Understanding and interpreting the trajectory (transport) charts:

A trajectory chart shows the pathway of movement, or transport, of air as it travels through the atmosphere. Unlike most weather charts which show weather conditions at one particular time, trajectory charts illustrate air movement over several days.

You can get a lot of information from trajectory charts. Most importantly, they tell you where the air is coming from or going to. Back trajectory charts, like the one you will use in this project, tell you where the air has been as it travels toward a particular location (the receptor). They also indicate the altitude of the air as it travels toward the receptor. If a trajectory passes over an area at low altitudes, it is more likely to collect pollution emissions from that area.

The chart below shows a sample back trajectory, along with notes about how it can be interpreted.

This back trajectory chart shows the movement of air toward a location in South Africa. Air arriving at the receptor was initially located to the north, but it traveled southward, turned toward the east, and arrived at the receptor from a southeasterly direction. Any pollution in the air could have come from any location along the trajectory path. Based on the chart at the bottom, as the air moved along its altitude varied between 0 and 500 meters above the ground. Air traveling at any altitude below 3000 meters can collect pollution emissions from the areas it passes over. However, air traveling very close to the ground has a better chance of collecting pollution emissions. In this case, the air probably got most of its pollution along the areas between the 4th and 10th triangle, as you count backwards from the receptor (the star).

Trajectory charts also give an indication of the speed of air movement. As air passes over different areas on its way to a receptor, it has a better chance of collecting pollution emissions from an area if it is traveling slowly. If the air is moving rapidly, it doesn’t stay in an area long enough to collect very much pollution. The chart below shows another sample back trajectory, with notes about how it can be interpreted.

This trajectory chart shows the movement of air towards a receptor in the Arctic Ocean north of Siberia. According to the back trajectory, the air traveled toward the southeast at first, then turned toward the northeast and eventually arrived at the receptor from the southeast. The altitude portion of the chart shows that the air traveling toward the receptor was between 200 and 900 meters above the ground. The altitude was close to 500 meters for all portions except most of the second day before arriving at the receptor, between the 3rd and 6th triangle, counting backwards from the receptor. The slowest wind speeds were found in the third day before the air arrived at the receptor, between the 7th and 12th triangles. This slow-moving air, which was also at relatively low altitudes, was likely to collect pollution emissions from the areas it passed over during this time.

Understanding and interpreting the cloud images:

It’s important to know whether air traveling to a receptor passed through clouds. If it did, then the chemical reactions that change pollutants into acids are likely to occur much faster than if there were no clouds along the air’s travel path.

Infrared cloud images tell you two important things: where the clouds are, and whether the cloud altitude is high or low. Lighter colors indicate the presence of clouds, and darker colors indicate clear skies. Within a cloudy area, brighter colors indicate high-altitude clouds and darker colors indicate low-altitude clouds. The chart below shows a sample infrared satellite image of clouds, along with notes about how it can be interpreted.

This image shows large areas of clouds in parts of the central and eastern United States, and clear skies in the west and south. Although the image shows a large geographical area, it still gives a lot of small-scale detail. For example, if you look carefully you can see that Wisconsin has clear skies throughout all portions of the state except for the extreme southeast and southwest corners. The image also shows areas of both low-altitude and high-altitude clouds. The states of Indiana and Ohio, for example, are completely covered by clouds. Looking carefully at the color in these two states, you can see that western Indiana and eastern Ohio are a bit darker, indicating low-altitude clouds. Eastern Indiana and western Ohio have high-altitude clouds.

Combining the trajectory and cloud information:

A cloud chart is like a snapshot of the atmosphere: it shows cloud conditions for a particular moment in time. A trajectory chart, on the other hand, shows air movement over several days. The trajectory chart you’ll use for this project shows air movement over a three-day period.

In order to combine the information you can get from trajectory and cloud charts, you’ll have to make some markings on the cloud charts, showing where the air was located at the time of that particular cloud chart. You can also mark up the trajectory chart. As an example, consider the following charts. The chart at the left shows a trajectory that arrived in southwestern Kansas. The top cloud chart shows cloud conditions at the time of trajectory arrival. The bottom cloud chart shows cloud conditions 24 hours earlier, when the air was located over central Nebraska. Click on the charts to see larger, marked-up versions.

Sample back trajectory chart, arriving at the receptor at 0000 UTC on March 15, 2004. Click on the chart to see a larger, marked-up version. In the altitude graph at the bottom of the trajectory chart, the units of altitude are “meters AGL”, which means “meters above ground level”.
Infrared satellite image showing clouds at 0000 UTC on March 15, 2004. Click on the chart to see a larger, marked-up version.	Infrared satellite image showing clouds at 0000 UTC on March 14, 2004. Click on the chart to see a larger, marked-up version.

Determining the population (or population density) of a region:

Knowing the population of areas underneath the trajectory is important because higher-population areas generally emit more pollutants than lower-population areas. In this way, you can use population information as a substitute for pollution emissions.

The following chart shows population density for Michigan. The darker colors indicate greater population density (more people per square mile), and the lighter colors show smaller population density. If a trajectory passed over the southeastern section of Michigan, it would probably collect a lot of pollution emissions.

Analysis checklist

Questions to consider	Guidelines for interpretation
What areas did the trajectory pass over?	Only areas along the trajectory can contribute pollution to the air that travels to the receptor.
Did the trajectory passage over these areas occur at high altitudes or low altitudes?	Air traveling closer to the ground has a greater chance of collecting pollution emissions from cars, smokestacks, etc.
Was the air moving quickly or slowly as it passed over these areas?	Slow-moving air spends more time as it travels over a particular area, giving it more time to collect pollution.
Was it cloudy in the areas of trajectory passage at the time the trajectory passed over?	Cloudy air turns pollutants into acids faster than cloud-free air.
Is the population of the areas of trajectory passage sufficiently high to assume that these areas contributed pollution to the air?	Areas with higher population generally emit more pollution into the atmosphere than low-population areas.