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How To Read a Nomogram Chart

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How To Read a Nomogram Chart

Post by Stovepipe on 2011-12-23, 12:53 am

Just when I think I've figured out most of the weather charts, the nerdiest of all charts emerges: The Nomogram.

Here is the key:


TREND’s Predominant P-type Nomogram

The nomogram shows the distribution of p-type trends as a function of partial thickness values. Close examination of precipitation events over the past 30 years accounts for the boundaries on the nomogram separating the various p-type trend areas. Mid level thickness values increase from left to right along the x axis. Low level thickness values increase from bottom to top along the y-axis.

The last revision to the predominant p-type nomogram (winter 2006/2007) was focused on the central region of the nomogram where many events occur with weak thermal advection and/or surface based melting layers that can result in near freezing isothermal layers. This part of the nomogram requires the forecaster to be especially aware of the so-called secondary p-type factors that can become larger players for controlling precipitation types. These factors include evaporation, melting, precipitation amounts, and precipitation rates.

TREND’s 6 hourly predominant p-type categories include:

All snow
All snow or snow/sleet mix
Icing – measurable sleet with freezing rain
Icing – freezing rain mixed with trace frozen
Freezing rain or rain
Measurable snow with rain
Wintry mixtures

Changes to or mixing with other p-types are in part determined by noting which predominant p-type category the thickness values are trending toward. When the partial thickness values are located near an adjacent p-type category and are trending toward that adjacent category, forecasters should reflect this trend in their 6 hourly forecast of p-type.
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