I want to calibrate a barometer using current pressure. I live in a town with altitude 540m (at the top of an escarpment) but the closest internet pressure readings are from a town which is 50m above sea level(at the bottom of the escarpment). Assuming the temperature varies linearly between the two altitudes, and the atmosphere decreases exponentially, what rule of thumb can I use to convert a pressure reading from the lower town to my upper town? (This isn't actually a homework questions! : )
To convert a pressure reading from the lower town to your upper town, you'll need to consider the variation in altitude and the temperature. Assuming a linear variation in temperature between the two altitudes and an exponential decrease in atmosphere, you can follow these steps:
1. Determine the height difference between the two towns.
- In this case, the height difference is 540m - (-50m) = 590m.
2. Calculate the pressure difference due to the change in altitude.
- The pressure decreases with increasing altitude. As a general rule of thumb, the pressure decreases by about 1 hPa (hectopascal) for every 8.5 meters of elevation gain.
- Divide the height difference by 8.5 to determine the pressure difference in hPa. In this case, the pressure difference would be 590m / 8.5m = 69.41 hPa.
3. Determine the temperature difference between the two towns.
- Since the temperature varies linearly between the altitudes, you'll need to estimate the temperature difference between the top and bottom of the escarpment. This can be done by finding the average temperature at each location and using the difference between the two average temperatures.
- You can use weather records or historical data for the towns to estimate the average temperatures.
4. Adjust the pressure difference based on the temperature difference.
- Calculate the temperature difference between the two towns. Let's say the temperature difference is ΔT°C.
- The pressure decreases with decreasing temperature. As a rough estimate, the pressure decreases by about 1 hPa for every 10°C temperature decrease.
- Divide the temperature difference by 10 to determine the pressure correction in hPa. In this case, you would divide ΔT°C by 10.
5. Calculate the final pressure adjustment.
- Add the pressure correction obtained from step 4 to the pressure difference obtained from step 2.
- For example, if the pressure difference due to altitude is 69.41 hPa and the pressure correction due to temperature is 3.2 hPa, then the final pressure adjustment would be 69.41 hPa + 3.2 hPa = 72.61 hPa.
Therefore, you can use the rule of thumb that for every 8.5 meters of elevation gain, the pressure decreases by about 1 hPa. However, since temperature also affects pressure, you should estimate the temperature difference between the two towns and adjust the pressure accordingly.