“Which witch knew whether the weather …

… was going to change.” Ever heard of that spelling challenge?

Spelling aside, how do we know whether the weather is going to change? Now that WeatherSignal on iPhone 6 and 6+ can collect barometric pressure, and given the foggy, rainy weather we’ve been experiencing in London recently, it’s a great time to explore what you can discover about weather and forecasting by measuring barometric (or atmospheric) pressure.

How well can we understand the weather with only barometric pressure? In their paper “Surface pressure observations from smartphones” (which uses WeatherSignal data!), Clifford Mass and Luke Madaus explain that pressure may be one of the most valuable weather measurements, as it is not easily affected by other atmospheric factors, and because it reflects the deep, 3D structure of the atmosphere.

(Of course, a fun project might be to attach an iPhone with WeatherSignal to a drone and measure pressure at different heights! That way, you could build your own model of surface pressure and how it relates to the pressure throughout the air column.)

Hobbyist drone Flying over field

Hobbyist drone flying over field; Photo Credit: Michael Khor

Okay, so we’ll use a robotic flying drone or a model airplane to take our pressure measurements! Well, maybe. In any case, what should we be looking at? What are the components of a forecast based on pressure?

I can think of a few variables that would be of interest (and welcome any comments):

  • The absolute value of pressure (WeatherSignal for iPhone shows you the absolute pressure)
  • The absolute pressure at sea-level, which is essentially absolute pressure normalised for altitude (if you sign up for the WeatherSignal on iOS beta-testing programme, you can get both the absolute pressure and the adjusted sea-level pressure).

  • The rate of change in pressure - how quickly pressure changes is reflected in the intensity of weather changes. This article give a nice overview on how different rates of change in pressure relate to weather.
  • How much the pressure changed - hurricanes are often accompanied by pressure drops of 30 to 70 mb (millibars).
  • The direction of weather changes – going from low to high likely means better weather, whilst if the pressure drops, get your umbrella! (or go inside!)

It would also be interesting to see how the pressure readings from WeatherSignal correlate with the forecast of your local meteorological office. In looking for pressure maps for London (provided by the UK’s MetOffice), I found some great resources to help you read weather maps of all sorts:

  1. UK’s Met Office Surface Pressure Maps
  2. Factsheet on interpreting weather charts

All of this is really interesting, but I feel that we can push the boundaries even further in terms of making connections between weather and other, non-weather phenomena. As it is, we’re still in the “Weathersphere”! Looking for fun or interesting connections, following are a few that I found.

Barometric pressure has an effect on water chemistry

According to the Environmental Monitor, pressure has an effect on water chemistry, specifically the amount of gas that can dissolve in water increases with pressure. This article on pressure points out that this applies to oxygen, which makes me think about the health of ecosystems and how changes in pressure affects animal populations.

Pressure as an indicator for man-made climate change

A new study led by Stanford climate scientist Noah Diffenbaugh finds that California’s drought, which has been marked by exceedingly high atmospheric pressure, is much more likely to have happened in conditions with modern concentrations of greenhouse gases. What if these findings also indicate that pressure could be an indicator of air quality? It would be interesting to see if there is a significant difference in pressure readings in cities versus rural areas (on top of from normal atmospheric changes).

Pressure as an indicator of animal migrations

Although it is understandable that weather affects animal migrations, and that pressure is directly related to the weather conditions, using pressure for understanding ecology and animal activity has interesting potential. What phenomena can we observe by superimposing pressure data by what we observe in our parks or gardens?

What connections do you observe? What analysis would you use barometric pressure for? Just starting out with your explorations of the Weathersphere? Let us know how WeatherSignal works for you!