Weather Charts. Natural History Museum of Utah Nature Unleashed Stefan Brems

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1 Across the world, many different charts of different formats are used by different governments. These charts can be anything from a simple prognostic chart, used to convey weather forecasts in a simple to read visual manner to the much more complex Wind and Temperature charts used by meteorologists and pilots to determine current and forecast weather conditions at high altitudes. When used properly these charts can be the key to accurately determining the weather conditions in the near future. This Write-Up will provide a brief introduction to several common types of charts. Prognostic Charts To the untrained eye, this chart looks like a strange piece of modern art that an angry mathematician scribbled numbers on. However, this chart is an extremely important resource when evaluating the movement of weather fronts and pressure areas. Fronts Depicted on the chart are weather front combined into four categories; Warm Fronts, Cold Fronts, Stationary Fronts and Occluded Fronts. Warm fronts are depicted by red line with red semi-circles covering one edge. The front movement is indicated by the direction the semicircles are pointing. The front follows the Semi-Circles. Since the example above has the semi-circles on the top, the front would be indicated as moving up. Cold fronts are depicted as a blue line with blue triangles along one side. Like warm fronts, the direction in which the blue triangles are pointing dictates the direction of the cold front. Stationary fronts are frontal systems which have stalled and are no longer moving. These fronts are often responsible for rain/snow storms that never seem to end or clouds that never seem to leave. These fronts are depicted by a combination of the warm front redline with semicircles on one side and the cold front blueline with the triangles on the other. Occluded fronts are a strange combination of a warm and cold front with one overtaking the other while still moving forward. These fronts are depicted as a purple line with purple triangles and semi-circles on one side of the line. Pressure Areas Pressure Areas dictate where all weather fronts move. When high and low pressure areas are interacting with one another the Coriolis Effect prevents air from flowing directly from a high pressure

2 area to a low pressure area. Consequentially, air spins out of high pressure areas as it flow to low pressure areas. At high altitudes this spinning is quite predictable. As you approach the surface, objects such as mountains and tall building can force the air to flow in different patterns. Air flows out of high pressure areas clockwise and into low pressure areas clockwise. High pressure areas are depicted by a red H with an underlined set of either three or four numbers directly next to it. This set of numbers ex: 1006 depicts the atmospheric pressure at the ground in millibars. Weather in high pressure areas is often sunny and nice. Low pressure areas are depicted as a blue L with the same set of four numbers next to it. You can compare how powerful the pressure difference is by subtracting the low pressure digits from the high pressure. The higher the sum, the greater the pressure difference. Weather is low pressure areas can vary from pleasant to extremely stormy. Some of the most powerful storms on record have occurred in areas of extremely low pressure. Isobars are depicted as grey, looping, nonintersecting lines on the chart. These lines depict areas of equal pressure where the area inside one loop is the same the entire way around. These isobars surround pressure peaks and valleys depicted as the H and the L. As a rule of thumb, the farther apart the Isobars are, the less powerful the wind. The closer the Isobars are, the more powerful the wind. Troughs and Ridges depict areas of constant low or high pressure. This can be incredibly helpful to meteorologists in determining the future movement of weather patterns. Troughs are depicted as a dashed orange line. Ridges are depicted as a segmented arrow yellow line. Radar Charts Across the US, weather radar sites constantly scan the sky for precipitation. Radar chart are used to see the current location of different types of precipitation. When used with historical radar reflections (several hours old) a good projection of locations that will get precipitation can be made. Weather radar has been an indispensable part of the US weather reporting system since WWII when radar operators noticed weather cells could show up on their scopes and mask enemy aircraft inbound. Though the issue was rectified, weather radar was born. Between 1950 and 1980, weather radar made its way around the world opening new possibilities for weather reporting. Radar towers like this one in Figure 1 contain a radar dish which scans the skies starting at 0 degrees above the horizon and increasing the pitch up to several degrees above the horizon before returning to 0. This constant scanning helps track precipitation at different altitudes. Figure 1: Weather Radar atop at Tower

3 Charts Radar charts are very simple to read. Areas of little reflectivity (drizzle) show up as blue-greed on the chart, as the precipitation gets more intense, the color moves up the color scale from blue-green to green to yellow to orange to red to purple as the most intense. You can see this scale at the left hand side of Figure 2. Snow is shown as blue, white and pink. In Figure 2, you can see a very severe storm approaching populated northern Utah from the west. You can see the standard makeup of storms with intensity building and the most intense weather in the center of the cell. In this particular image, you can see bright yellow boxes which are Severe Thunderstorm Warnings issued by the National Weather Service. Figure 2: Severe storm activity over northern Utah Satellite Charts Where radar charts fall short, satellite charts pick up. Radar charts are only able to see physical precipitation, not clouds. Satellite charts, which are pictures taken from a geostationary satellite focused on the US are used to find the location and track of clouds only. After taken, these images are then cleaned by a computer which removes or represses terrain features leaving only clouds on the chart. Ion some rare occasions, after large snow storms, the satellite may pick up snow on the ground after a storm has passed. Figure 3: Satellite Chart of the US In figure three, you can see the continental United States as well as parts of southern Canada, northern Mexico and Cuba. At the top of the chart, you can see a scale ranging from 0 to 99. This scale allows a person to measure the density (opacity) of the clouds of a certain system. Turn your attention to Kansas; you can see on Figure 3 that Kansas is almost completely covered by white and light grey, meaning Kansas is completely covered by clouds. Notice off the coast of central California down to

4 central Baja, you can see fog rolling in off the ocean and colliding with the land. Since fog rarely passes far inland, the fog seems to cease directly at the coast. Satellite charts are a relatively new part of the weather reporting system in the US, and have become widely available for countries across the globe. Selected Questions for Gallery Interpreters 1. Do you recognize any of the weather maps shown? What is the most distinguishing feature of each? 2. Do you use the National Weather Service to pinpoint forecast your location? ( 3. How have radar images helped you decide the proper attire for certain days? 4. How have Prognostic Chart Frontal Symbols helped you decide weather you should wear long pants/shorts or long sleeved/short sleeved shirts? Images of Interest Figure 4: Early radar image of a tornado producing storm over the Minneapolis-St. Paul. 1965

5 Figure 5: Radar image of Superstorm Sandy, Note the severe snowfall in the center of the storm