PHYS 105 Air, Weather and ClimateLab 9. Forecasting I: Barometric Tendency (Does Falling Pressure Really Mean Stormy?)In this lab we will use the barometer and a classic weather rule to forecast the weather.The barometer has been used for hundreds of years to forecast the weather. In fact, it was pretty muchthe ONLY way to predict the weather, until the development of the telegraph in the mid-1800
PHYS 105 Air, Weather and Climate
Lab 9. Forecasting I: Barometric Tendency (Does Falling Pressure Really Mean Stormy?)
In this lab we will use the barometer and a classic weather rule to forecast the weather.
The barometer has been used for hundreds of years to forecast the weather. In fact, it was pretty much
the ONLY way to predict the weather, until the development of the telegraph in the mid-1800’s made
rapid long-distance communication possible (“A storm is coming!!”).
In this lab we will the classic pressure tendency rule and test if it successfully forecasts the weather
this week. This rule has been used for hundreds of year, and is supported by modern theories about air
pressure, cloud formation, and precipitation. Part of your assignment in this lab is to explain why this rule
The pressure tendency rule (this is the hypothesis for this lab):
Rising pressure leads to clearing skies and falling pressure leads to increasing clouds and precipitation
and steady pressure leads to no change.
rising pressure tendency is when the pressure increases by more than 2 millibars over 24 hours, and
falling pressure tendency is when the pressure decreases by more than 2 millibars over 24 hours, and
steady pressure tendency is when the pressure changes by 2 millibars or less over 24 hours.
Equipment & Materials
Procedure (data collection)
1) Read the lab instructions below and visualize the activities described. THIS LAB TAKES 4 DAYS.
2) Collect all necessary equipment and materials.
3) Record the air pressure in millibars at the same time each day for 4 days. To read the pressure, look at
the black arrow and then tap the face of the barometer gently. Notice if the arrow ‘rises’ (that is,
moves in a clockwise direction) or ‘falls’ (counter-clockwise). Then move the gold hand so it is
directly over with the black arrow. That will help you see tomorrow if the pressure has risen or fallen.
4) Record the weather at the same time that you record the air pressure. Notice if the sky is cloudy or
clear, and if it is raining or snowing or not. If there are clouds, estimate the % of the sky that is
covered, and try to identify the clouds by name.
5) Repeat this for 4 days. This will give you 3 data points, one for each day after the first day. (If
possible, these should be 4 days in a row. If you miss a day, you must divide the pressure change by 2,
to calculate the pressure change per day. Even if you skip a day, you still need 4 days of data.)
Procedure (data analysis)
6) For each day, classify the barometric pressure tendency as rising, falling or steady.
7) Each day make a specific prediction for the next day’s weather based on the pressure tendency rule.
Notice that this rule predicts the weather after the barometric pressure reading, not at the same time.
8) Compare the observed weather to the predicted weather, and for each day decide if the prediction was
correct or not.
9) Count how many times the prediction was correct. Count how many times the prediction was wrong.
10) Read the textbook again, and think about why this rule is generally correct. You might consult other
sources as well, such as web pages. If you do, make note of them and cite them in your report.
11) Pay attention to the weather patterns and what various meteorologists have to say about the weather
where you are this week. For example, accuweather.com or weather.com, or newspapers, or tv or
radio stations. Look for ideas about other factors that influenced the weather this week.
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