Chapter 5 - Fronts, Lows, and Highs

My father took the Sunday New York Times which, while I was in my early teens, was the only paper which had a relatively detailed weather map. While it primarily had the analysis features, the fronts, highs, and lows on it, it also had a few isobars which indicated how real weather maps worked. One Sunday in late spring the map indicated a cold front would be heading our way and the forecast indicated it would pass through the next afternoon. I remember sitting next to an open window in study hall watching for it when it hit. The wind which had been light and blowing out of the window suddenly changed as a cold gust came in the window scattering papers all over the room. After that I started paying attention to fronts.

One thing I found out was that cold fronts are often very pronounced. The change in weather is usually dramatic. Before the front hits, the winds are usually out of the south, the temperatures are usually warm, and the air is humid. Clouds abound and thunderstorms usually occur before the front passes. After the front hits, the winds usually blow out of the northwest and are cooler and less humid. Of course, with everything in nature, exceptions arise. While most cold fronts are relatively distinct, a few cold fronts are diffuse and the changes are gradual over a 12 hour period. And, the "back door" cold front of summer rarely has even a cloud associated with it, only a change from hazy, hot and humid to warm and dry. With these, the winds usually don't even change more than 30 degrees.

Low pressure areas are easiest to determine from the data. Simply contour the atmospheric pressure adjusted to mean sea level and the concentric curves stand out. If the central pressure is lower than the surrounding pressure, in other words it is a dimple, then we tentatively place an L in the center. If the central pressure is higher than the surrounding pressure, then the feature is a high. And, yes, it is possible for a low somewhere to have a higher central pressure than a high someplace else. The central pressures are relative to the surrounding area not to the average of the atmosphere as a whole. Figure 5-1 is a sample of a surface weather map. The dotted line across the southeastern states is a construction line for later.

I looked for warm fronts as they came through and was disappointed. They weren't easy to find. They still aren't. The best advice is to look for foggy areas east of the low pressure area. Usually they aren't well defined by the pressure field; the best way to identify them is the pressure change over the last three hours and the location of fog, low ceiling or haze. Since warm fronts are hazardous to aviation, let's look in more detail about fronts in contact with the ground.

There are two other types of surface fronts which are mentioned, the stationary and the occluded fronts. Both can provide some nasty weather when you are flying in their neighborhoods. Stationary fronts are simply fronts where the cold air is moving parallel to the front. They don't move very rapidly, hence the adjective "stationary." Occluded fronts are complex fronts which generally occur where a cold front has caught up with a warm front and shifted the boundaries aloft. They can be anything from benign to providing some very heavy precipitation. Since they are almost always found near the center of low pressure areas, the normal VFR pilot should already have made the decision not to go when these are in the neighborhood.

In the simple air mass ideas, a cold front is simply a demarcation between a warm and a cold air mass where the cold air is moving towards the front. Since the cold air mass is more dense than the warm, in a cold front, the cold air is moving into the warm air mass. It burrows under the warm air like an avalanche. In fact, the shape of a cold front looks very similar to shape of an avalanche or a mud slide or even a raindrop sliding down a pane of window glass. One person compared the cold air to the proverbial 600 pound gorilla. What does the warm air do when the 600 pound gorilla approaches? Anything the gorilla wants.

The warm front is the area where the cold air is retreating from the front. This allows the warm air to approach the front. Since the cold air is near the surface, the last remnants near the ground can be eroded by warm air aloft. Figure 5-2 shows a schematic of a warm front at the surface and the frontal zone above. Notice there are waves in the frontal zone aloft. Sometimes these waves get big and erode the cold air below mixing with it and taking it aloft. Sometimes erosion doesn't happen, sometimes it does. If it does, the fog will probably evaporate where the warm air descends to the ground and the warm front reforms someplace else nearby. When this happens, the warm front is said to have jumped. These can be disconcerting to say the least. If you plan to try to land when a warm front is near your destination, carry plenty of reserve fuel and check your destination weather frequently.

