Trajectories are aerial maps showing the path taken by an air
parcel. These estimates of air movement are important to better
understand the behavior and potential impact of air pollution.
Two types of trajectories are presented for each location:
forward and backward. Choose a location from the table below.
These trajectories were obtained from the National Oceanic and
Atmospheric Administration's Air
Resources Laboratory 
and present an estimate of air movement
based on one model. A very different picture of air movement can be
gained by using other models or by putting different information
into the same model. Compare these trajectories with those
available at CAPITA,
Ozone Transport Assessment Group .
Where Did the Air Come From and Where Is
It Going?
The maps available from this page illustrate estimates of the
general path air traveled over 48 hours to arrive at a particular
Texas location and where it is going immediately afterward. The
images are calculated every day for 2100 hours Zulu (Z) or
Coordinated Universal Time (UTC), which is 3 p.m. Central Standard
Time (or 2 p.m. Mountain Standard Time). They are posted to the
TCEQ's Internet Web server the following hour. Trajectories are
available for the past 30 days.
These estimates of air movement are important to better
understand the behavior of air pollution. The meteorological
dynamics that cause air to rise or fall, and that determine its
path can affect air quality by carrying air pollutants many miles
from their sources.
How to Read the Trajectory Maps
Each backward trajectory map shows an aerial (plan) view of the
path taken by an air parcel, and a vertical view of its movement at
different altitudes, before arriving at 164 feet (50 meters) above
the earth's surface at a particular location. Similarly, the
forward trajectories show the same information starting at 164 feet
for the ensuing 36 hours. The forward trajectories start at 2100Z
and end at 0900Z (3 a.m. Central Standard Time) the second calendar
day after the trajectory started.
The aerial view traces the path the air parcel followed over 48
hours (36 hours for forward trajectories). Tick marks and the
numbers 00, 06, 12 and 18 from the 24-hour clock are used along
each trajectory to indicate the position of the air parcel over the
48-hour or 36-hour period. The vertical view at the bottom of each
map shows the height of the air measured at these times by
corresponding tick marks.
The tick marks, except for the first and last marks, indicate
six-hour increments. The first tick mark along the trajectory from
the air's final destination marks the parcel of air's location
three hours prior to its arrival. The last tick mark on the
trajectory shows where it was three hours into its journey. The
height of the air is measured in hecto pascals (HPA). Eight hundred
and fifty HPA is about 5,000 feet.
How the Trajectories Were Created
The trajectories were calculated using the HYSPLIT
trajectory model written by Roland
Draxler. (For a discussion about one application of the model and
the algorithms used, see Draxler, R. R., 1996: Trajectory
Optimization for Balloon Flight Planning. Weather and
Forecasting, No.11, Pages 111-114.)
The input for the HYSPLIT model is the output from the eta
model, which is run at the National Centers for Environmental
Prediction (NCEP--formerly known as National Meteorological
Center). Trajectory and weather forecasting models have error.
Clearly, any error in the eta weather forecasting model is passed
along into the HYSPLIT trajectory model. Moreover, as the input
parameters to HYSPLIT vary, significantly different trajectories
are generated.
