From: Subject: Monday Weather Topic in Greater Depth Date: Thu, 22 Oct 2009 10:28:05 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/online/archive/course/09_fall/f09w05m_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Monday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Monday, 5 October = 2009=20

FOLLOWING THE WEATHER WITH A METEOROGRAM

Most of the current surface weather data that you have inspected so = far has=20 been displayed either on weather maps or as tabular data for a single=20 observation time. Often times we would like to see how the weather has = changed=20 at particular location. A metgram, or more correctly, = a=20 meteorogram is a graph ical means of displaying the time = variations of=20 one or more weather elements observed at a given location. A 24-hour = plot of=20 observed data for selected stations throughout the country could be = obtained=20 from the AMS Weather Studies website through the Metgrams for = Selected=20 Cities link. Meteorograms will be especially useful when we inspect and=20 interpret such weather sequences as those associated with frontal = passages.

COMPONENTS OF THE METEOROGRAM

The meteorogram is divided into four panels, each of which is used = for=20 plotting a different weather element with the base panel being = essentially an=20 x-y graph. All panels have a common horizontal axis that represents time = in=20 hours (in Z or UTC time), increasing to the right on the graph. The = vertical=20 axis is scaled according to the range in the variation of the particular = weather=20 element at that location.

TEMPERATURE & DEWPOINT

The top panel of the meteorogram is designed to display the air = temperature=20 and dewpoint temperature (a measure of the atmospheric water vapor = content)=20 variations over the interval, using the units of degrees Fahrenheit. = Since we=20 can assume that the air temperature and dewpoint temperature are = continuous in=20 time we can connect the hourly observations with lines. A red line is = drawn=20 connecting the temperature data points to form a continuous thermograph = for the=20 temperature and a green line to form a continuous time sequence for the=20 dewpoint.

SKY COVER & WIND DATA

The top of the second panel has a series of 24 circles that depict = the hourly=20 sky cover observations or the fraction of the local sky = hemisphere being=20 covered by clouds below 12,000 feet altitude. The cloud amount symbol = conforms=20 to the station model code as plotted on the surface analysis. (See the = selected=20 Weather Map Symbols.)

The lower portion of this panel contains the observed hourly wind = data=20 plotted in the same conventional format as appearing on the surface = analysis. A=20 wind shaft is oriented with the wind direction. Wind speeds are plotted = with a=20 series of wind barbs, to the nearest 5 knots. Each barb represents 10 = knots and=20 a half barb represents 5 knots.

Both the sky cover and wind information will be assumed to be = discrete hourly=20 observations. No attempt is made to connect these observations to form a = continuous time trace of wind speed.

WEATHER & VISIBILITY

The bottom portion of the third panel would contain, when = appropriate,=20 standard symbols in green that are used to identify the current = "weather", which=20 includes precipitation and other significant weather phenomena as needed = for=20 aviation interests. The weather symbols correspond to the official = station model=20 code of symbols. (See the selected Weather Map Symbols.)

The top portion of the chart is a plot of the prevailing visibility = in=20 statute miles. A gold colored line connects the hourly values.

AIR PRESSURE

The bottom panel is a plot of the sea-level corrected air pressure in = millibars (mb). A blue line is used to connect the hourly data to form a = barogram, since the barometric pressure trace is assumed to be = continuous with=20 time.

INTERPRETING THE METEOROGRAM

The meteorograms are especially useful to see how the various weather = elements change, especially if a front were passing the station. Frontal = passages may cause rapid changes in several of the weather elements over = short=20 time intervals. Sequences of weather elements associated with cold = frontal=20 passages often differ from those associated with warm frontal passages. =

Take time to study the variations of each weather element separately = and then=20 in conjunction with the other weather elements. Try to explain why these = variations took place. With experience, you should be able to notice = that=20 various clues can be detected which help in your explanation.

Since a 24-hour interval is plotted, a diurnal variation in the = elements may=20 be apparent. Usually, the temperature will show a distinct diurnal = cycle, with=20 the highest values within an hour or two following local noon and the = lowest=20 values within the hour closest to local sunrise. To help you visualize = this=20 diurnal variation, you may want to ascertain the time of local solar = noon on=20 your plot as well as the times of sunrise and sunset. However, = complications=20 such as clouds, the direction and speed of the wind, may influence the = timing of=20 the diurnal temperature cycle or completely obliterate it. For example, = a low=20 cloud layer may elevate the nighttime temperature and suppress the = daytime=20 temperature. Strong winds may be responsible for transporting warm or = cold air=20 into the region, causing a modification of the anticipated diurnal = temperature=20 curve.

Return to the Monday=20 Daily Weather Summary

Prepared by Edward J. Hopkins, Ph.D., email mailto:hopkins@meteor.wisc.edu
=A9=20 Copyright, 2009, The American Meteorological Society. =