Sunday, May 15, 2011

How to read a surface chart, Happy hurricane season, 3rd AOI for the season

And the day we have all been waiting for, it is May 15, 2011, the start of the 2011 Pacific hurricane season. Now, today we'll tell you how to read a surface chart, but before we get started my readers can guess where I got my information from (Hint: It is one of the link on the top of this page). The winner will get a free invitation to the ##hurricanes IRC channel. 

Let' get started:

What is a Surface Chart? ( Click here to see the current Eastern Pacific Surface Chart) It is a map of weather conditions and forecasts for the next period. According to a Surface Chart is as follows:
"An analyzed synoptic chart of surface weather observations; essentially, a surface chart shows the distribution of sea-level pressure (therefore, the positions of highs, lows, ridges, and troughs) and the location and nature of fronts and air masses, plus the symbols of occurring weather phenomena, analysis of pressure tendency (isobars), and indications of the movement of pressure systems and fronts. Also known as sea-level chart; sea-level-pressure chart; surface map. "
High Pressure SymbolLow Pressure symbolThe most noticeable features on a Surface Chart are the High and Low pressure systems. Indicated by large letters, and when in color Highs are blue and Lows are red. In the northern hemisphere High pressure systems orbit clockwise, Low pressure systems orbit counter-clockwise. Movement of a Low pressure system is indicated by an arrow indicating direction of movement and a Low Pressure end point, located at the forecast position of the system at the end of the forecast period. Movement of a High is also indicated with an arrow of direction of movement and a High pressure end point. The direction of movement arrow is sometimes omitted if space doesn't allow. If the system is not moving "STATIONARY" is usually printed next to the system. The current barometric pressure reading for the system is indicated in millibars located usually above the symbol and underlined, (i.e. 1006). The future barometric pressure can be indicated by an underlined two digit number, near the future location of the system (i.e. 04 ) If the system is new, rapidly strengthening or this can be indicated with the written words such as "DISSIPATING, NEW or RAPIDLY STRENGTHENING" Hurricane strength conditions are indicated with "XX".
Isobars and Wind on Surface ChartsThe reason our Eastern Pacific Hurricane usually move off into the Pacific is this counter-clockwise rotation of arrow SWthese extreme Low pressures. Like a rolling tire, the rotating action drives them to the southwest. Other environmental factors push them more to the north, and as the northern hemisphere cools the jet stream pushes them back toward the northeast and sometimes into Baja after late July.
Isobars are the long lines forming unusual shapes on a surface chart. and are directly associated with Highs and Lows and relative barometric pressures. indicated in solid or dotted lines, brown on color charts, these are rough demarcations of barometric pressure, taken from reporting stations. The closer together the lines, the greater the wind in that area. Wind does not blow parallel to these lines, because of the earth's rotation, but slightly off axis to a point of lower pressure. In the case of intense winds "DEVELOPING GALE" or "GALE" will be written on the chart
Type of fronts on Surface ChartsFronts are a very visible and important feature too, although not usually for hurricane weather. In a color chart cold fronts are indicated in blue, Warm fronts in red, stationary fronts in red and blue and occluded fronts in purple. Fronts are a line, indicating position of the front. Warm Fronts have semi-circles, Cold Fronts triangles and stationary fronts have both. The side of the line on which the added symbols exist indicate the direction of flow from the front. Stationary fronts have both semi-circles and triangle on opposing sides of the line, indicating that the front isn't moving. Occluded front is where cold air has overtaken a warm front, using both triangles and semi-circles in purple, indicating the movement of the front."

We have our 3rd AOI of the season. Does not looks like much, but an organized burst of convection is present. The air is too dry for significant development, but it is at east something to watch. It is located near 111 W, 9 N.

Our next post will be this afternoon.

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