Field note: A droplet's journey

On a warm afternoon, the Sun heats water in a coastal bay. Some liquid water changes into water vapour and rises; this process moves water from the surface into the air. Plants on the nearby hillside also release water vapour from their leaves through transpiration. As the moist air rises, it cools. Water vapour condenses into tiny liquid droplets that gather to form clouds. When droplets grow large enough, water falls as precipitation—often rain in this coastal region. Some rain flows over the ground into streams and back to the bay as runoff. Some soaks into the soil by infiltration and may travel underground as groundwater. Plants can absorb some of this water, while some eventually reaches the bay. Water is continually moving between stores such as oceans, clouds, soil, groundwater, rivers, and living things. The Sun supplies energy for much of this movement; gravity helps water fall and flow downhill.