The term advection refers to the transport of something from one region to another. Meteorologists are most interested in the advection of variables like temperature, moisture and vorticity. Assessing advection on weather maps is dependent upon two factors; 1) the strength of the wind and 2) the angle of the wind relative to the lines of equal value (isolines) of the variable being advected. The strongest advection occurs when the winds are oriented perpendicular (at 90 degrees) relative to the isolines. No advection occurs if the winds are parallel to the isolines. The figures below depict three different examples of temperature advection. The arrows are wind vectors and the horizontal lines are isotherms (lines of constant temperature) in degrees Fahrenheit.
The wind vectors are longer in Figure A than they are in Figure B, which implies that the winds are stronger in Figure A. Since in both cases the winds are aligned perpendicular to the isotherms, stronger advection is occurring in Figure A than Figure B, because of the stronger winds in A. In Figure C, no advection is occurring because the wind vectors are parallel to the isotherms, indicating no horizontal transport of temperature.
Positive and Negative Advection:
Animation by: Van Dorn
On the other hand, Figure E shows negative advection, since lower values of a variable (in this case temperature) are being advected towards higher values of the same variable. The end result of negative advection is to decrease the variable values in the direction the wind is blowing.