One of my favorite weather resource web sites is Unisys Weather (weather.unisys.com). I’ll explain why.
First, below is a current weather map for North America. If you examine the color bar at the bottom, you’ll notice a color gradient from black (-100dBZ) to light blue (250dBZ). The color is based on the energy returned to the weather radar. Weather radar blasts a beam of energy out and then waits for that energy to be returned. The amount of energy returned back to the radar receiver is directly related to what is in the air. If the air is clear, little if any energy is returned back to the radar. Water drops, raindrops, snow, and hail suspended in the air will return an amount of energy back to the radar which is directly proportional to the density of that substance in the air. The more raindrops there are, the more energy returned.
Now, back to why I favor Unisys Weather.
Unisys Weather provides most every type of weather map and chart one needs to be able to undertake “armchair weather forecasting.” Everyone could potentially turn into a TV weatherperson using Unisys Weather. Unisys Weather (and they do not pay me for their endorsement) provides a regular person enough information to track weather systems and atmospheric variables.
This map represents current weather. I know the time/date stamp says 0115Z 28 JAN 13, however “Z” stands for “Zulu” time, and Z-time is 5 hours ahead of Eastern Time Zone, and 6 hours ahead of Central Time. The map represents conditions at 8ET/7CT. All maps will be labeled such, unless the maps have been ‘doctored’ for mass-consumption, like Weather Channel maps.
Note the lack of station models, the lack of frontal boundaries, and all the other information except for areas of snow and rain. Obviously this map is excellent for finding out what is going on at the surface, around your location, or family, or upstream, or downstream.
We really are interested in weather at the surface. After all, that is where we come into direct physical contact with weather, right? To do a better job of predicting weather and planning in advance for changes in the weather, other maps might help.
Unisys offers maps which illustrate conditions “aloft,” over our heads where atmospheric conditions are also constantly in a state of flux, of change. Conditions above drive conditions at the surface. And, conditions at the surface, particularly heating and cooling, affect atmospheric conditions aloft.
This map illustrates atmospheric pressure at the 500mb level. The 500mb pressure maps communicate to weather forecasters the directions most weather systems will travel. In meteorology, the 500mb pressure level is important as winds at this level control the speed and course of all weather systems which affect the surface, for the most part.
Remember, air pressures decreases as one’s altitude above the surface increases, right? 500mb is roughly 19,000ft. However, pressure also varies with latitude. As one moves north to the poles (or south, whatever the case may be) the air becomes cooler. Cooler air is more dense. Being more dense, colder air is heavier and settles towards the Earth’s surface. The 500mb pressure level is closer to the Earth’s surface at the poles since the colder, denser air is closer to the Earth’s surface, and little air is aloft to generate air pressure. The white lines are “isobars,” connecting altitudes of equal pressure. If you begin in Texas, the highest altitude of the 500mb level is 5770m (about 17,400ft over your head). Each line is a drop in altitude of about 10m (60ft). If you were to graph this out in 3-dimensions, you would have a slope.
This “slope” is called a “pressure gradient.” Pressure gradients can tell us about wind speed. The larger the slope, the faster the wind speeds. Just like slopes of the ground, the steeper the slope the faster your sled moves, or your soap-box derby car. Atmospheric pressure gradients work the same way. The little arrows tell us wind speed and direction. The tip of the arrow points in the direction of movement. The arrow’s fletching tells us wind speed. Each “barb” is about 10knots; 1/2 a bard is 5 knots. A solid barb (triangle) is about 50knots. A knot is 1.15mph, so a 50 knot wind is about 57mph.
Commercial aircraft like to travel with the Jet Stream. The Jet Stream is a manifestation of the Rossby waves, which we will cover much later in the material. But, everyone has at least heard of the Jet Stream, I assume, and probably knows it is a very fast “stream of air” aloft.
Unisys Weather has these weather charts available, as well. These charts do not look much different than the 500mb charts, that is, until you look at the weather station symbols and the respective wind speed arrows. Examine the fletching on the arrows, especially in the blue zones near the United States / Canada border. If you look over the Upper Peninsular of Michigan, you can see upper air wind speeds over 100knots (115mph).
Now, a commercial airline pilot flying from Seattle, Washington to New York City might appreciate the extra boost. The pilot’s counterpart flying from New York City to Seattle would be less than amused. 300mb is roughly an altitude of 30,000ft (9360m). Remember, though, how cooler, denser air settles to Earth. Now, look at the map again. All of these lines connect points of pressure equal to 300mb. Over Texas, to reach the 300mb pressure aloft you would have to ascend to an altitude of 30,840ft. At Kansas City, Missouri, you would find the 300mb pressure level at 30,420ft. And, in the northern part of Canada’s Manitoba province, you would have to ascend to about 28,000ft to find the 300mb pressure level, thus illustrating how the Earth’s atmosphere is closer and denser toward the poles than at the Equator.
A few other web sites, like WeatherUnderground, provide similar maps. Unisys Weather maps are update at regular intervals. Each type of map is updated at a different interval, though. Surface maps are updated about every 3 hrs, for example, while upper air maps might be updated every 6-9-12 hrs.
This is an adaptation of a recent posting I made to my online Weather and Climate course. I use my Announcements to provide my own commentary in addition to what my students receive in the textbook, my homegrown weather and climate videos (posted to YouTube), and my own homegrown notes and exercises. I believe it helps provide a more personalized course. A few students have indicated my discussion has helped communicate the material in what I would term a “different” way, not a “better” way, as those are qualitative judgments which may not be true for all people.