Winter Weather

1. Winter Weather October 17, 2007

2. Winter Weather • A winter storm is a low-pressure system that covers a large area and contains weather fronts. • In the Northern Hemisphere, circulation is counterclockwise • Fronts can produce rain, sleet, freezing rain, or snow

3. Winter Cyclones • Winter cyclones are produced by (or spin off of) the Aleutian and Icelandic Lows (both semi-permanent pressure cells) • They can impact weather between September and May • Most intense in January and February • The severity of the storm can be determined by checking the pressure at the storm’s center – anything below 1000mb is severe

4. Winter Lows

5. The severity of winter cyclones depends on • Latitude • Distance from the ocean • Source region • Continentality • Southern Italy and Nebraska are at the same latitude • Topography • Mountains alter wind patterns • Altitude • Influence temperature distribution

6. Jet Stream • Parks over the US during the winter • Can change position and contour often, or stay in the same position for months • Persistent winter ridges produce a warm, dry winter • Persistent winter troughs produce a cold, wet winter and spring floods

7. Sleet and Freezing Rain • The most dangerous winter weather because no one takes them seriously • Ice pellets or snowflakes that melt as they fall, but refreeze before hitting the surface • Freezing rain freezes on the first thing it hits: power lines, roads, trees, geese… • If more than 1 in of ice forms at the surface, it is termed an ice storm

8. Freezing Rain

9. Sleet

10. Hail • Hail consists of ice pellets formed in roughly concentric layers • Formed when water is frozen in the atmosphere. The ice pellet falls and encounters water, which freezes to the ice pellet forming a second layer • The size of hail is determined by the strength of the updraft • Hail has more water volume, snowflakes have more air volume

11. Hailstones

12. Snow • Snow is precipitation in the form of frozen water with a crystalline structure • For powdery snow, 8-12 inches of snow will melt into 1 inch of water • For wet snow, 4-6 inches of snow will melt into 1 inch of water

13. Winter Precipitation

14. Drifting Snow • Drift control used to prevent roads from becoming impassible • Temporary snow fences erected to capture snow blowing from the west and northwest • Rows of conifers also used

15. Blizzards • Blizzards are characterized by blowing and drifting snow, limited visibility, and cold temperatures • Occur on the polar sides of fronts behind cold fronts • Temperatures below 25°F and winds above 35mph • Blizzards are strongest when reaching the farthest point south in the jet stream

16. Rain into snow • Precipitation changes from rain into snow as a cold front passes • Wind changes from SSW to WNW as the front passes • Snow into rain • When a center of low pressure passes over an area before the warm front • Snow is melted into rain • Causes freezing rain and ice storms

17. Nor’easters • Aka Northeasterns • When trough sets up over Atlantic seaboard • Atlantic Ocean feeds extreme snow storms along the coast • Winds out of the NE • Snow 1-4 feet deep, coastal flooding, beach erosion

18. Cold Air Damming • When high pressure sets up over the NE US or eastern Canada, it forces cold air into the Appalachian Mountains • The air is too dense to rise over the mountains, so it becomes trapped • This produces extreme temperatures, ice storms, freezing rain, and/or blizzards • Example, Blizzard of 1993

19. Cold Air Damming

20. FYI • Alcohol increases the danger of hypothermia (which is extremely low body temperature) • It impairs judgment • It causes small blood vessels in the skin to dilate, increasing the rate of heat loss from the body

21. Folklore • The wooly boogers! • Their coloring predicts winter weather • Hornets • Placement of nests predicts winter weather • Punxsutawney Phil