During a stormy situation, there is a reasonably standard arrangement of the warm and cold fronts and the low pressure area. Figure 5.3 gives the classic model of the surface fronts associated with a storm, the low pressure area. Here the cold front is drawn with its standard triangular sharp pointed arrowheads which are pointed in the direction the front is moving. The warm front is drawn with the rounded arrowheads also pointed in its direction of movement. If you keep this picture in mind as you plan about flying with an eye on avoiding storms, you won't be far wrong. True, we often have fronts without lows and occasionally there will be a low without a front. However, these are usually relatively inactive systems.

Of course, these systems move sometimes fast and sometimes very slowly. The speed of the system is usually quite slow, moving slower than a Cessna 140; however, since the systems rotate, parts of them move quite fast. Cold fronts a long way from the low pressure area may move along at 20 miles per hour. And, as they pass, the weather changes. In summer, the nastiest weather is usually found ahead of the cold front and south of the warm front. In winter, the situation is usually reversed with the heaviest snowfall to the north and west of the storm.

Fronts, highs and lows are analysis features, things to look for in the myriad of data that give you a clue as to what's going on. These help in using that conceptual model which you carry around with you. Most of the time lows are storms, with the attendant clouds, rain or snow, and so forth. Fronts are only one of the analysis features available. More recently, trough lines, dry lines, vorticity maxes and areas of diffluence are names which have popped up in briefings.

It is rare that a significant low pressure area is not associated with at least one front. Usually, a young low will be found along a stationary front, at the intersection of a cold and warm front, or, as it gets old, at the end of an occluded front which usually branches into a cold and a warm front. Since low pressure areas affect the winds and the resulting circulation produces winds moving upward, the lows will be the places where the clouds and precipitation are most active.

For a developing low pressure area, the angle between the cold and warm fronts is often closer to 180 degrees. A well developed cyclone, another name for a low pressure area, will have an angle of roughly 90 degrees between the warm Front and the cold Front. An old storm will frequently have an angle between the fronts of much less then 90 degrees and when the fronts converge, the result is usually a complex front called an occluded front. There can still be significant weather in a storm with an occluded front. The angle does give a clue to the developmental stage of the storm, yet, like many things, it is not a firm rule, only a "rule of thumb". Another "rule of thumb" is that the low pressure area will move parallel to the isobars in the warm sector. These both have good theoretical backing, but one or more of the assumptions in the theory may break down in the real world.

Most people look at a surface weather map and think in terms of the weather at the surface. Pilots, perhaps more than anybody else, know the atmosphere is three dimensional. Figure 5-4 shows the three dimensional nature of a cold front . The dotted line in the figure 5-1 shows where the cross section of figure 5-4 is drawn. This gives a side view across the cold front from the warm sector on the right to the cold air in the surface high on the left. The 500 millibar surface seen in earlier chapters, and in the inset at the top, is now a curved line going from a high elevation on the right to a lower elevation on the left, where the cold air in the lower half of the atmosphere by weight is the most dense. Here the chilly molecules aren't moving as rapidly as the ones over the Gulf Stream. The result is the pressure gradient in the upper air is strong and winds must also be strong. If you've tried to hold a heading of 210 degrees on the back side of a cold front, you know that your ground speed has been reduced. Trying to fly southwest at 10,000 feet on the warm side of the cold front doesn't usually help either because there are usually some strong winds on the warm side as well. The center of the jet stream often crosses the front usually just south of the cyclone or low pressure area.

The volume of air east of the surface front and frontal zones aloft is called the warm sector, primarily because this is where the warm, moisture laden air comes streaming northward to fuel the storm. The cold sector is found to the west and north of the fronts. Figure 5-4 is a cross section of a cold front. There are four graphs of temperature as a function of height shown spaced out along the ground. Each graph tops out at the tropopause and the warm air from the warm sector is above the cold air to the left of the front. The cold front on the surface is sharp causing abrupt changes in wind speed and direction often in a few seconds. Aloft, the front is really a zone of turbulence between the two layers of air blowing in different directions. Below the frontal zone is the cold sector.

The jet stream isn't constant in speed and direction. The winds in the jet will be fastest where the temperature contrast across a moving cold front is the greatest. Figure 5-5 illustrates a typical jet Stream at 500 mb. The dotted line is drawn along the core of the jet, the line of maximum wind speed at this level. The dot-dash line outlines the area which has wind speed greater than 50 knots. Wind speeds diminish on either side of the center line.

