Winter Weather

October 17, 2007

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

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

Winter Lows

• 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

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

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

Freezing Rain

Sleet

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

Hailstones

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

Winter Precipitation

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

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

• 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

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

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

Cold Air Damming

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

Folklore• The wooly boogers!

– Their coloring predicts winter weather

• Hornets– Placement of nests predicts winter weather

• Punxsutawney Phil