Guide to the Meteograms

One very useful way to look at a time series of meteorological data at a particular point is by ploting the data in a "meteogram." In a meteogram, time progresses from left to right across the figure. One or more quantites may be plotted, either as line graphs, bar graphs, symbols, numbers, shading, etc. Using a meteogram, one can get a feel of how conditions change and evolve at a stationary point (termed the "Eulerian" perspective). This is handy because most people are rather stationary with respect to the atmosphere (unless you do a lot of traveling), so a person's natural perspective is Eulerian. Weather maps, or satellite "movies" are good for giving an overall perspective of the "big picture", but it can be difficult to tell what will transpire in your particular locality.

Keep in mind that like all the other model forecast information provided by COLA, these data are "as is." They are straight from the computer models at the National Centers for Environmental Research (NCEP) of the National Weather Service. No interpretation, corrections, or other objective or subjective changes have been made. These are not the "official" forecasts, though in most cases they should resemble them rather closely since the official forecasts for your region are based on these models to various extents.

Forecast meteograms are given for selected US cities. In fact, the the forecasts are based on the grid point nearest the city in question. The models cannot directly discern any features of the terrain or atmosphere smaller than the grid resolution (although certain aspects such as the small-scale roughness of the land surface, and the sub-grid scale distribution of thunderstorms are represented indirectly by using parameterizations). Thus these models may not do a good job of forecasting very localized weather such as might be associated with things like mountainous terrain, or sea breezes.

Tropospheric Time-Height Cross Section

Conditions for the lower troposphere (up to 500 millibars) are shown in profile. The model data we receive is interpolated down to 1000 millibars, but over high terrain only data that is near or above ground level is displayed. Data below ground level has no physical meaning and is omitted.

1000-500 mb Thickness

Thickness is the vertical distance between two pressure levels. In general it is true that the distance will be a function of the density of the air between the two pressure levels, which is itself directly related to the temperature of the air. Thus, thickness is a good indicator of the mean temperature in the layer of atmosphere between the two levels -- greater thickness = warmer air.

Stability Indices

The stability indices are measures of the potential for strong or severe weather. The indices shown here are the Lifted Index (LI) and the Convective Available Potential Energy (CAPE).

Sea Level Pressure

Sea level pressure (SLP) is the surface pressure interpolated down to sea level from the altitude of the grid box of the model. This corresponds to the barometric pressure one hears reported on local radio or TV weather reports.

10-Meter Winds

10-Meter Winds (about 33 feet above the ground) correspond to typically measured winds at weather stations.

2-meter Temperatures

Air temperature and dew point temperature are given at the 2-meter level (6½ feet above ground). The dew point temperature is the temperature that a sample of air would have if it was cooled (at constant pressure) until it reached saturation. The dew point temperature is an alternative way to describe the amount of moisture or humidity in the air. If the dew-point temperature is close to the air temperature, the relative humidity is high, and if the dew point is well below the air temperature, the relative humidity is low. One technique for forecasting overnight low temperature is to look at the daytime dew point: if no fronts are expected to come through, tonight's low temperature will not get much below today's dew point.

2-Meter Reletive Humidity

Cloud Cover

This panel has a blue background to show the cloud-free areas. The panel is divided into three horizontal layers for the display of low, middle, and high cloud cover, which are drawn as white bars. If the white bar covers the full height of its layer, that is 100% cloudiness. The white bars have no gap between them to better simulate the appearance of cloudiness in the panel.


The bar graph at the bottom of the figure indicates predicted precipitation types and amounts.