D-values are used by pilots in planning pressure pattern flights. However, because of the size of pressure systems and the high speed of air-craft, as compared to wind speeds, pressure pattern flying is impractical for distances less than 1,000 miles. Today, most weather offices provide computer-generated flight plans that include d-values. You should, however, be familiar with d-values and know how they are computed.  

A d-value is simply the departure from the height of a pressure surface. It is calculated by the formula

For example, a radiosonde indicates the actual height of the 700-millibar level to be 2,990 meters (9,809 feet), while the standard height of the 700-millibar surface is 3,010 meters (9,875 feet). First, the height in meters was converted into feet by using table 6-1-7. Plugging these values into the formula, we find that D equals 66 feet, as follows:

The minus sign means the actual height is lower than standard, while the difference of 66 feet tells us how much lower it is than standard.

The values may also be calculated from an upper air sounding as reported in meters, then the difference converted into feet.

To a pilot, the parameter d represents the difference between an aircrafts pressure altimeter reading and the actual altitude of the aircraft as shown on the aircrafts radar altimeter.

NOTE: Pressure altimeters are calibrated to read "0" when the air pressure is 29.92 inches of mercury. Radar altimeters are calibrated to read height-above-the-ground.

Since 29.92 Hg represents the standard atmospheric pressure at sea level, a pressure altimeter indicates the height of an aircraft in reference to sea level. Prior to takeoff, pilots set the pressure altimeter by inputting the airfields current altimeter reading (the indicated altitude is the height of the airfield, as well as that of the aircraft, above sea level). Radar altimeters are calibrated to read "0" when aircraft are on the ground. Therefore, if an airfield is 65 feet above sea level, the pressure altimeter should read 65 feet, and the radar altimeter, zero.

How does all this relate to d-values? Since all aircraft are required to update their altimeter settings when flying below 18,000 feet MSL, d-values are of little use in this range overland.

However, aircraft flying above 18,000 feet MSL or over ocean areas use Standard Sea Level Pressure (29.92 inches) for their altimeter setting. In most cases, these aircraft will fly their assigned flight level based on the pressure altimeter, regardless of what the radar altimeter shows, since use of the pressure altimeter is the standard. Not all aircraft have a radar altimeter. The d-value will give them the appropriate information to calculate their actual flight altitude if desired. More importantly, it is used by jet aircraft flight engineers who need the information to calculate fuel-burn rates. Fuel-burn tables are calculated for the various flight levels assuming a standard atmosphere. The d-value supplies an adjustment factor to compute more accurate fuel-use predictions. You will most often be asked to provide d-values for large jet aircraft flying at the higher altitudes.

For meteorological flight-planning, d-values are normally computed for standard levels within the atmosphere. For other levels, the pressure-altitude curve on the Skew T, Log P Diagram can be used to determine the height. The U.S. Standard Atmosphere heights are listed in table 6-1-6.

The actual heights of constant-pressure surfaces are determined from a radiosonde sounding. We will use the following sounding to compute a few d-values:

Remember, with the exception of the 1,000-millibar level, the heights of constant-pressure surfaces encoded in theradiosonde code do not contain all the digits. Digits are added as follows:

Prefix a "2 or "3 to the height of the 700-millibar level. A "2" is prefixed if the first digit is "5" or greater, while a "3" is prefixed if the first digit is less than "5".

The procedure for computing the d-value for a constant pressure surface is as follows: 

1. Find the encoded height of the constant-pressure level.

2. Add the proper prefix and/or suffix to the encoded value.

3. Convert the height value in meters into feet. (Use conversion table 6-1-7.)

4. Find the standard height of the constant-pressure level.

5. Use the formula or determine the difference between the actual height and the standard height. If the actual height is higher than the standard height, prefix the value with a plus (+) sign. If the actual height is lower than the stan-dard height, prefix the difference with a minus () sign.

Using the following steps, we can now compute the d-value for the 850-millibar level found in the above sounding.

1. The height of the 850-millibar level is encoded as 490.

2. To get the actual height of the level, we must prefix the encoded value with a "1." The actual height is 1,490 meters.

3. 1,490 meters converts to 4,889 feet from the table.

4. The standard height of the 850-millibar level is 4,781 feet.

5. The difference between the actual height and the standard height is 108 feet. Because the actual height is greater than the height of the standard level, the d-value is +108 feet.

Learning Objective: Identify the three types of condensation trails, and recognize how they form.

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