Concentrate on the area to the left and ahead of the max for a moment, the area over Eastern Ohio. If you were in that balloon and could see the air on both sides of you, at the average speed of those winds, the winds toward the axis would be moving faster than you. The winds away from you would be moving slower. The net effect is that from your traveling point of view, the winds would have a spin, or vorticity, relative to the balloon. Just below the center of this spinning effect is the surface low pressure area, the storm.

On the other side of the jet, in this case over north central North Carolina, a circulation is forced but this one is anti-cyclonic, a high. As air moves downward in a high, clouds don't form there. Similar circulations are found to the rear of the jet maximum, forcing air upward in the tightly curved air just south of the 500 mb low in Mississippi. This tightly curved air flow coupled with the jet max circulations produces some very interesting thunderstorms over the Gulf states. South of the jet max to the rear over the panhandle of Florida, the curvature is anti-cyclonic but the spin of cyclonic. The balance between the two curvatures keeps the clouds subdued in the Gulf. Evaporation continues, perhaps enhanced by plenty of sunshine streaming to saturate the air in the clear to partly cloudy skies.

The lines of constant pressure curve downward most dramatically just to the west of the front. In the area near the top of the figure is a schematic 500 millibar chart which would be typical of the area denoted by the finely dotted horizontal line in the lower part of the diagram. The arrows length is proportional to the wind speeds. It is here and at levels above where the winds aloft are the strongest, the jet stream.

Jet streams are tied to fronts. Without the temperature difference, there would be no height difference (or no pressure gradient which is the same thing), and, hence, no wind. So, if you see a cold front on a weather map, you can almost always expect that the center jet stream will be found a few hundred miles in the cold air behind the front gradually centering on the low. Further, the winds above the cold air will blow nearly parallel to the front and will be strongest above the frontal zone.

The cloudy area around cyclones or the low pressure area says one major thing to the meteorologist. Where there are clouds, air is moving up. Where skies are clear, air is not moving up. It may be moving down or exactly sideways in the clear areas. Most pilots would agree with a corollary to this idea; what goes up must come down. And air that goes up does come down. For the forecaster, the questions are where and when.

Robert Buck, in his book Weather Flying, reports using this idea as a way of gaining altitude during commercial runs. The upward moving air in a low gave him a small boost after takeoff to assist in getting up to the higher elevations. I shuddered when I read that, but then I am not equipped for icing and my stomach is not up to the turbulence.

By the same token, flying in the clear, blue weather of a high pressure area, you will find it harder to maintain altitude because the air is slowly subsiding around you. Yet, the relatively smooth air is caused by the subsiding air in the high. (Descending air becomes more stable whereas ascending air becomes less stable)

Incidently, the center of the low does not have to reach any particular reduced pressure for it to be a storm. All it has to be is a place where a few stations (for confirmation) have pressure below their neighbors. Similarly, a small high pressure area may be quite local; however, since these rarely have adverse weather, nobody makes a fuss about them. One exception; bubble highs are often found as outflows from thunderstorms and are a tool the forecaster uses to try to figure out what's going to happen next.

Figure 5-6 shows the position of a classic low pressure area and associated fronts for three days. On day 1, the system has moved out of Texas and is moving northeast. By day 2, the system is now over the Appalachians and on day 3, the system is moving towards New England. The path the system takes is called the storm track. The storm track is one of the things most meteorologists try to predict. The path of the center of low pressure is sensitive to the amount of cold air to the west, its distribution, and the effect this cold air has on the upper level atmosphere, especially the distribution of the winds. The reason that the storm track is important is that the distribution of the weather, the snow and ice is relatively well known.

Figure 5-7 shows the average distribution of ice storms and heaviest snowfall around a typical wintertime storm. The heaviest snows typically fall along a line 100 to 200 miles northwest of the storm track. Of course, there is usually some snow anywhere close to the left of the line. The storm described at the beginning of this chapter had a track which, at its closest point, tracked almost exactly 100 miles southeast of the farm. At the farm, the air aloft was closest to the optimal temperature, -12 degrees C, for snowflakes to grow into stellar flakes which have the fanciest shapes and the most air within the dimensions of the crystal. The heat released and the subsequent addition of the heat to the dynamics of the storm are maximized here also. Of course, each storm is different so the forecaster must look at the temperatures and forecast temperatures to decide on the area where the maximum amounts are possible and look at the forecast humidities and the vertical velocities to decide on amounts and types.

