|Low-level Stratus and Fog Development|
in Houston, Texas
An endless blanket of grey. No end in sight. No edges. No boundaries. A thick mist. Nothing is
visible. Like a scene from a horror movie. The sun explodes from the horizon. Its rays beam to
the Earth-burn through the mist, break through the blanket of grey. Another day in Houston, Texas
and along the Texas Gulf Coast has arrived.
Stratus finds its root meaning from the Latin stratum meaning to stretch out. From Oxford's Dictionary
of Weather, stratus is a "predominantly grey, low-level layer cloud that is relatively featureless and
has a fairly uniform base." Fog is "a visible suspension of water droplets in the atmosphere near
the surface." Therefore, fog is merely a stratus cloud deck with its base at the surface.
There are several types of stratus cloud decks. One of these is the basic stratus cloud, low and grey with
uniform bases and tops, much like a quilt. Stratus can also combine with cumulus to produce a nearly
uniform cloud with a somewhat lumpy appearance. Nimbostratus is a dark, grey cloud from which
continuous stratiform precipitation (not associated with thunderstorms) originates. Stratus
can also be viewed in the middle layers of the troposphere as altostratus and in the upper
levels as cirrostratus, both of which produce a blanket appearance across the sky. This
article will focus upon low-level stratus.
There are also several types of fog development. Radiation fog develops during a clear night when an
inversion develops near the surface producing a shallow area of cooler air near the surface and
warmer air aloft. The cooler air near the surface provides an environment conducive to saturation
and therefore fog development. Advection fog develops when warm air moves over cold air, a
condition that occurs frequently in the Houston metropolitan area. This type of fog will be
the focus within this article. Other types of fog include upslope fog, found in mountainous
regions whereas air flows upward until it cools and condenses into fog; and steam fog,
which forms when cold air moves over warm water and condenses with the addition of moisture
from the rising warm air from below.
Several atmospheric conditions produce stratus and fog along the Northeast Texas Gulf Coast. Frontal
boundaries typically lose jet support once they become positioned across the Gulf States. The jet
stream becomes parallel to the frontal boundary and no longer is capable of being a catalyst. Therefore,
a cold front transitions to a quasi-stationary front. With continued convergence along the boundary
and residual moisture, stratus and fog often develop and linger in the region for several days
until the front slowly drifts into the Gulf or frontolysizes across the southern United States.
Post-frontal fog often develops as frontal boundaries become stationary across the northern Gulf. If
the boundary moves through Houston, cold air advection in conjunction with a dampened surface (from
stratiform precipitation) will produce post-frontal fog.
The Bermuda High often positions itself over the eastern Gulf or western Atlantic Ocean. With a
clockwise flow over the Gulf, Houston receives onshore flow due to southeasterly surface winds. This
constant feed of low-level moisture sets the stage for low-level stratus as well as advection fog
development. This pattern is especially true during the fall and early winter months whereas
the Gulf waters remain warm and coastal areas begin to cool. With onshore flow, the warmer
air (relative to the land surface) is advected over the coastal regions. As the air cools
over the land surface, it condenses and creates low level stratus and fog in the Houston
During the spring, the low level jet will develop over the eastern Texas and western Louisiana
area due to strong flow around the western periphery of the Bermuda High. The low-level jet
forms above an inversion layer whereas lighter winds slowed by earth's friction are capped.
With westerly winds funneling cool, dry air from the Rockies to the Mississippi Valley and
the Bermuda High pumping warm, moist air from the Gulf into the Southeast United States,
a strong pressure gradient develops at the meeting point of these two air masses, thereby
creating the low-level jet. This strong influx of warm, moist air often yields low stratus
across eastern Texas to include Houston, particularly during the spring. This stratus
develops during the overnight and early morning hours and often lasts through the afternoon.
Low-level stratus and fog development are detrimental to aviation and marine industry operations. As
an aviation meteorologist for Air Routing International, I closely monitor low ceilings and
visibility restrictions due to fog as both can determine whether or not our customers (private
commercial aircraft) can take-off from their departure airport. As an Air Force aviation weather
forecaster, I observed flights being delayed, altered, and even canceled due to low cloud decks
and restricted visibility, inhibiting the pilots from completing their missions.
