Potential Instability
ept500mb = equivalent potential temperature at 500hPa [K]
ept850mb = equivalent potential temperature at 850hPa [K]
'd ept500mb-ept850mb'
ept500mb = equivalent potential temperature at 500hPa [K]
ept850mb = equivalent potential temperature at 850hPa [K]
'd ept500mb-ept850mb'