3.3: Advection

Types of Advection

Advection is defined as the horizontal movement of a substance or property due to the bulk motion of a fluid, such as air or water. Imagine smoke from a chimney being transported horizontally towards you by the wind. While many factors, such as humidity, dust, etc., can advect, weather forecasters often focus on the advection of heat in the atmosphere. There are two types of advection: Warm air and Cold air advection.

• Warm air advection occurs when warmer air is moving into a region, pushing cooler air out.
• Cold air advection occurs when cooler air is moving into a region, pushing warmer air out.

We can use a combination of Isotherms and wind barbs to determine what, if any, type of advection is occurring. This is summed up in Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): Advection shown using isotherms and wind barbs. (CC BY 4.0; Alicia Mullens) Alternative description of image.

First, suppose the wind barbs are perpendicular (or nearly perpendicular) to their nearest isotherm. In that case, some type of advection is occurring, as the wind appears to be blowing from warmer to cooler or from cooler to warmer temperature regions. Warm air advection (station C) occurs when the wind barbs are pointing towards warmer air or higher isotherm values. Cold air advection (station A) occurs when the wind barbs are pointing towards cooler air or lower isotherm values. If the wind barbs are parallel (or nearly parallel) to their nearest isotherm, we say that the station isn’t experiencing any type of advection (station B).

Now, let's investigate advection on a U.S. map, with city weather stations represented by letters A through F. Figure \(\PageIndex{2}\) is a simplified map of select isotherms, wind barbs, and frontal systems. A cold front is defined as the leading edge of a cooler, denser air mass that replaces a warmer air mass, causing a noticeable drop in temperature. It is represented on a weather map by a long, blue, curved line with spikes to indicate direction of movement. A warm front is defined as the leading edge of a warmer, less dense air mass that is replacing a cooler air mass, causing a noticeable increase in temperatures. On a weather map, it is represented by a long, red, curved line with semicircular humps on it. Cold and warm fronts help illustrate the movement of air and should corroborate surface station wind directions. Using this information, let's answer the following questions.

Figure \(\PageIndex{2}\): Simplified map of Isotherms, Fronts and Wind Barbs. (CC BY 4.0; Alicia Mullens) Alternative description of image.

8. In Figure \(\PageIndex{2}\), cities ___ are experiencing Warm Air advection on this map.
   1. A & D
   2. E & F
   3. A & C
   4. B & C

9. Cities ___ are experiencing Cold Air advection on this map.
   1. A & D
   2. B & F
   3. C & D
   4. E & F

10.  Cities ___ aren't experiencing any advection on this map.
    1. A & D
    2. C & F
    3. E & F
    4. C & B

Now let’s use what we learnt to investigate a real-life weather map set-up in Figure \(\PageIndex{3}\). The map displays Isotherms, Station Models, and Fronts for the Continental United States at 1300 UTC on February 8, 2017.

Figure \(\PageIndex{3}\): Isotherms, Station Models, and Fronts for the Continental United States at 1300 UTC on February 8, 2017. (Public Domain; American Meteorological Society via NCEP/NWS) Alternative description of image.

11.  Areas in the south-eastern United States, ahead of the cold front, primarily had temperatures in the:
    1. 30s and 40s
    2. 50s and 60s
    3. 70s and 80s

12.  New Orleans, in southeastern Louisiana, had a temperature of 68°F and a dew point of 68°F. Winds were blowing from the South at: 
    1. 5 knots
    2. 10 knots
    3. 15 knots

13. Notice how New Orleans’ wind barb is almost perpendicular to the 70 isotherm it intersects. That is a sign of advection. Based on where New Orleans’ wind barb is pointing, in New Orleans, the air is blowing: 
    1. From colder air in the south to warmer air in the north
    2. From warmer air in the south to colder air in the north

14. Therefore, New Orleans is experiencing ____ advection, as is much of the southeastern United States. 
    1. warm air
    2. cold air
    3. no

15. Now let's investigate Sioux Falls, in eastern South Dakota. Sioux Falls' air temperature is 1°F with a dew point of -4°F. Winds at Sioux Falls are blowing from the northwest at ____.
    1. 10 knots
    2. 20 knots
    3. 30 knots

16. Similar to the wind barb at New Orleans, Sioux Falls' wind barb is almost perpendicular to the 0°F isotherm nearby. Based on where the wind barb is pointing, winds are blowing ____ at Sioux Falls. 
    1. from warm air to cold air
    2. from cold air to warm air

17. Therefore, Sioux Falls is experiencing ____ advection. 
    1. warm air
    2. cold air
    3. no

Finally, let's infer an observation based on the overall pattern of winds both ahead and behind the cold front in Figure \(\PageIndex{3}\). You can also refer to Figure \(\PageIndex{2}\) for a more simplified version. Based on the information, we can typically expect warm air advection ahead and cold air advection behind a Cold Front. Similarly, one can typically expect cold air advection ahead and warm air advection behind a warm front.