Upon completion of this section, you should be able to compare typical pressure gradients in the tropics with those in the middle latitudes, and be able to interpret frequency of wind directions and speeds from wind rose diagrams.
Just as the tropics display a "Type B" personality with respect to temperatures, they generally maintain that same personality when it comes to pressure. In the tropics, pressure gradients tend to be much more relaxed than they do in the "Type-A" middle latitudes (resuming our analogy from the previous page). If you look at the surface analysis over the northern Atlantic Basin at 06Z on September 8, 2003 below, you should note that the middle latitudes (toward the top of the image) have much tighter pressure gradients than low latitudes. The exception to the rule is tropical cyclones, of course. The tightly-packed isobars around Hurricane Isabel indicate a strong pressure gradient, as do those around Hurricane Fabian (which was on the doorstep of the middle latitudes).
So it is pretty apparent that the relatively small (large) temperature gradients over the tropics (middle latitudes) go hand in hand with relatively small (large) pressure gradients. At this point, the only flies in the ointment seem to be tropical cyclones. Indeed, tropical cyclones do something that is unheard of in the middle latitudes: they form in an environment bereft of large temperature gradients yet somehow develop very large pressure gradients around their centers.
The overwhelming message, however, is that pressure gradients are typically weak across the tropics. That also means that changes in surface pressure with time at any given location are usually puny compared to the larger increases and decreases that regularly accompany the approach and passage of mid-latitude high- and low-pressure systems. In the equable tropics, pressure patterns can persist for very long periods (weeks and even months). Yet, almost mysteriously, there is a regular daily rhythm of changes in surface pressure that meteorologists detect in the tropics. If you're intrigued, check out the "Explore Further" section at the end of the page.
For a broader view of pressures across the tropics and how they compare to those in the middle latitudes, check out the chart of average sea-level pressures at 00Z on February 12, 1998 below. At the time, there were intense northern hemispheric low-pressure systems over the Gulf of Alaska, the Great Lakes, the middle Atlantic Ocean, northern Russia and the east coast of Asia (splotches of blues and purples on the map). Meanwhile, robust high-pressure systems (bigger blobs of greens, yellows, oranges and reds) were interspersed between the intense lows.
In the tropics, on the other hand, pressure patterns are much more relaxed and much more equable than the middle latitudes. In other words, prominent centers of high and low barometric pressure are more difficult to find, especially equator-ward of latitudes 30 degrees north and south, which mark the very outer fringes of the tropics. The most notable exception to the generally more relaxed pattern of pressure in the tropics on February 12, 1998, was a tropical cyclone, not surprisingly. The spot of relatively low pressure (blue splotch) just to the east of Madagascar in the southwest Indian Ocean is the signature of Tropical Cyclone Ancelle, which reigned over the southwest Indian Ocean from February 5 to February 13, 1998 (during the southern hemisphere's summer).
The height patterns on constant pressure surfaces over the tropics are similarly relaxed. Consistent with the general lack of temperature gradients at 500 mb over the tropics, note the absence of strong gradients between 30 degrees latitude (north and south) on this chart of long-term mean 500-mb heights. Height contours on the other mandatory pressure levels in the tropical troposphere show a similarly relaxed pattern.
Since the pressure gradient force is a primary driver of wind speed, you might think that the winds are almost always weak in the tropics (outside of tropical cyclones, that is), with the weak pressure gradients at the surface and aloft. But, that's far from the truth! To help you visualize the fact that many places in the tropics are quite breezy, despite weak surface pressure gradients, I'm going to introduce a new type of plot -- the wind rose. Wind roses display the observed frequency of wind directions (and sometimes speeds) at a particular location. On the left below is a histogram displaying frequencies of observed wind speeds (in meters per second) at an ocean buoy moored at 8 degrees South, 95 degrees West during the year 2002. On the right is the corresponding wind rose for the buoy, which shows the frequency of observed wind directions during the same year.
From these two images, we can quickly get two important messages. First, wind speeds at the buoy were between five and nine meters per second (roughly 10 to 20 mph) the vast majority of the time, which hardly constitutes "weak" winds. Second, the direction from which the wind blew during the year was remarkably consistent. To get your bearings with the wind rose, note that each concentric ring represents a ten-percentage point increase in the relative frequency of the observed wind direction. Thus, the daily mean wind direction of 130 degrees (from the southeast) occurred on nearly 45% of the days, and the daily mean wind direction of 140 degrees occurred on about 28% of the days! The wind rose clearly demonstrates that winds retained their overall southeasterly direction for almost the entire year (and didn't deviate much from 130 degrees). Small variations in wind direction and breezy conditions are fairly typical in tropical locations because of the famous belt of "trade winds," which we'll cover formally in a later lesson.
Many wind roses that you'll encounter also include wind-speed data right on the wind rose plot. For example, check out this wind rose plot for the month of March at Grand Rapids, Michigan. At first glance, it's easy to see that it looks much different than the wind rose from the tropical ocean buoy shown above. Wind directions are much more variable during the month, which is more common in the middle latitudes thanks to the parade of high- and low-pressure systems that march around the globe. On this wind rose, each concentric ring represents a two-percent increase in the relative frequency of the observed wind direction, so the most common wind direction at Grand Rapids during March (from 90 degrees -- due east) occurs a little less than 10% of the time.
