Advection Fog - Advection fog often looks like radiation fog and is also the result of condensation. However, the condensation in this case is caused not by a reduction in surface temperature, but rather by the horizontal movement of warm moist air over a cold surface. This is very common during the winter when warm moist winds blow over cold snowcover. Advection fog may also form when moist maritime air masses drift over a cold inland area. This usually happens at night when the temperature of the land drops due to radiational cooling.

Isobars - Isobars are lines on a weather map joining points that have the same air pressure at a given altitude. Isobars are typically drawn in for surface plots. When isobars are tightly packed, windy conditions can be expected.

Blue Jet - An upper-level atmospheric electrical discharge, usually blue or bluish-white in color, that occur high above thunderstorms. Similar to sprites. This phenomenon is believed to connect the tops of thunderstorms with the ionosphere.

Perihelion - The point on the annual orbit of a body (about the sun) that is closest to the sun; at present, the earth reaches this point on about 5 January. Opposite of aphelion.

Transpiration - Process by which water vapors escapes from plants through leaf pores

Back-building Thunderstorm - A thunderstorm in which new development takes place on the upwind side of the storm. Upwind refers to the west or southwest side of the storm. In situations where back-building storms occur, the storm can seem to remain stationary or even move backwards against the mean wind flow. Meteorologists often call these types of storms training thunderstorms.

Solstice - Time of year when sun is at its maximum poleward location