What kind of pressure ?
My guess would be atmospheric pressure, particularly lower than average atmospheric pressure.http://en.wikipedia.org/wiki/Low_pressure_area#Associated_weather There exist gizmos for predicting near-term weather, usually with a microprocessor and a pressure transducer, the kind that can sense absolute pressure, essentially a barometer. The history of the pressure signal may be part of it too. That is a sudden drop in barometric pressure is more ominous than a gradual one. I think. Or I might have that backwards. But I remember thinking it was something more complicated than just the instantaneous value for atmospheric pressure.
are you talking about a umbrella with a button that you press one time and the umbrella stays lighted until you press again?If so, just use a on off press switch.
"Pressure activated"? Isn't that just a switch?