Caution: Gradient Ahead

Driving to the summit of the beautiful mountains in central Pennsylvania, motorists are greeted by signs that warn of a "steep gradient" ahead. Clearly, "steep gradient" refers to the abrupt downhill "grade" of the road. Let me express a steep (large) gradient in a slightly different way. For the mountain road to have a large gradient, the elevation of the road must change relatively rapidly over a short distance. In terms of driving, the road is steep. Using parallel thinking, a small gradient means that the elevation of the road changes very little with distance. In other words, the road is relatively flat.

How do we distinguish between a large and small gradient on a topographical map? Let's return to the interactive Hawaiian topography you worked with earlier in the lesson. Start with the plan view of the Big Island and focus your attention on the summit of Mauna Kea, where, of course, the terrain can be fairly steep. Reassure yourself by rotating the view and then changing your perspective from a plan view ("top-down") to a head-on view of Mauna Kea. Looks like a pretty steep grade in places, doesn't it? Now flip back to the plan view. What do you notice about the contours of constant elevation near the summit? You're absolutely correct! They're packed close together. Looks like you "made the grade"! Thus, a large gradient on a topographical map, which marks a large change in elevation over a relatively short horizontal distance (the terrain is steep), corresponds to a tight packing of contours.

Now return to the plan view and focus your attention on the valley that lies between Mauna Loa and Mauna Kea. You'll note that the "packing" of contours of constant elevation is rather loose in the valley. In other words, the gradient is small. Now change your perspective from a plan view to a head-on view. The terrain is relatively flat. Thus, a small gradient, which marks little change in elevation over a relatively short horizontal distance (the terrain is flat), corresponds to a loose packing of contours.

Now let's take what we've learned from topographical maps and generalize it to weather maps. On the colorized pressure map on the U.S. below (please be aware that isobars are drawn every two mb), you'll note that isobars are packed fairly tightly over much of eastern New England. This tight packing of isobars, which translates to a large gradient in mean sea-level pressure, arises from the large contrast between a low pressure center over northeast Maine ("X" marks the spot) and a high-pressure center over Iowa. If you now look at the twin plot of Northeast station models, you'll note that winds were sustained as high as 30 knots along the seaboard of New England. It turns out that tight packings of isobars correspond to strong surface winds. If you demand more proof, take a look at this analysis of mean sea-level isobars and winds at 00Z on March 14, 1993. Folks, the analysis is a flash "roll-over". To help you navigate, move your mouse outside the pop-up frame and you'll see the pattern of mean sea-level isobars at 00Z on March 14, 1993. Now move your mouse over the image and you'll see the corresponding winds at a height of 40 meters above the ground (a proxy for surface winds). By the way, contours of equal wind speed that you see on this analysis are called isotachs.

Now focus your attention on the low pressure system centered over the DelMarva Peninsula. Talk about a large pressure gradient! As some of you might remember, this low-pressure system, which was almost at full throttle at this time, was the catalyst for the Blizzard of 1993. Besides dumping prodigous amounts of snow, the behemoth low lashed the Eastern Seaboard with hurricane-force winds. Note the red isotachs of wind speeds greater than 60 knots in areas of large pressure gradients.

I will delve into the details of the connection between large pressure gradients and strong winds later in the course.

Isobars of mean sea-level pressure (MSLP, for short). Click here to get the current data.

Tightly packed isotherms mark large gradients (large horizontal changes in temperature), which, as we will learn in later lessons, often indicates the presence of cold and warm fronts, which you've probably heard weather presenters talk about on television. Please look at the national temperature map that we discussed in an earlier section. Note the tightly packed isotherms over the panhandles of Texas and Oklahoma and southeastern Kansas. Imagine traveling rapidly from Dallas, Texas ("D" marks the spot) to Scottsbluff, Nebraska (marked by an "S") you would experience a temperature change of over 50 degrees Fahrenheit. While taking your imaginary journey, you crossed a rather strong front. As I pointed out in Lesson 1, fronts, in a classic sense, are sites for clouds and precipitation (recall that a large temperature gradient associated with a front provided the breeding grounds for the record ice storm of January, 1998, over northern New England and southeast Canada). So, as a conscientious weather forecaster, you'll need to pay close attention to large temperature gradients.

All of my words of advice will come together and make sense to you in good time. For now, what's one of the best presents you can give your favorite meteorologists to help them organize weather data at the city where they work? Send them a meteogram!