Z - see Zulu Time

Zeroth Law of Thermodynamics - if two systems are put in contact with each other, there is a net exchange of energy between them unless their thermal energy per unit volume is equal (at thermal equilibrium). This law states that if two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. The Zeroth law was added after the First, Second, and Third laws.

ZL - see Freezing Drizzle

ZNS - zones

Zonal Flow - large-scale atmospheric motion along a mostly east-west path (nearly along a line of latitude).

ZR - see Freezing Rain

Z-R Relationship - a mathematical equation that converts radar-measured reflectivity to an estimated rainfall rate. The reflectivity factor (Z) is related to both the drop-size distribution and the 6th power of the drop diameters. However, the radar only measures reflectivity, not the drop-size distribution. Hence, the drop-size distribution must be assumed and can be wrong. Unfortunately, different drop-size distributions can produce the same reflectivity factor but have markedly different rainfall rates. For example, many small drops can produce the same reflectivity as one large drop; however, the rainfall rate from the small drops is very different from the one large drop.

Zulu Time (Z) - global time scale based on the local time observed on the Prime Meridian; also called Greenwich Mean Time (GMT) or Universal Coordinated Time (UTC)