National Weather Service United States Department of Commerce

Learning Lesson: That Sinking Feeling


Ocean water is salty and, in general, cold. Differences in the water density associated with temperature and salinity are vital in shaping the Great Ocean Conveyor. This lesson plan demonstrates how salinity and temperature affect density of water.

TOTAL TIME 10 minutes
SUPPLIES Fish bowl (or large, deep glass container such as fish tank, bowl or pitcher); Food coloring; Table salt; Ice cube tray
Make the ice cubes the evening before. Into one-half of the ice cube tray pour fresh, dark food-colored water. Using a different color, pour dark-colored, very salty water into the other half of the tray. Freeze the solutions.
SAFETY FOCUS Thunderstorm safety rules


  1. Fill the fish bowl nearly full with tap water. Allow the water to settle for a minute or two.
  2. Gently add one cube of each color into the water.
  3. Have the students describe and explain what occurs.


In both cases, the colored waters will begin to sink to the bottom of the bowl. This is due to the temperature of water from the melting cube is denser than the surrounding tap water and therefore sinks. However, the colored water from the salty cube should sink faster than the fresh water cube because the addition of salt.

Each winter, in the far North Atlantic Ocean as new sea ice forms, the salt that is left behind in the ocean makes the water very dense. This dense water sinks to the ocean floor and is the "engine" for Great Ocean Conveyor's motion.

Building a Weather-Ready Nation

Differences in air density help develop thunderstorms. Cold, sinking air can lead to rising, relatively warmer, air and thunderstorms. This often occurs on the West Coast in winter as cold pools of sink air accompany a low pressure system.

A Severe Thunderstorm Warning is an urgent announcement that a severe thunderstorm has been reported or is imminent and warns you to take cover. Severe thunderstorm warnings are issued by local National Weather Service offices. What you can do before a storm strikes...

  • Know the county you live in and the names of the major nearby cities or towns. Severe weather warnings and statements are issued by county and reference major cities.
  • Check the latest weather forecast and hazardous weather outlook.
  • Watch for signs of an approaching thunderstorm.
  • If a storm is approaching, keep a NOAA Weather Radio and/or AM/FM radio with you.
  • Postpone outdoor activities if thunderstorms are imminent. This is the best way to avoid being caught in a dangerous situation.