Most of the current surface weather data that you have inspected so far has been displayed either on weather maps or as tabular data for a single observation time. Often times we would like to see how the weather has changed at particular location. A metgram, or more correctly, a meteorogram is a graphical means of displaying the time variations of one or more weather elements observed at a given location. A 24-hour plot of observed data for selected stations throughout the country could be obtained from the RealTime Weather Portal through the Metgrams for Selected Cities link. Meteorograms will be especially useful when we inspect and interpret such weather sequences as those associated with frontal passages.
The meteorogram is divided into four panels, each of which is used for plotting a different weather element with the base panel being essentially an x-y graph. All panels have a common horizontal axis that represents time in hours (in Z or UTC time), increasing to the right on the graph. The vertical axis is scaled according to the range in the variation of the particular weather element at that location.
The top panel of the meteorogram is designed to display the air temperature and dew point temperature (a measure of the atmospheric water vapor content) variations over the interval, using the units of degrees Fahrenheit. Since we can assume that the air temperature and dewpoint temperature are continuous in time we can connect the hourly observations with lines. A red line is drawn connecting the temperature data points to form a continuous thermograph for the temperature and a green line to form a continuous time sequence for the dewpoint.
The top of the second panel has a series of 24 circles that depict the hourly sky cover observations or the fraction of the local sky hemisphere being covered by clouds below 12,000 feet altitude. The cloud amount symbol conforms to the station model code as plotted on the surface analysis. (See the selected Weather Map Symbols.)
The lower portion of this panel contains the observed hourly wind data plotted in the same conventional format as appearing on the surface analysis. A wind shaft is oriented with the wind direction. Wind speeds are plotted with a series of wind barbs, to the nearest 5 knots. Each barb represents 10 knots and a half barb represents 5 knots.
Both the sky cover and wind information will be assumed to be discrete hourly observations. No attempt is made to connect these observations to form a continuous time trace of wind speed.
The bottom portion of the third panel would contain, when appropriate, standard symbols in green that are used to identify the current "weather", which includes precipitation and other significant weather phenomena as needed for aviation interests. The weather symbols correspond to the official station model code of symbols. (See the selected Weather Map Symbols.)
The top portion of the chart is a plot of the prevailing visibility in statute miles. A gold-colored line connects the hourly values.
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 time.
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 time intervals. Sequences of weather elements associated with cold frontal passages often differ from those associated with warm frontal passages.
Take time to study the variations of each weather element separately and then in conjunction with the other weather elements. Try to explain why these variations took place. With experience, you should be able to notice that various clues can be detected which help in your explanation.
Since a 24-hour interval is plotted, a diurnal variation in the elements may be apparent. Usually, the temperature will show a distinct diurnal cycle, with the highest values within an hour or two following local noon and the lowest values within the hour closest to local sunrise. To help you visualize this diurnal variation, you may want to ascertain the time of local solar noon on your plot as well as the times of sunrise and sunset. However, complications such as clouds, the direction and speed of the wind, may influence the timing of the diurnal temperature cycle or completely obliterate it. For example, a low cloud layer may elevate the nighttime temperature and suppress the daytime temperature. Strong winds may be responsible for transporting warm or cold air into the region, causing a modification of the anticipated diurnal temperature curve.