How Far Away are Those Clouds?

Saturday 200 PM

It's a beautiful sunny day here in central Pennsylvania. So what am I doing? Satisfying my curiosity about something. You may have had the same question: How far away are those clouds I'm looking at? Here's a picture of the edge of a cloud deck that's not far above the southern and southwestern horizon:

Now, how can I estimate how far it is from here to a point that would directly underneath the edge of the clouds? Perhaps a satellite picture would help. Hold on a moment. Thanks. I found one:

Calculating the distance using a standard map, it appears that I was looking at a cloud edge very close to 100 miles away. As you can see from the photograph, we have a fairly open view to the south. However, 10-15 miles to our south and southeast, Tussey Mountain is located. You can make it out just barely by looking just above the roof line on the house on the left side of the photograph. The ridge varies in elevation somewhat, but the top is a little over 2000 feet above sea level. Our home is at 1380 feet. We could calculate how much our view of the horizon is blocked out by the mountain, but give me a break. I have some more things on my to-do list for this afternoon.

In truth, I should really have a detailed list of tasks. My wife operates this way and is highly organized. I keep some lists and notes, but at any time they might be on scattered pieces of paper. In an effort to help myself become better organized, I bought a book published in 2003 by Penguin Books entitled Getting Things Done by David Allen. Sadly, I have not found time to read it. The bookmark I left the last time I read part of it is a coupon for dog biscuits that expired more than 2 years ago. If I remember, I'll take the book to the beach in a few weeks. Pardon me while I look for my list.

Okay. let's look at the weather situation. First, here's a national enhanced IR satellite picture:

The cloud area we were looking at to the south is part of a band of moisture that thickens to the west northwest. It was raining at Chicago at midday. Now we'll look at the NMM-WRF and GFS model outputs from 8AM EDT today (Saturday). They basically show a warm to hot increasingly humid pattern becoming established from Monday through Wednesday across all of the Middle Atlantic region and much of New England. This is inferred by noting how the red-dahed thickness lines advaince to the northeast (thickness is proportional to the mean temperature in the layer under examination (in this case the layer between the 1000 mb level and the 500mb level)). At the surface, a high pressure area sets up residence on the East Coast and creates a south to southwesterly flow that would draw warmer and more huimid air into the Northeast. The NMM-WRF model shows a fairly large area of what would likely be scattered showers and thunderstorms in Pennsylvania on Monday. The GFS is not as showery looking on Monday, but becomes that way on Tuesday. The GFS continues to show a cold front slicing into the the heat and replacing it wilth cooler and drier air from the Great Lakes to the Northeast region by the end of the week.

The last item for today is a brief discussion of the relationship between temperature, dewpoint and relative humidity. Temperature measures how warm or cold it is. Dewpoint is a measure of how much moistiure there actually is in the atmosphere at the measuring point. It is also the temperature at which saturation would occur. In other words, if it is 70 degrees and the dewpoint is 50, the temperature would have to drop to 50 for saturation to occur. If the temperature and dewpoint are both 50, the airmass at the measuring point is saturated and the relative humidity is 100 percent. I referred to the measuring point because that is important. I have heard people remarking that the relative humidity is 100 percent so why isn't it raining? The answer: just because the relative humidity at ground level (where we are measuring values for this little discussion) is 100 percent, that doesn't mean it is the same all the way up through the atmosphere (and in particular, the levels at which precipitation would form).

A confusing aspect of the relationship between temperature, dewpoint and relative humidity is that you cannot just take the temperature and dewpoint, make a fraction, and convert that to percent. It turns out that for each (approximately) 20 degree increase in temperature, the air can accomodate twice as much water as a vapor. Suppose the temperature and dewpoint are both 50 degrees at sunrise. Now, assume the actual amount of water vapor in the air remains exactly the same all day. That means the dewpoint holds at 50 degrees. If the temperature climbs 20 degrees to 70, the air will be able to accomodate twice the amount of water vapor as is actually present. Do the math, and you see the relative humidity drops to 50%. If the temperature climbs another 20 degrees to 90 as the dewpoint stays at 50, the relative humidity drops by half again to 25%. So, the relative humidity value does not really tell you how much moisture there is... because the value is related to temperature and moisture.

In the summer, we tend to feel that the air is humid once the dewpoint reaches 65 or higher. At levels above this, the amount of cooling caused by the evaporation of perspiration is curtailed. But suppose the dewpoint is 65 and the temperature is 85. What is the relative humidity? That's right, it is about 50%. That doesn't sound very "humid", does it? And if the temperature soars to 105, the relative humidity would drop to 25%. Yet, it would still feel rather humid. That is because it is the actual amount of moiusture in the air (as represented by the dewpoint), and not the relative humidity, that helps determine our level of comfort.

Have a fine rest of the weekend!!!

Elliot

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