Hi Jen, here's an information article on Smoke Detectors that may shed some light on your question. Basically from what I've heard before, there are two types of detectors... one being heat sensitive, the other being particle sensitive. It's possible that in your situation you have a particle sensor and the moisture in the air is setting it off. In that case switching to a heat sensitive detector would alleviate your problem, BUT, these are less effective because the particle sensor will detect things (smoke, etc.) earlier than waiting for the flames to get out of control.
All that said, read the following information I found.
About Smoke Detectors
There are two common types of smoke detectors: photoelectric, which detects only visible products of combustion, and ionizing, which detects both the visible and invisible products of combustion. Other less common types of detectors are projected beam smoke detectors and aspirating (air sampling) smoke detectors.
Sensors, detectors and transducers covers a wide category of devices used to monitor, measure, test, record, analyze and/or display data as generated due to changes in a measured norm. Major sensor and sensor switch categories include acceleration and vibration, acoustic, analytical, density and specific gravity, electrical and electromagnetic, encoders and resolvers, environmental, flow, force, gas, humidity and moisture, level, linear and orientation position, pressure, proximity or presence, rotary position, temperature, tension, tilt, torque, velocity, viscosity, and weather sensors.
Environmental sensors include a wide variety of devices designed to measure, test, and/or alert for changes in environmental conditions, including radiation (both as a visible / invisible wavelength and as a hazardous emission), temperature, moisture and dew point, smoke, dust and opacity, light, weather, and water quality.
Temperature sensing products are used to measure or maintain changes in temperature, heat patterns, radiation, or infrared wavelengths. They include temperature probes, RTDs, thermocouples, thermometers (dial, digital, glass, laboratory and industrial), thermal cutoffs, thermistors, thermostats and thermal switches, thermowells, and smoke and flame detectors.
Gas sensors and analyzers are used to measure the amount of a specific gas or gases in a given environment, or, as with gas chromatography, are used to separate them. Aside from sensors, instruments and switches, residual gas analyzers and total organic carbon analyzers are included in this family.