Low-level stratus and fog development are two of the most difficult weather phenomena for which to
forecast. This difficulty is due to model variations. Although forecast models may depict a
chance of stratus development, often times the levels are much higher than what is actually
observed or the timing is off by several hours. This timing issue is particularly evident when
a surface front stalls along the Gulf Coast. Because these fronts are quasi-stationary, they
often meander with diurnal changes. A strong low-level flow may create a warm sector whereas
the front will move inland. However, a boost of cooler air from the north-northwest overnight
may move the frontal boundary into the Gulf. Forecasting stratus levels and fog restrictions
on visibility is extremely valuable to a pilot since he or she has certain criteria in
which he or she can take-off and land-dependent upon his or her experience level and
company flying policies.
Stratus and fog along the Gulf Coast have long been a topic of concern. For decades, Galveston has
been the major seaport for the southern United States. Houston is a hub for two major airlines and
also serves as host to a plethora of private companies. "Airplane accidents due to low ceilings
and visibility claim more lives per year than any other cause, including engine trouble. Low
clouds and fog are also a major factor in airport delays. These delays cost the airlines close
to $24 million per year." (Kyle and Levert) Therefore, research and further study have been
accomplished on the matter to better serve the forecaster.
The United States Air Force conducted and completed the Lyle Study in January 1969. The study utilized
the pressure gradient rather than the wind velocities to forecast stratus for Randolph Air Force Base
and surrounding areas. The accuracy was uncanny. The 00 Zulu model run pressure data yielded a
nearly ninety percent accuracy at forecasting the presence of a stratus deck. And, the pressure
gradient data forecasted a ceiling within one hour and fifteen minutes as well as within three
hundred feet of the base over sixty percent of the time. The study presents the forecaster with
a listing of questions about the pressure gradients across the southern Texas area for the
aviation forecaster. I personally utilized this study when forecasting for Randolph and
Lackland Air Force Bases. The answers provided to these questions were compiled to produce
a likelihood of stratus across the San Antonio area. (Lyle)
The National Weather Service in Houston conducted a study of fog development along the upper Texas
Gulf Coast. Their analysis concluded that wind velocities and relative humidity were directly
related to fog development. At least seventy percent relative humidity was observed in cases
of fog development. Low level wind speeds were higher on days with fog, signifying the presence
of the low-level jet. According to the study, an aviation forecaster should forecast low
ceilings and fog development in the following atmospheric conditions: low-level relative humidity
of at least seventy percent and easterly to southeasterly winds (onshore flow) greater than
five knots with increasing speeds and veering direction aloft. (Kyle and Levert)
Meteorology is a young science. With its origin in the World Wars and model dependency upon
recent technological advances, our capabilities of forecasting complex phenomena like low-level
stratus and fog are limited. Future research will need to focus on strengths of pressure gradients
as they give an indication of the strength of the low-level jet and general low-level flow from
the Gulf. The Lyle study depicted that using pressure gradients is a sound means of understanding
the strength of low-level flow and therefore moisture advection from the Gulf. More research
is also required on modeling stationary boundaries and their slight but impeding movement once
positioned along the Gulf Coast. With so many inconsistencies in the separate forecast models
available, research should focus on utilizing the correct biases and disregarding the
erroneous biases, a process known as ensemble forecasting.
Dunlop, Storm. Oxford Dictionary of Weather. New York: Oxford University Press, 2001.
Kyle, Bryan, and Carolyn Levert. "A Study of Fog along the Upper Texas Coast."
Online Posting. National Weather Service Office, Houston/Galveston, Texas. 01 May 1998.
Lyle, Richard W. "Lyle Stratus Study for Forecasting Gulf Stratus at Randolph Air Force Base."
24th Weather Squadron, Detachment 10, Randolph Air Force Base. January 1969.