Tag Archives: winter

Weather and Health: Influenza

Posted on by

Last week, the Center for Disease Control and Prevention announced that the flu-season is officially under way in the United States.  This is the flu’s latest arrival in twenty-four years.

Flu-season typically lasts from November to March, winter in the U.S.  For this reason, many people believe that cold weather causes illness.  However, the fact is that viruses, not weather, cause influenza, a respiratory infection that is commonly called the flu.

In the colder months, people tend to gather indoors and interact in close proximity.  This increases the chance for spreading germs.  This winter, however, mild conditions have dominated the weather across much of the country.  As a result, people have been spending more time outdoors. Some medical experts cite this as one of the reasons for the reduced spread of the virus. Nonetheless, the flu is still active.  Cases have been reported in all fifty states with Colorado and California showing widespread activity.

Influenza is an infectious disease that usually affects millions of people each year. To defend against it, the CDC recommends getting an annual vaccination.  Other preventative actions include staying away from anyone who is infected and washing your hands frequently.

Image Credit: Center for Disease Control and Prevention

Avalanches

Posted on by

Backcountry winter activities such as skiing and snowmobiling are exhilarating adventures, but can also have serious risks.  This winter, avalanches in the Unites States have claimed the lives of seventeen people, including four this past weekend.

A product of weather and topography, avalanches depend on the steepness of a terrain. A slope between 25° and 60° is considered most favorable for a dangerous slide.  A steeper gradient will cause more frequent, but smaller events.  Snow, the other key ingredient, accumulates on mountain slopes in layers, forming a snow-pack over time.  Each layer is different in texture and weight, depending on the moisture content of the snow. Once on the ground, the snow remains subject to atmospheric conditions.  As temperatures rise and fall, the process of melting and re-freezing can form seams of instability in the snow-pack. Eventually, something will trigger an unstable slab to overcome the delicate friction holding it in place.

Triggers can be natural or man-made.  Some natural triggers include the weight of additional snow, sudden changes in temperature, and falling trees.  People can set off avalanches when their activities traverse an unstable snow slab.  Once a slab breaks away, it will cascade down the mountain engulfing anything in its path. So, if you are heading out to the back woods, it is vital to heed the avalanche warnings in your area.

A Year of Snow, a Year of Plenty

Posted on by

There is an old weather proverb that says, “A year of snow, a year of plenty.”  Rooted in agriculture, it refers to the traditional process of cold winters providing snow cover for fields that prevent crops from sprouting too early.  That snow then melts in the spring and provides moisture for a successful growing season.

As we now know, this winter has been anything but traditional with most of the United States experiencing uncommonly dry and mild conditions. In fact, today the U.S. Drought Monitor showed that 38% of the country is experiencing moderate drought.  This is up from 28% in December.  Some of the states most seriously affected by the lack of precipitation include, Texas, Georgia, Florida, and parts of California.

One of the reasons for the quiet weather this winter is the northern position of the Jet Stream.  Also known as the “storm track”, the Jet Stream usually dips south in the winter bringing cold air and snowstorms to the continental U.S.  The current La Niña episode, however, has pushed the Jet Stream north keeping most of the lower forty-eight states warmer and drier than normal.

Forecasters are predicting an end to this pattern in the spring when La Niña is expected to subside.

On the Ice

Posted on by

Winter is usually the time for outdoor activities like ice-skating or cross-country skiing. This year, however, the unseasonably mild conditions have limited the opportunities for many traditional winter sports across the U.S.

A brief cold blast of winter, like the one forecast for the New York City area this weekend, is generally not enough to form ice capable of supporting significant weight.   Below is a guideline on how thick ice needs to be to support different activities. The measurements refer only to new, clear, solid ice.  White ice usually has air trapped inside it and is consequently not very strong.

The thickness of ice can vary dramatically from spot to spot on the same pond or lake. Therefore, it is always best to follow the instructions of local officials and posted signs.  When in doubt, stay off the ice.  It is better to be safe than sorry.

Chart Source: Minnesota Department of Natural Resources

Snow in North Africa

Posted on by

The severe cold snap that has been affecting a large part of Europe reached the coast of North Africa earlier this week.  As a result, some towns in Libya and Algeria experienced a rare snow event.

Algiers, the capitol city of Algeria, along with a number of neighboring provinces, saw snow and ice accumulations measuring between six and eight inches. Several parts of Libya, including the capitol, Tripoli, received between two and three inches of snow. This was the heaviest snowfall the area has seen in many years. Snow in these cities is very uncommon given their southerly latitudes and position along the relatively warm Mediterranean Sea.

