Building upon several years of deploying outdoor wireless sensor networks, I would like to add the following hint:
Think ahead and do not underestimate the problems arising from humidity!
I will answer your question by providing some pitfalls with humidity in outdoor devices. However, please consider these general guidelines when planning a wireless sensor network.
It is very simple to build a housing that keeps the rain from falling onto your sensor, but as @Ghanima pointed out it very much depends on what you really want to measure.
However, the main problem is not rain but humidity. This is roughly what happens with not properly constructed devices in outdoor environments, starting with a dry housing:
- The outdoor temperature rises. The temperature of the air in your device rises even more (especially when placed in the sun).
- Therefore, the air pressure in your device rises, it expands and some part diffuses out of the device.
- The temperature falls. Thus humid (!) air diffuses back into the device.
- The humid air condensates inside the device.
- The cycle repeats, but the temperature inside the device is not high enough to evaporate the water again. Therefore, water accumulates inside the device. Every day a very tiny amount of water is added.
Several possible approaches to counteract this problem:
Sealing the device Trust me, that does not work with household items. Even if your freezer box has a seal and silicone works good in the bathroom, both do not prevent the flow of humid air! Dipping the device into water without any water coming in, does not mean that humid air will stay out, too! If you want to go that way, buy industrial grade (IP rated) housings and do not drill additional holes into it!
Drilling a hole into the bottom What, you just said the opposite? The reason is simple: If you can not avoid that humid air comes into your device, you should at least avoid that water accumulates inside. For many electronic components some humidity is not such a big problem, but accumulating water is!
Air-conditioning Heating up the device prevents condensation. This is the common approach for large and sensitive outdoor devices, especially if the condensation itself induces problems (for example for optics).
A pressure-compensating diaphragm Not that cheap but works quite good in practice by equalizing the air pressures. There are also cable feed-throughs with an integrated diaphragm.