If the cyclone produces ice at the ground, it will usually be found tight in along the cold side of the storm track in late fall, winter, and spring and may be, depending on the case, a little ways into the warm sector. Near the storm track, the probability of freezing rain increases.

Freezing rain or clear icing is probably the most dangerous precipitation that pilots encounter. As shown in figure 5-8, it most often starts as snow high above the surface. The snow then melts in a warm layer; the wet bulb temperature must be above zero Celsius. Then the rain falls through air where the wet bulb temperature is below freezing. The rain then supercools and will freeze on contact with the ground or airframe. Of course, freezing drizzle will start as rain aloft and then fall into the colder air where it supercools.

Since the air temperature can be above 0 degrees C, all that is really needed is for there to be layer which is somewhat drier than those above or below. It can be raining at the ground, but a thousand or so feet above that in drier air, the freezing rain will coat anything that flies through with clear ice. While clear icing can occur in other weather situations, it is most common near the center of the low, just a tad northwest of the storm track.

When ice forms, it is well to be on the ground. One week icing was forecast so we opted to drive to the farm in the Catskills. It was well that we did. But it was nice and I can remember it well...

* * *

It was cold and frosty, with a crunch underfoot when I took some trash to the garage. The big bear was climbing up in the crystal clear evening sky, so bright that one was tempted to reach up and grab it by it's tail. The thermometer announced a temperature of 22 degrees F with its narrow red tongue. I stopped at the wood pile on my way back to pick up an armload for the fireplace. We would want a fire when the kids came back from the slopes. The season and vacation had been good. Just enough packed powder to make the slopes interesting; not too much ice.

The morning was clear, not the indigo skies of severe clear which is common here in the dead of winter; there was just a hint of undefinable cloudiness somewhere up there. After breakfast, the wife and I bundled up and took a walk in the early morning when the winds were calm and the day was new. The kids would be sleeping until late as they had fallen asleep in front of the fire. Vacation is like that.

Yesterday's warm sunshine had congealed the snow from a few days ago into loaves on the stone wall. Most of it had come off the bare branches of the maples but here and there a clump of white still stuck to the bare branches. Some of the snow was still sticking to the dried crowns of the Queen Ann's Lace in the meadow and icicles were hanging from the eves along the barn roof. The bumper crop of rabbits had left their tracks where they had been about their daily business, and a curious fox had been investigating their endeavors. There was no evidence in the snow that they had been there at the same time. At the bridge over the brook the ice and frost hung on the stems along the edge of the brook where last summer we had found a quart of blackberries. As we walked the quiet pastures, the crunch of our boots on the two inches of snow and the dead stalks of the grass underfoot were the only things marring the morning. After a while the chill was starting to penetrate so we headed back to the house to the promise of a hot cup of coffee. As we breasted the hill back of the house, I noticed that the hint of overcast was starting to thicken. No halo, but a bit of corona around the sun which was beginning to look a little feeble.

While Muriel put the pot on, I roused the kids. Amid much grumble and mumble, they arose and set about getting ready for the day's activity at the slopes.

After lunch Muriel and I headed to town for supplies. As I came out of the hardware store, I noticed that the sun was considerably dimmer than it had been. A definite stratus layer had developed. We walked over to the market to lay in sufficient supplies for the hungry horde. Both the chocolate and popcorn supplies were low, so they had to be replenished. As we walked out carrying four bags of extras, we both agreed that "it felt like snow."

The forecast for the day had been for snow late in the day. Last night, the forecast showed the low, the center of the storm, passing south of us through Pennsylvania. The Albany, New York, area wasn't supposed to get much, but the Catskills could get an appreciable amount. It was expected to be a relatively slow storm. I was reminded of the old rule: You get the most snow where it snows the longest and snows the hardest. Common sense. And it works.

As we headed up the hill to the farm, I saw the first flakes.

The temperature had risen to 28 when we unloaded the car. In another hour, the slopes would be closed and the kids would be headed home, so we put on the cross country skis and headed out for some afternoon exercise.

