ONLINE WEATHER SUPPLEMENTAL INFORMATION

To complement the Daily Summary for Tuesday, 23 February 1999

ESTIMATING WARM or COLD AIR ADVECTION

NOTE: Modify 2nd sentence in paragraph 3 and 1st sentence of paragraph 4 for recent weather events associated with cold air and warm air advection.

The term "advection" is a generic term used to describe the horizontal transport of any atmospheric property by the wind. While the most common examples are warm air or cold air advection, the term can be used to describe other horizontal transport processes such as moisture advection, where atmospheric water vapor is transported by wind. Occasionally, warm or cold air advection is described simply as "temperature advection".

How can we tell if warm or cold air advection were occurring? In theory, in order for advection of air of different thermal characteristics to occur several factors must work in concert. One consideration is that the atmosphere must be in horizontal motion, with stronger winds increasing the chances for advection to occur. Calm conditions do not produce thermal advection because no horizontal transport takes place. Secondly, some horizontal differences in air temperature must exist over the region. A large air mass in its source region will have horizontally homogeneous thermal properties. Consequently, not much temperature advection will be exhibited within the center of the air mass itself. Finally, the winds must flow fairly directly across the zone of strongest horizontal temperature contrast in order for the horizontal transport of atmospheric thermal properties to occur.

Practically, one way to identify the type of thermal advection is to look at how the air temperature at a given location changes over a given time interval and look at the wind direction. Consider the cold air that moved southward on northerly winds (winds from the north) across the northern Plains at the end of last week and contributed to the blizzard conditions over the Dakotas. Another example would involve cold air following a cold front. Over the course of the day, strong cold northwesterly winds after frontal passage would cause air temperatures at individual stations to decrease as the cold air moved southward. Some stations may report falling temperatures throughout the daylight hours. Much of this cooling at individual stations occurred over a 24 hour interval and could be attributed to cold advection. However, one must be careful with selecting the time interval for temperature comparison. A cooling in the late afternoon and early evening may be as much the part of the usual diurnal heating cycle associated with nighttime radiational cooling rather than with cold advection.

On the other hand, warm advection occurred with southerly winds (winds from the south) across the Great Lakes region over the weekend that established new record high temperatures in several Michigan cities. In this case, the winds were transporting warmer (and more humid) air northward, with an attendant increase in the air temperature over the region.

How can we tell where temperature advection patterns are occurring from a surface weather map? You will have to look at both the air temperature and wind flow patterns on the map. To determine the temperature pattern, an isotherm analysis of the map will be needed. Remember that when isotherms are drawn connecting those stations having the same temperatures, the isotherm patterns will describe the overall temperature patterns observed across a geographic region.

Next you will need to estimate the wind speed and wind direction for the region in question. If you have a map with plotted wind information (with the traditional wind barbs and wind arrows on the standard surface station model), you should lightly draw some arrows on your chart that parallel the general wind direction over the region so as to represent the typical wind flow for that region. The wind speeds can be read from the number of wind barbs. Alternatively, if you had a map with isobars that analyzed the pressure pattern, you can estimate the winds from an extension of the "hand-twist" model. The winds would spiral inward in a counterclockwise fashion around a Low in the Northern Hemisphere. At any location application of this model means the surface wind direction would be along the isobar lines but at a slight angle across the line toward lower pressure values. (Low pressure is on the left.) The wind speeds are related to the isobar spacing. Higher wind speeds are associated with a closer spacing of the isobars and conversely, low wind speeds occur with when the isobars are far apart.

Now look at how the isotherms and the general wind flow appear together. The map and description of the temperature advection appearing on page 70 of Part A - Narrative, shows that you will be able to identify the occurrence of warm air advection when the wind blows from a warm region with isotherms having larger numbers toward a colder region where the isotherms have lower numbers. Conversely, cold air advection occurs where the wind flow is from colder regions delineated by lower-valued isotherms toward warmer regions with higher-valued isotherms.

Several simple rules of thumb may help in the location of temperature advection from a typical surface weather map:

• Little temperature advection occurs on either side of a stationary front. As its name suggests, the front and the two air masses that the front separates do not move horizontally in any quick fashion and are essentially stationary. Typically, the isotherms and winds on either side of the front parallel the frontal zone because the wind does not carry air of a different temperature across the isotherms.
• Little temperature advection occurs near the center of a surface high pressure cell because of the weak winds typically found there. Furthermore, the centers of high pressure systems tend to have weak horizontal temperature gradients because the air sinks and spreads out from a common region aloft, leading to relative homogeneity. As a result, large, slowly moving high pressure systems are often described in terms of an air mass, whose properties have been defined by the nature of the airmass source region. On the other hand, mid latitude low pressure systems experience many different thermal advection patterns because of the converging flow of winds carrying air of quite different thermal properties on different sides of the low.
• Warm advection occurs either:
  • In the vicinity of a warm front. A warm front marks the leading edge of a warm air mass that is replacing a colder air mass. If the warm front were oriented in an east-west fashion and if it were moving toward the northeast, then the region of warm advection near the surface would appear to the south of (or "behind") the advancing front. This warm advection would be caused by warm southerly winds. As the warm air is carried aloft over the cooler air, warm advection is carried aloft in what is often called "overrunning"; or,
  • Along the western flank of many high pressure cells where southerly winds flow northward and carry warm air from the south toward the typically colder north. High pressure systems that are of tropical origin or carry winds from off the oceans (or Gulf of Mexico) often provide warm advection to interior regions of the U.S.
• Cold advection occurs either:
  • In the vicinity of a cold front. Since a cold front marks the leading edge of a cold air mass, the cold advection would occur after the front has passed, with the cold westerly or northwesterly wind flow; or,
  • Along the eastern flank of many high pressure cells. The clockwise circulation around the high will bring colder air southward on northwesterly and northerly winds on the east side of the high. A cold high pressure system, often described as a polar or arctic air mass, that moves southward from Canada produces cold advection within regions in the U.S.

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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological Society.