Part 1: Surface Maps and Wind
By Airwaves Writer Eric Tobias
To move on to Part II, Sea Breeze, click HERE
To skip to Part II, Precipitation, click HERE
I was waiting at a train station in the rain the other day, and I overheard a woman complaining, “They said only 20% chance of rain today, wouldn’t you love to get paid what they make for all of their mistakes?” As an atmospheric scientist, this bothers me. The problem with uncertainty in meteorology isn’t because of the weatherman or the science. The problem lies with the lack of measurement of the ever-changing chaotic atmosphere. If we could measure every single atmospheric molecule, at every single second, we could perfectly predict the wind, temperature, precipitation, cloud cover and every single weather phenomenon possible. That kind of measurement and technology is of course impossible, so we have to make do with what we have.
My professor once told me, “Take a large, almost round, rotating sphere 8000 miles in diameter. Surround it with a murky, viscous atmosphere of gases mixed with water vapor. Tilt its axis so it wobbles back and forth with respect to a source of heat and light. Freeze it at both ends and roast it in the middle. Cover most of its surface with liquid that constantly feeds vapor into the atmosphere as the sphere tosses billions of gallons up and down to the rhythmic pulling of a captive satellite and the sun. Then try to predict the conditions of that atmosphere over a small area within a 5-mile radius for a period of one to five days in advance.” With weather forecasting, it’s obvious where all the uncertainty comes from. We sailors know better than random train station lady. We know that the weather affects us a whole lot more than just our hair getting a little damp. Here I’m going to provide a 3-part guide to help you better understand the uncertainty in your weather forecast, so you can use it to your maximum advantage at your next regatta. I’ll try not to get too scientifically complicated, I promise.
Wind is arguably the most difficult variable for a meteorologist to predict. This can be frustrating as a sailor because it is quite obviously the most important one. It not only affects what sail we’re going to put up or what side of the course we’re going to go to, but it also influences the waves and sea conditions we’re going to be dealing with. To better understand wind, you must first learn about the forces dictating it. The geostrophic forces that determine the wind speed and direction are the pressure gradient force and the coriolis force (I know, I promised I wouldn’t get too technical). Basically, the large-scale forces of the wind are due to the balance of pressure differences (which can arise from temperature differences) and the rotation of the Earth. The ageostrophic forces that influence the wind are smaller scale forces near the surface, such as thermals, sea breeze, wind shadows, land/topography influences and local phenomena. The true wind speed and direction is a combination of the two. All right, cool. So how do we apply this knowledge? Let’s take a look at a surface analysis map:
A good resource for retrieving surface analysis maps is the NWC Weather Prediction Center website at http://www.hpc.ncep.noaa.gov/index.shtml . The solid red lines are isobars, lines of constant atmospheric pressure. The geostrophic wind (remember, the large-scale “basic” wind) goes parallel to these isobar lines. In the northern hemisphere, the wind rotates counterclockwise around a low pressure system and clockwise around a high pressure system; the opposite is true for the southern hemisphere. Just by looking at these lines, we can see the wind direction (for the most part). Don’t forget, the true wind speed and direction can still be altered by ageostrophic (we’ll call them “local”) forces. When the isobar lines are closer together, it means we have a stronger pressure gradient force, therefore stronger winds and likely a more accurate wind forecast.
When the isobar lines are more spaced out, you have a weaker pressure gradient force, and weaker geostrophic winds. Recall that the true wind speed and direction is a combination of the large-scale geostrophic wind and the local-scale ageostrophic wind, so when the geostrophic wind is weak, the true wind is mostly influenced by the ageostrophic wind (a mouthful, I know). I’ll repeat that because this is important: When the pressure lines are more spaced out, your wind speed and direction is likely to be more influenced by local forces such as sea breeze, thermals and topography. So when you see a forecast map that has really spaced out isobar lines in your area, maybe it’s time to take your wind forecast with a grain of salt, and rely on local knowledge or maybe poke the boat up into the wind a few times to get a feel for the wind tendencies that day.
Looking at this particular surface analysis map, we can see that the low off the coast of North Carolina causes a strong pressure gradient (meaning the lines are close together) between its location and Maryland. We can expect strong winds from the north in the Maryland/Virginia area at this time (remember the wind goes counterclockwise around a low). There’s also a very strong pressure gradient extending from Montana up into Canada, where there’s likely to be strong winds from the northwest. However, for the Central Plains region, extending all the way from Texas to Michigan, a large high pressure system provides a significantly weak pressure gradient (the lines are spaced out and squiggly), so the winds here are likely to be lighter, more disorganized and more susceptible to local forces.
If you don’t already know how to read fronts on a weather map, the red lines with semicircles on them represent warm fronts and the blue lines with triangles on them represent cold fronts. Fronts are often accompanied by turbulent, unstable air, often causing thunderstorms and heavy precipitation. Cold fronts can be especially violent because as the cold air moves in, it pushes underneath the current warm air and causes a turbulent lifting motion that often results in intense storms. You can think of a cold front as a snow shovel pushing snow along a sidewalk, where the snow piling up in the shovel represents your massive cumulonimbus thunderstorm clouds. If you look at a weather map and see a strong cold front sweeping through your area, be ready to experience some severe weather and wind.
You can also predict the wind direction based on fronts. In the North America region, there are typical weather patterns associated with the passing of fronts. Before the passing of a warm front, the wind direction is typically east or southeast. While the warm front passes, there is a steady rise in temperature and a wind shift to a south or southwest direction. Before the passing of a cold front, the wind is typically south or southwest, and as the front passes there is a steady drop in temperature as well as a wind shift to a west or northwest direction. It can be a game changer to look at a weather map in the morning and know if a front is going to pass through your racecourse that day. Later, when you’re out on the water and you feel a sudden temperature change, you can know the wind shift direction associated with it.
A surface map can be a useful tool to know about the fronts in your area and also to know about how much faith to put in your wind forecast. In part 2 of this series, we’ll check out sea breezes and how to utilize them to scoot past the other boats in the fleet.
To move on to Part II, Sea Breeze, click HERE
Part 1: Surface Maps and Wind