Setup an APC AP9319 Environment monitoring unit today. It has temperature and humidity probes to do basic environmental monitoring of the are surrounding a rack. I’ve got it rigged up to a light beacon to flash an orange strobe and send an email if the alert criteria is met. See AP9319

According to APC, the AP9319 has since been replaced by their uber expensive NetBotz Rack Monitor 200. It seems like it offers more than you would actually need for individual zone monitoring.