The various colors along each "spoke" represent wind speed ranges according to the color key at the bottom of the image. So, along the 90-degree "spoke," winds between 1.80 meters per second and 3.34 meters per second (roughly 3.5 - 6.5 knots) marked by the yellow shaded area occurred approximately 2% of the time. Winds between 3.34 meters per second and 5.40 meters per second (roughly 6.5 knots - 10.5 knots) marked by the red shaded area occurred about 3.5% of the time (5.5% - 2%). Winds between 5.40 meters per second and 8.49 meters per second (roughly 10.5 - 16.5 knots) marked by the blue shaded area occurred about 3.5% of the time (9% - 3.5% - 2%), and so on. Along any given spoke, the individual percentages for each range of wind speeds should sum to the total percentage associated with the entire spoke.
I strongly recommend taking some time to practice extracting information from wind roses (you can start with the "Key Skill" section below). Wind roses can provide lots of practical information. For example, consulting meteorologists use wind roses when the work on the design of airports (runways should be built to avoid strong crosswinds), and skilled forecasters regularly use wind roses when studying the climatology of a particular location. After you're comfortable with interpreting wind roses (and check out the "Explore Further" section, if you wish), you'll be ready to examine another difference between the tropics and the middle latitudes -- the structure of mid-latitude cyclones versus the structure of tropical cyclones.
You'll need to interpret wind roses not only in this course, but future courses, so it's a good idea to spend a little time making sure you're comfortable with gathering basic information from them. Consider the March wind rose from Grand Rapids, Michigan and answer the following questions. If you do not understand the answers to these questions, be sure to review the guidelines for interpreting wind roses above and / or ask your instructor for clarification.
During the month of March at Grand Rapids, which wind direction is observed the least frequently on average? What percentage of the time is this wind direction observed?
Answer: North-northeasterly winds are observed least frequently at Grand Rapids during March. Winds from the north-northeast are only observed slightly less than 3% of the time.
Which wind direction most frequently produces wind speeds greater than 11.06 meters per second (roughly 21.5 knots)?
Answer: West-southwesterly winds most frequently produce speeds greater than 21.5 knots (almost 1% of the time), followed closely by southwesterly winds. The light blue shaded area corresponding to these speeds is largest along the west-southwesterly and southwesterly spokes.
What percentage of the time do winds blow from the west-southwest between 3.34 meters per second and 8.49 meters per second (roughly 6.5 - 16.5 knots)?
Answer: Winds blow from the west-southwest between 6.5 knots and 16.5 knots slightly more than 5% of the time. We have to add the percentages that correspond to the red shading (slightly less than 3%) and blue shading (more than 2%).
As you learned on this page, pressure gradients in the tropics tend to be very relaxed, and changes in surface pressure with time at any given location are usually puny compared to the larger variations that regularly accompany the approach and passage of high and low-pressure in the middle latitudes. In the equable tropics, pressure patterns can persist for very long periods (weeks and even months). Yet, almost mysteriously, there is a regular daily rhythm of changes in surface pressure that meteorologists detect in the tropics.
To see what I mean, focus your attention on the time-trace of barometric pressure at Nauru, a tropical island in the western Pacific just a tad south of the equator. For the record, the trace in barometric pressure spans from midnight on April 16, 2003, to midnight on April 25, 2003. Although the fluctuations in pressure are relatively small in the grand scheme of weather (only a few millibars), there is an undeniable rhythm to the ebb and flow of the barometer. Indeed, much like the tides of the oceans, there are two high and two low "tides" in pressure that occur each day. In other words, there is a persistent oscillation in barometric pressure at Nauru that has a period of half a day (one high tide and one low tide in 12 hours). To better see this "semi-diurnal" oscillation in pressure at Nauru, check out this annotated version of the barograph trace. This semi-diurnal oscillation in barometric pressure is a staple of the tropics.
As it turns out, the amplitude of the pressure tides is largest in the tropics, where pressure variations generated by passing weather systems are routinely small. So, it's no wonder that these pressure tides stand out on barograph traces. In contrast, the amplitude of pressure tides is much smaller over the middle latitudes (the amplitude of the semi-diurnal pressure tide falls off dramatically with increasing latitude), so they are usually dwarfed by much larger pressure variations produced by passing weather systems (making them difficult or impossible to detect on barograph traces).
For the record, the greatest amplitude of the semi-diurnal pressure tide, which is a approximately one or two millibars, occurs at the equator. So, why do they exist? In a nutshell, the atmosphere absorbs only about 10 percent of the incoming solar energy. Ozone in the stratosphere accounts for a large fraction of the atmosphere's absorption, while, to a lesser degree, tropospheric water vapor accounts for most of the rest of the atmosphere's absorption of solar energy. At any rate, the resulting warming of the atmosphere after sunrise (and cooling on the other side of the earth) creates sufficient changes in air density that internal gravity waves form and propagate both vertically and horizontally. As these density-driven waves reach the earth's surface, they induce noticeable changes in pressure over the equable tropics. At higher latitudes, these gravity waves become "vertically trapped" and their affects on surface pressure become increasingly unimportant (the proof of the vertical trapping of internal gravity waves at higher latitudes involves very sophisticated mathematics and is beyond the scope of this course).