These unusual wintry conditions in North Africa coupled with the snow-drought in much of the United States, highlights how exceptional this winter season has been around the globe.

Wild Winter Weather in Europe

Posted on by

Last week, following a mild start to the season, winter found Europe.

From the U.K. to Russia and as far south as Italy, temperatures plummeted well below freezing and snowfall accumulated in substantial amounts.  Rome was blanketed by eight inches of snow. That is the most snow the city has seen in over twenty-five years.  Venice was so cold that its famous canals partially froze.  The hardest hit areas, however, were in Eastern Europe.  Places in Ukraine reached temperatures of 33° Celsius below zero, that is -27°F.  These are some of the harshest winter conditions that Europe has experienced in decades.

This intense cold snap caused serious problems in a region that is used to more moderate winters.  Power outages, transportation delays, and sadly over 300 weather-related deaths have been reported across Europe.

While this deep dip in the Jet Stream remains in place, the cold and in some cases, severe weather are forecast to continue.

Jet Stream dipping south over Europe.

Image Credit: wdtn.com

Winter and the Jet Stream

Posted on by

The local forecast is calling for a big dip in the jet stream this weekend.  This will drive cold air south over New York City and send temperatures plummeting from their above average readings. You may wonder, what is this meteorological phenomenon that is bringing winter back to the city so abruptly?

Jet streams are bands of strong winds in the upper atmosphere that mark the location of the strongest temperature contrast at the surface. They are born out of the complex interactions among a variety of atmospheric conditions, including the position of warm and cold air masses, and the location of areas of high and low pressure.

In the U.S., the polar jet stream, one of four major global jets, marks the divide between cold arctic air and mild mid-latitude air. Its position is a function of temperature contrasts and therefore shifts throughout the year, depending on the season. In the warm summer months the jet is typically located around 50° to 60°N latitude.  In the winter, when temperature contrasts are increased, it usually shifts to the south.  It has been known to plunge as far south as 30°N latitude.

This winter, however, the jet stream has been staying well to the north with only a few arctic outbreaks so far.  When they do come though, the sudden drop in temperature can be very jarring.

Average seasonal positions of the Polar Jet Stream in the northern hemisphere.

Image Credit: ncsu.edu

A Brief Blast of Winter

Posted on by

Earlier this week, New York City was sent into a deep freeze.  After a very mild December, winter finally found us, if only for a short while.

This arctic plunge brought the city its coldest air of the season to date. Temperatures remained below freezing for two consecutive days. Wednesday marked our coldest day since late January of last year with a low temperature of 13°F.

This cold weather, however, did not last long.  Warmer air returned by the end of the week and temperatures quickly rebounded to above average readings.  This warm weather is very odd for January.  Nonetheless, the local forecast is calling for a continuation of these spring-like conditions into next week.

We will have to wait for the Jet Stream to dip south again for winter to return.

Graphics Credit: MF at The Weather Gamut

La Nina Returns

Posted on by

La Nina has returned for a second year in a row.  According to NOAA’s winter climate outlook, this oceanic-atmospheric phenomenon will strongly shape our upcoming winter season.

La Nina is a climatic episode associated with the larger El Nino-Southern Oscillation (ENSO) climate pattern in the Pacific Ocean.   During a La Nina event, sea surface temperatures in the eastern Pacific are cooler than normal.  This ocean temperature anomaly influences weather around the globe.

In the US,  La Nina will impact both the temperature and precipitation in many parts of the country. The southern states are likely to see conditions that are both warmer and dryer than normal.  This is not good news for the drought-stricken state of Texas.  The northern tier is expected to experience  below average temperatures with the northwest getting above average precipitation.  The northeast and mid-Atlantic states have a 50/50 chance of seeing irregular conditions from La Nina.  In this region, the Arctic Oscillation (AO), a different oceanic-atmospheric pattern, has a stronger influence on winter weather.

Less predictable than La Nina, the AO continually transitions between positive and negative phases. A negative phase will bring cold arctic air and snowy conditions to the eastern US.  The cold snaps and heavy snow we saw last winter in the northeast were influenced by a very strong negative phase of the AO.  These strong phases can last anywhere from a few days to a few weeks and can be difficult to anticipate in long term forecasts.

The current La Nina event is forecast to last through February.

Photo Credit: NOAA