evaporating from Earth’s surface pr

evaporating from Earth’s surface produces watervapor (atmospheric gas). The water vapor eventuallycondenses in the atmosphere to form aerosol waterdroplets (clouds). The droplets combine to form raindrops or snowflakes (atmospheric precipitation).Snowflakes can accumulate to form ice (cryosphere)that sublimates back into the atmosphere or meltsback into water. Both rainwater and meltwater soakinto the ground (to form groundwater), evaporateback into the atmosphere, drain back into the ocean,or are consumed by plants and animals (which releasethe water back to the atmosphere via the process oftranspiration).In addition to water that is moving about theEarth system, there is also water that is stored and not circulating at any given time. For example, a very small portion of Earth’s water (about 2% of the water volume in oceans) is currently stored insnow and glacial ice at the poles and on highmountaintops. Additional water (perhaps as much as 80% of the water now in oceans) is also stored in“hydrous” (water-bearing) minerals inside Earth.When glaciers melt, or rocks melt, the water canreturn to active circulation.The endless exchange of energy and recycling ofwater undoubtedly has occurred since the first waterbodies formed on Earth billions of years ago. Yournext drink may include water molecules that oncewere part of a hydrous (water-bearing) mineral insideEarth or that once were consumed by a thirstydinosaur!Relating Scales of UnderstandingThe hydrologic cycle is a reminder that each thing onEarth is somehow related to everything else in space,time, or process. Geologists seek to understand thesecomplex relationships relative to human lifetimes andthe geologic time scale. For you to think like a geologist, you must consider many materials and processesover a broad range of temporal and spatial scales ofobservation. Some of these scales of observation maybe unfamiliar to you, so you will need to convert unfamiliar sizes and rates to familiar ones. For example,a rate of “1000 meters per million years” is much easier to conceptualize if it is converted to “1 millimeterper year.” You can also make scale models of thingsthat are too large or small to visualize. A scale modelis a physical representation of something that is actually much larger or smaller and has the same proportions as the actual object. For example, a toy car is asmall model of an actual car. The scale of the model isthe ratio by which the actual object was enlarged orreduced to make the scale model. If a toy car is 20 centimeters long and the actual car was 800 centimeterslong, then the ratio scale of model to actual car is 20:800,which reduces to 1:40. The model has a fractionalscale of 1/40, meaning that the actual car is 40 times(40*) larger than the model.