in the atmosphere, and no longer available to be evaporated.
Therefore at 3000 feet we switch back to the DAR of 5.5 degrees per 1000 feet. I'm not going to give away the answer which you should be able to calculate easily, but it should be clear here that the temperature is greater in Waimea bay than it is in Hanalei Bay. It is also much drier, because the relative humidity is lower and the air is trying to evaporate moisture at this point, which leads to dry conditions.
What does this have to do with Mt. Ka'ala?
|Map from Hawaii DLNR site on Ka'ala.|
Anyway, after eating our lunch and horsing around for a bit on thelee side of the mountain the air moves down the slope, warming along the way. As it warms it can take on more water vapor. First it evaporates the clouds, but since some of the air's moisture has been lost in the form of rain, the clouds are quickly evaporated. This is why the cloud base on the lee side is higher than the cloud base on the windward side. This is also why the lee side is usually quite dry; the air descending from the mountains is constantly evaporating available moisture.
|Geography John demonstrating proper|
sling psychrometer technique
That about does it for this post. In our next post one of the geography crew will explain about the vegetation zones you typically encounter as your elevation increases in Hawai'i, providing some examples from the Ka'ala trip. Stay tuned!
Gradient: a change of a particular variable over distance. For example, the higher you go up a mountain the colder it is. There is a strong temperature gradient. Another example can be seen with rainfall; Hawai'i has very noticeable rainfall gradients. If you start in Waikiki, there is very little rain, but once you make it to the back of Manoa valley it is very rainy. There is a very sharp change over distance, hence a strong gradient.
Leeward: The leeward side of the island is the one that the prevailing wind blows away from. In Hawai'i the prevailing winds are the trades, which blow from the northeast and east, so the leeward side is the south and east sides of the islands.
Windward: The windward side of the island is the one on which the prevailing wind blows. In Hawai'i this is normally the northeast and east sides of the islands.
Question 1: If the temperature increases, what would happen to the relative humidity? What about if the temperature decreases?
(1) We would generally refer to this as a northwesterly wind. We always name the wind for the direction from which it blows. So a wind blowing from the south is a southerly wind, one from the east is an easterly wind, and so forth.
(2) In geography we deal with water in three states: solid (ice), liquid (water), and gas (water vapor). It is the last of these that concerns us here. The air always has a certain amount of water vapor in it, and this water vapor is invisible.
(3) Rising air cools, sinking air warms up.