It was snowing very lightly when we returned to the bright kitchen and some hot chocolate. There was a light dusting on the roads as I laid the fire in the fireplace. I was just getting it started when the kids drove up. There is nothing to compare to the smell of hemlock kindling burning. It burns with snaps and crackles and occasional flare up. It does get the cured maple started.

The kids emerged slowly from the car. They appeared to be moving in slow motion compared to when they had left. They and their accouterments arrived in the kitchen with a series of crashes and exhausted excitement. Apparently, the slopes had been fast. Apparently too, the restaurant had been efficient and well stocked. The kids began to tell about their day. As they sipped hot chocolate, I manned the corn popper. Pretty soon the normal distribution of sounds emerged from the popping corn.

After supper, I went out to check the cars. In the dim light of the day, the snow was coming down with authority. The setting sun was totally obscured by the stratus layer, now invisible because of the gently falling snow. A rabbit, out on some errand, scurried in the field. Off in the distance I could hear the roar of the plow working through the inch or two on the road.

Just before we went to bed, I went out on the porch and it was still coming down. The land lay silent, not even the noises from town penetrated the falling crystals. I put out a glove and caught a few. Bringing them under the porch light, most of them were the big six-pointed stellar crystals which are the ones usually portrayed in pictures. There were few grainy columns and platelets which we had seen earlier. It seemed to me like the snowflakes were singing, just perceptibly, as they completed their fall from the clouds above. There was a good four inches of new snow on top of the mailbox. The plow had come through and left its trail of snow along the edge of the road, and in the driveway the kids had shoveled out yesterday as well. Oh well, clearing that would be for the morning. The temperature hovered at 28 F.

The temperature had dropped to 20 the next morning, and every snowflake that had fallen by bedtime had been joined by their sisters and their cousins and their aunts. Looking out at the mailbox, the snow had drifted almost up to the mailbox. The wind had come up in the night and there were small swirls and pockets of snow in the air. Nothing as bad as I had seen before.

I put the bacon and pot on. On vacation, I have the honors for breakfast. The sounds of the wind in the night alerted me to the possibility of an interesting day, so I, and the rest of the tribe, had slept late. The kids had already decided that this would be a day to rest up from the slopes, so time wasn't a problem.

Measuring the depth of snow when the winds have piled it up here and cleared it off a little there is not easy. Taking an average of about ten soundings with the yardstick, it appeared that we had been the recipient of about 14 inches of snow. The plows had given up sometime in the night and the road had a significant covering of the white stuff. It seemed that they had delivered the most to the driveway.

It was snowing lightly from scuddy looking clouds I would be hard pressed to call cumulus but they were certainly not the stratus of yesterday. There were at least two layers, the broken cumuli below and a broken layer of stratus above. The lower layer was moving in from the west running like white dragons skimming over the mountains looking for game. The other layer was coming out of the southwest. It was clear by looking at them that it was going to get colder.

Since it was vacation, there was no need to go anywhere. After lunch the wind seemed to go down so the wife and I strapped on the cross country skis to see what had happened. I'm not sure if breaking trail with these is easier with the skis on or off, but I kept them on. After half an hour of exercise, the sun started to break through the clouds and the snow began to sparkle. Looking closely, the flakes on top were the big stellar crystals with almost perfectly flat top and bottom. The sunlight reflecting off the tops made the field below us a sheet of diamonds.

The evening news provided a summary of the storm. As forecast, the path the center of the low pressure area, often called the storm track, moved south of the farm through Central and Northern Pennsylvania, with its closest approach about 100 miles or so. Interstate 80, about 60 miles south of us, had been covered with a sheet of ice but the road sanders and salters had been able to keep pace with it so there weren't any big pileups. Probably the bigger company truckers, forewarned by their private meteorological services, had taken the northern route along the New York State Thruway which hadn't had much snow or gone further south or stayed home.

As we drove back to the Washington area two days later, limbs from the once stately maples littered the ground along the road and there were, in some of the high elevations, still a glimmer or two of ice casing the branches. The bent birches, some of them stooping clear to the ground in the southern Poconos reminded me of the Robert Frost poem. Near Hazelton, where Interstates 81 and 80 cross, we ran out of the effects of the ice. Evidently this was where the storm track crossed the highway.

There is certainly a lot of energy involved in a storm like this. The next chapter looks at the energy flow in the atmosphere and in storms.
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