Morning Dew

I woke up this morning for my 7 am flight to a lovely AWOS report of less than 1/4 mile, and 100 overcast in fog. I called Rob and told him the fog would burn off around 10 per the forecast and that we should fly later once it cleared up. Bob Kierein, the flight service guy, and the weather channel all declared that the fog would clear out around 10 once it warmed up. It never did.

The formation for fog is mainly due to a close dew point/temperature spread. Take a look at the Dulles metars from this morning. You will see that the temp and dewpoint started off eaqual but never had a chance to break apart. After the rain we had the other day and the light winds as this cold front approaches the air is laden with moisture.

KIAD 191605Z 18003KT 2 1/2SM BR OVC002 18/16 A2986 RMK AO2
KIAD 191552Z 00000KT 1 3/4SM BR OVC002 17/16 A2986 RMK AO2 SLP110 VIS W 3/4 T01720161
KIAD 191512Z 12003KT 1 1/2SM BR OVC002 17/16 A2986 RMK AO2 VIS W 3/4
KIAD 191452Z 00000KT 1SM BR OVC002 17/16 A2988 RMK AO2 SLP115 VIS W 1/2 T01670161 50001
KIAD 191352Z 00000KT 1SM BR VV002 17/16 A2988 RMK AO2 SLP116 VIS W 1/2 T01670161KIAD 191340Z 12003KT 1SM BR VV002 16/16 A2988 RMK AO2 VIS W 1/2
KIAD 191311Z 00000KT 3/4SM BR VV002 16/16 A2988 RMK AO2
KIAD 191310Z 00000KT 1 1/4SM BR VV002 16/16 A2988 RMK AO2 VIS 1/4 WEST
KIAD 191255Z 00000KT 1/4SM FG VV001 16/16 A2988 RMK AO2
KIAD 191252Z 00000KT 0SM FG VV000 16/16 A2988 RMK AO2 SLP115 T01610156
KIAD 191152Z 00000KT 0SM FG VV000 16/15 A2987 RMK AO2 SLP114 T01560150 10156 20128 53003 $
KIAD 191052Z 32003KT 0SM FG VV000 16/15 A2987 RMK AO2 SLP112 T01560150 $

The problem with this weather is that only the most sophisticated airliners can make it into the airport. Visibility is the controlling factor in shooting an instrument approach in the part 121 world. Larger airports with category 2 or cat 3 approach systems have RVR transmissometers installed for particular runways. RVR is runway visual range. It measures how far you can see when looking straight down the runway. There are usually 3 RVR sensors--touchdown, midfield and rollout. Only two need be operational before the required weather mins go up. Today was one of those days where the RVR was right on the edge. 1/4 mile vis is equal to about 1200 rvr. Imagine landing at 130 knots only being able to see 1200 ft in front of you. And then best part is, if you are inside the final approach fix and the rvr goes below mins you are still authorized to continue on down and take a look and land if possible.

Next time you are in the back of an airliner and you dont see the ground until you are over the runway, tell him/her good work. Your flight crew had their work cut out for them. Here is a wikipedia definition of RVR.

From Aeronautical Information Manual 7-1-16
There are currently two configurations of RVR in the NAS commonly identified as Taskers and New Generation RVR. The Taskers are the existing configuration which uses transmissometer technology. The New Generation RVRs were deployed in November 1994 and use forward scatter technology. The New Generation RVRs are currently being deployed in the NAS to replace the existing Taskers.
RVR values are measured by transmissometers mounted on 14-foot towers along the runway. A full RVR system consists of:
Transmissometer projector and related items.
Transmissometer receiver (detector) and related items.
Analogue recorder.
Signal data converter and related items.
Remote digital or remote display programmer.
The transmissometer projector and receiver are mounted on towers 250 feet apart. A known intensity of light is emitted from the projector and is measured by the receiver. Any obscuring matter such as rain, snow, dust, fog, haze or smoke reduces the light intensity arriving at the receiver. The resultant intensity measurement is then converted to an RVR value by the signal data converter. These values are displayed by readout equipment in the associated air traffic facility and updated approximately once every minute for controller issuance to pilots.
The signal data converter receives information on the high intensity runway edge light setting in use (step 3, 4, or 5); transmission values from the transmissometer and the sensing of day or night conditions. From the three data sources, the system will compute appropriate RVR values.
An RVR transmissometer established on a 250 foot baseline provides digital readouts to a minimum of 600 feet, which are displayed in 200 foot increments to 3,000 feet and in 500 foot increments from 3,000 feet to a maximum value of 6,000 feet.
RVR values for Category IIIa operations extend down to 700 feet RVR; however, only 600 and 800 feet are reportable RVR increments. The 800 RVR reportable value covers a range of 701 feet to 900 feet and is therefore a valid minimum indication of Category IIIa operations.
Approach categories with the corresponding minimum RVR values.
Nonprecision 2,400 feet
Category I 1,800 feet
Category II 1,200 feet
Category IIIa 700 feet
Category IIIb 150 feet
Category IIIc 0 feet
Ten minute maximum and minimum RVR values for the designated RVR runway are reported in the body of the aviation weather report when the prevailing visibility is less than one mile and/or the RVR is 6,000 feet or less. ATCTs report RVR when the prevailing visibility is 1 mile or less and/or the RVR is 6,000 feet or less.
Details on the requirements for the operational use of RVR are contained in FAA AC 97-1, "Runway Visual Range (RVR)." Pilots are responsible for compliance with minimums prescribed for their class of operations in the appropriate CFRs and/or operations specifications.
RVR values are also measured by forward scatter meters mounted on 14-foot frangible fiberglass poles. A full RVR system consists of:
Forward scatter meter with a transmitter, receiver and associated items.
A runway light intensity monitor (RLIM).
An ambient light sensor (ALS).
A data processor unit (DPU).
Controller display (CD).
The forward scatter meter is mounted on a 14-foot frangible pole. Infrared light is emitted from the transmitter and received by the receiver. Any obscuring matter such as rain, snow, dust, fog, haze or smoke increases the amount of scattered light reaching the receiver. The resulting measurement along with inputs from the runway light intensity monitor and the ambient light sensor are forwarded to the DPU which calculates the proper RVR value. The RVR values are displayed locally and remotely on controller displays.
The runway light intensity monitors both the runway edge and centerline light step settings (steps 1 through 5). Centerline light step settings are used for CAT IIIb operations. Edge Light step settings are used for CAT I, II, and IIIa operations.
New Generation RVRs can measure and display RVR values down to the lowest limits of Category IIIb operations (150 feet RVR). RVR values are displayed in 100 feet increments and are reported as follows:
100-feet increments for products below 800 feet.
200-feet increments for products between 800 feet and 3,000 feet.
500-feet increments for products between 3,000 feet and 6,500 feet.
25-meter increments for products below 150 meters.
50-meter increments for products between 150 meters and 800 meters.
100-meter increments for products between 800 meters and 1,200 meters.
200-meter increments for products between 1,200 meters and 2,000 meters.
Retrieved from "http://en.wikipedia.org/wiki/Runway_visual_range"