Weather Stations 101

Have you ever wondered what a drag racing weather station does, how it works and how it helps bracket racers win more rounds?

Every drag racing weather station basically works like this:

1. It samples and displays the current weather conditions
2. It allows you to record your runs from your time slip along with the weather conditions at the time of the run
3. It predicts your ET based upon the current weather conditions and your previously recorded runs

Sampling and Displaying the Current Weather Conditions

The most common weather conditions sampled are:

• Temperature - in degrees Fahrenheit
• Barometric Pressure - in inches of mercury, inHg
• Relative Humidity - as a percentage

These three values are combined to calculate Density Altitude, or DA. Density altitude is a theoretical figure, calculated in feet, and used to illustrate changes in the weather conditions. For example, a temperature of 73.5ºF, relative humidity of 68.9% and a barometric pressure of 29.97" equates to a density altitude of approximately 1693 FT.

As a general rule, the higher the DA the slower your ET:

• Temperature increase = DA increase
• Barometric pressure decrease = DA increase
• Humidity increase = DA increase

Recording Your Runs

Now that you can see how the current weather conditions are sampled and displayed, the next step is to enter your runs into the weather station.

Each weather station can usually be configured to store runs from different tracks and for different vehicles, often referred to as databases. For example, the Eclipse weather station from Altronics can have up to 10 databases and store 400 runs in total. Databases should be created using the following guidelines:

• A different database for each track you run at
• A different database for each vehicle that you run

For example, if you run two cars and race at Sydney Dragway and Willowbank Raceway you would create 4 databases:

• Sydney Dragway - Car 1
• Sydney Dragway - Car 2
• Willowbank Raceway - Car 1
• Willowbank Raceway - Car 2

Most weather stations have the ability to hold the current weather conditions for use when entering a run. This is very important so you can get an accurate reading as close as possible to the time of your run. Often you will see crew members on the start line pressing buttons on a weather station - they are most likely holding the current conditions for entry once they get back to the pits.

Once you are back in the pits it is time to enter the run into the weather station. For the Eclipse, the values entered consist of the following:

• DA - current, held or manual. Here you enter the DA, either based on a current reading, a held reading (e.g. from the start line) or you can enter the figure manually
• 1000' ET - Your 1000' ET from the time slip. This figure is normally entered for run completion analysis and will be discussed in a future article
• Run ET - Your ET from the time slip
• Wind Speed and Direction - Many weather stations can account for wind factors and their affect on your ET. This will be discussed in a future article

Garbage In = Garbage Out

When entering runs into the weather station it is important that they all make sense given the following guidelines:

• When DA goes up your ET should be slower
• When DA goes down your ET should be faster

So looking at these three runs, which one does not make sense?

• Run 1 - ET = 10.00, DA = 2000 FT
• Run 2 - ET = 10.05, DA = 3000 FT
• Run 3 - ET = 10.06, DA = 2800 FT

If you picked run 3 you would be correct. With a lower DA of 2800 FT, the run should have been quicker than run 2 with a DA of 3000. Leaving these 'bad' runs in the weather station will affect its ability to accurately predict your ET. In this instance you need to investigate why run 3 is slower. Did you turn the tyres off the start line, did the wind pick up or did you back-off before the finish line? Whatever the case may be, the run should be removed. Many weather stations provide a function to locate these 'bad' runs.

Predicting Your ET

Once you have entered a minimum number of runs (usually 3) the weather station will be ready to start predicting your ET. When you sample the current weather conditions, the weather station will now display an ET prediction.

With DA, ET predictions are based upon the runs you have already entered into the weather station and the current weather conditions. For example, you have entered the following 5 good runs into your weather station:

• Run 1 - ET = 10.01, DA = 1010
• Run 2 - ET = 10.06, DA = 2237
• Run 3 - ET = 10.11, DA = 2567
• Run 4 - ET = 10.12, DA = 2870
• Run 5 - ET = 10.15, DA = 3602

With these values plotted on a graph you can see how as DA changes, ET changes in a predictable manner. Refer to the graph below.

Prediction Graph

Looking at the graph, the weather station will now be able to predict what your ET will be as the DA changes. For example, if you were going out for first round of eliminations and the weather station displayed a DA of 2000 FT, the predicted ET would be approximately 10.064. So now, with this accurate prediction, you can go into eliminations confident you can run your number.

Golden Rules for Going Rounds

Here are some golden rules to make the most of a drag racing weather station:

• Make sure your vehicle is running consistently. For a weather station to be accurate your vehicle must run similar times at similar DAs
• Once your vehicle is running consistently, STOP changing things. I know this is hard for some, but as the saying goes..."if it ain't broke don't fix it"
• Track conditions change from meeting to meeting, day to day and lane to lane. Be flexible and make allowances for these changes
• Remove bad runs from your weather station. They will have a major effect on the accuracy of the predictions
• Analyse your runs before you enter them into your weather station. Are they representative of the vehicle's usual performance?
• Use different databases for each track and each vehicle
• Do not leave your weather station in the blazing sun or on the wet grass. These environmental factors will influence the weather station readings
• Use a log book to record data, whether it is pencil and paper, smart phone or a laptop. This will help you learn more about your vehicle and how it responds to changing conditions. You will then be able to use the weather station combined with your own knowledge to make predictions