Professional Pilot, February 2019

Closely spaced isobars and the position of the magenta 5280 m 1000 500 mb thickness line suggest an impending blizzard for southern Norway in this weather model forecast chart Tropospheric thickness lower than 5400 m blue dashed line suggests air cold enough to support snow PROFESSIONAL PILOT February 2019 59 Blizzards and similar winter storm systems occur throughout the world generally between about 40 and 60 degrees latitude in both hemispheres They even occur in the high latitudes well above the arctic and antarctic circles but these blizzards tend to form by different means than the ones experienced in the middle latitudes Many blizzards such as those found in Antarctica or in mountainous regions are due to katabatic wind storms Katabatic winds occur as cold dense air flows downhill They normally do not last long enough to create blizzards but can attain speeds of up to 100 kts and may quickly mobilize already fallen snow to generate a ground blizzard Ground blizzards happen when already fallen snow is blown about by the wind achieving the same effect on visibility as snow falling through high winds As with fog often pilots may see blue sky above the blowing snow or the blizzard conditions are masked by the underlying snow pack when viewed from above thus ground blizzards can be deceptive to pilots Close attention should be paid to surface reports whenever there is freshly fallen snow and winds are significant or are expected to be so A sudden gust across the runway on takeoff or landing can quickly reduce or eliminate forward visibility Midlatitude blizzards Because blizzards depend primarily on snow and strong winds the atmospheric condition that most commonly produces them is the midlatitude cyclone These storm systems are signified on surface maps with a big letter L for Low and have cold and warm fronts extending from that central low pressure These cyclones begin as a disturbance in the polar front the boundary between the frigid polar air and the warmer subtropical air Divergence in the flow of air in the polar vortex the polar jet stream allows surface air to ascend lowering the surface pressure This in turn generates the cyclonic circulation that draws warm air poleward ahead of the low and pulls cold air equatorward behind it As the warm and often more humid air flows poleward it rises up over the colder air While this may produce rain or mixed precipitation ahead of the warm front eventually the air spirals around the top of the low rising over ever colder air cooling further and dropping its precipitation through the coldest region of the storm frequently as heavy snow The other component of the blizzard mix is high wind Unfortunately the same region of the cyclone that experiences cold and snow is also the region in which winds are likely to be strongest This is thanks to the high pressure center that is frequently found poleward and trailing the low The positioning of the low and high results in the strongest pressure difference of the storm being in the line between the 2 pressure centers Wind speed in this area is a function of that pressure gradient The greater the change in pressure over a distance the stronger the wind Strong lows are normally coupled with equally strong highs making the winds even stronger The strongest systems can support winds in this region that exceed 50 kts with gusts surpassing tropical storm strength What can enhance the blizzard effect is when there are few obstacles at the surface that may act as wind breaks Such landscapes are common across Siberia and central North America as well as along western continental coastlines allowing the winds to sustain full force for hours on end Blizzard conditions Given the defining conditions of winds over 35 mph 30 kts 56 kph and airborne snow there exist several factors that will affect aviation operations The most obvious is visibility Snow is very effective at reducing visibility Each snowflake acts as a tiny opaque obstacle to vision When those flakes join millions of their cousins in the air forward visibility is quickly reduced and in some cases eliminated entirely Snowflakes are also highly reflective and can scatter light in any direction depending on their orientation This makes the use of aircraft lighting a challenge Though not generally to the same degree as fog shining a landing or taxi light ahead into falling snow is just as likely to reflect the light back into the cockpit rather than illuminate the path ahead Image courtesy IPS Meteostar