Jade - A common name for two minerals that are used as gemstones: Nephrite and Jadeite.

Jellyfish - Are free-swimming, simple invertebrate animals that live worldwide in our planet's oceans and seas. A few species live in freshwater aquatic systems. Some species in this group have primitive nervous and digestive systems.  About 1,200 to 2,000 different species of jellyfish exist. Jellyfish have existed since the Cambrian Explosion, about 570 million years ago.

Jet Stream - Relatively fast, uniform winds concentrated within the upper atmosphere in a narrow band. Several jet streams have been identified in the atmosphere. The polar jet stream exists in the mid-latitudes at an altitude of approximately 10 kilometers (6.2 miles). This jet stream flows from west to east at average speeds, depending on the time of year, between 110 and 185 kilometers per hour (68 and 115 miles per hour). Another strong jet stream occurs above the Subtropical Highs at an altitude of 13 kilometers (8.1 miles). This jet stream is commonly called the subtropical jet stream. The subtropical jet stream's winds are not as strong as the polar jet stream.

Joint - A fracture in a rock where no movement has taken place or where no movement has taken place perpendicular to the fracture's surface. Important in rock weathering because it increases the exposed surface area for physical decomposition or chemical reactions.

Jökulhlaup - Icelandic term used to describe the rapid drainage of a subglacial large mass of water or ice-dammed lake. These events usually result in flooding.

Joule (J) - Common unit used to measure energy. One joule is the energy required to move a force of one newton one meter in the direction of the force.

June Solstice - Date during the year when the declination of the Sun is at 23.5° North of the equator. During the June Solstice, locations in the Northern Hemisphere experience their longest day. The June Solstice is also the first day of summer in the Northern Hemisphere. Locations in the Southern Hemisphere have their shortest day on the June Solstice. This date also marks the first day of winter in the Southern Hemisphere.

Jungle - Commonly used term to describe dense or undisturbed tropical rainforest.

Jurassic - A geologic period that occurred roughly 144 to 208 million years ago. During this period, the first birds and mammals appeared, shallow seas covered large areas of the continents, and conifers were the dominant land plants. Also, large herbivorous dinosaurs are common at the end of this period. One of three periods during the Mesozoic. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.

Juvenile Water - Mineral-rich water that originates from deep within the Earth's interior, where magma occurs. Often associated with volcanic eruptions. Also called magmatic water.