Anatomy of an Ice Storm

The ice storm of December 2007 will not soon be forgotten in central Oklahoma, or anywhere else in the central plains. Two low-pressure systems moved over the plains in quick succession, spawning thunderstorms and freezing rain that occurred over a wide swath for nearly 48 hours. Ice accumulations of greater than ½ inch were common throughout much of Oklahoma, Kansas, and Missouri, and localized ice totals of greater than 1 inch were common.

Effects of the storm were widespread and major. The ice glaze knocked out power to over a million customers, including more than 600,000 in Oklahoma, the largest power outage in the state's history. Many were without power for nearly a week. 38 fatalities have been directly attributed to the storm, including 27 within Oklahoma. Most of these fatalities occurred in traffic accidents on ice-coated roads. Trees also felt the brunt of the storm, with branches breaking under the weight of the ice. Anyone who went outside on the night of December 9 was surrounded by claps of thunder and the continual sounds of tree limbs cracking and crashing to the ground. Knowing all this, several questions come to mind: Why was this ice storm so strong and so long-lived? Why was such a large area affected? Just what does it take to produce a major ice storm, anyway? Let's dig in and answer these questions, starting off with a look inside the anatomy of an ice storm.

Forecasting winter storms is a unique challenge in meteorology, because a change of just a couple degrees in the temperature either at the surface or aloft will make the difference between rain, freezing rain, sleet, or snow. Ice storms in particular require a delicate balance of atmospheric conditions in order to produce the freezing rain that coats everything in a solid ice glaze. First off, it must be below freezing at the surface, so that rain that falls will freeze on contact. Secondly, there must be a relatively deep layer of warm, above-freezing air aloft so that precipitation falls as rain instead of sleet or snow. There must also be sufficient moisture present, and an atmospheric disturbance (low pressure system) to convert that moisture into precipitation.

Here in the central plains, this setup is most commonly realized when we have a cold, Canadian airmass that moves south into the region during the winter, displacing a warm, moist airmass influenced by the Gulf of Mexico. Being denser than the warm air it is displacing, the cold air tends to "ooze" south in a shallow layer perhaps only a few hundred meters deep. The warm, moist gulf air remains in place not far above the ground - though it may be only 30F at the surface, just a kilometer aloft it could be close to 50F. Precipitation falls first through the warm air, melting into rain. This rain then passes through the thin, sub-freezing layer near the surface quickly enough that it is still liquid when it reaches the surface. When it lands on cold trees, or power lines, or sidewalks and roads, it freezes on contact, producing the familiar ice glaze.

But what made this December's ice storm so strong and widespread? The answer is twofold - a very widespread, warm, and moist airmass aloft, and instability present in the atmosphere. Prior to the ice storm, conditions were very warm and humid for December throughout much of the southern and central plains - temperatures in central Oklahoma were near 70 degrees just a few days prior. When a cold airmass penetrated south into the central plains, this very warm, moist air was forced aloft, providing the warm-over-cold temperature structure needed for a major ice storm over a large portion of the central plains. Secondly, the airmass aloft was sufficiently warm that moist convective instability was present, allowing thunderstorms to form.

Moist convective instability occurs when a saturated parcel of air remains warmer than its surroundings when it is lifted. Air closely follows the ideal gas law, which explains that air naturally cools as it is lifted due to a decrease in pressure. For saturated air, this is partially offset by heat released when water vapor condenses. This heat, associated with the phase changed from gas to liquid, is called the "latent heat of vaporization". All this adds up to a decrease in temperature of about 6.5C/km as a parcel of air is lifted. If this decrease is less than the ambient decrease in temperature with height, the parcel will end up warmer (and less dense) than its surroundings and continue to rise, giving birth to thunderstorms. This process occurred during the December ice storm, and allowed for very heavy freezing rain to fall - convective rainfall rates are much greater than those usually seen in winter storms.


For more information on (and photos of) the December 2007 ice storm, check out this summary by the Tulsa NWS forecast office .



Story is © Nate Snook, 2007