|"Climate is what you expect; weather is what you get"|
Virginia is a four-season state. August days can be - on the average - hot and humid, while January days can be cold and dry. The warmth in the air in early Spring, and the chill in the air in the Fall, "feel" different from other seasons. That's the climate pattern. However, if you're planning an outdoor event for any particular day in August or January, you'll have to check the weather report within a few days of the event to know the temperature and chance of precipitation for that one day.
Climate is defined by long-term averages. In contrast, weather is what's happening now. Weather changes rapidly within 24 hours, but climate changes gradually over the decades. It requires many measurements to document if we are trending towards an ice age or global warming.
Climate is measured in 30-year averages known as "climate normals." The 1961-1990 normals and the 1971-2000 normals are different, because the climate shifted slightly. The National Weather Service has divided the continental United States into 344 climate divisions, with six in Virginia, to document monthly temperature and precipitation values.
The Virginians paying the most attention to climate change may be on the coastline, anticipating sea level rise - but the operators of ski resorts are also concerned. As the climate warms, the capacity to offer good skiing before the Christmas holidays becomes less reliable and the ski resorts have to emphasize summertime hiking, zip lines, etc.
Just to the west of Winchester, Canaan Valley (West Virginia) gets 100 inches of snow annually. That helps to support looooooooong ski seasons that are the envy of ski resort owners less than 70 miles away in Virginia. The Virginia ski resorts get 25-50% of that total, and must manufacture snow at Massanutten, Bryce, or Wintergreen.
winter storms from the Great Lakes and Gulf Coast drop 100 inches of snow annually in West Virginia's mountains (redder zones indicate greater snowfall)
Source: National Weather Service Baltimore/Washington Forecast Office, Normal Annual Snowfall Map
On average, it rains or snows in Virginia for roughly 110 days each year. The number of overcast days, is greater, especially in the winter when gray clouds cover the sky for days at a time.
Colorado cities advertise that the sun shines 300 days/year, but remember that there are lies, damned lies, and statistics. Denver's claim is based on the sun being visible for just a minimum of one hour per day. Denver gets less than 16 inches of rain each year, but precipitation can be measured there nearly 90 days/year on average.1
Statewide averages will not help in planning a backyard picnic, but do provide a base for comparing Virginia with other places and for comparing places within Virginia.
Average rainfall for the entire state is 43-44" annually, but when it rains in Bristol the sun could be shining in Chincoteague. The Shenandoah Valley is relatively dry, a "rain shadow" getting only 33" of rain each year as storms soak the mountains on either side. In contrast, the higher elevations in southwestern Virginia get over 60" of rain annually.
Storms moving west-to-east across the North American continent have their moisture squeezed out by the mountains in West Virginia. Moist air drops in temperature 3.3 degrees Fahrenheit for every 1,000 feet increase in elevation (the "adiabatic lapse rate"). For each 3,000 feet increase in elevation, the air will be 10 degrees cooler on average.
Orographic (mountain-caused) cooling occurs as air masses are forced upward, and colder air can hold less moisture in suspension. Water droplets condense into raindrops and snowflakes. By the time the air masses reach the Shenandoah Valley, they are relatively dry.2
Storms totally ignore political boundaries. Hurricane Floyd dumped 17 inches of rain on Williamsburg on September 16, 1999, but the city of Roanoke received just 1 inch of rain that day.
Political boundaries do affect water management. Roanoke kept its water restrictions in place after September 16, 1999, since the Carvin's Cove reservoir was still 25 feet below the spillway. Roanoke County's Spring Hollow Reservoir near Salem, just a few miles away from the city reservoir at Carvin's Cove, received enough runoff from its watershed to supply sufficient water all summer for county residents. (Since then, the two jurisdictions have joined the Western Virginia Water Authority and water operations are synchronized.)
Based on the averages since 1885, it would appear Virginia is getting hotter and wetter.
trend of average annual temperature in Virginia, 1895-2010
(red line indicates average temperature is increasing by 0.5°F per century)
Source: National Oceanic and Atmospheric Administration/National Climatic Data Center Temperature, Precipitation, and Drought Time Series
trend of average annual precipitation in Virginia, 1895-2010
(red line indicates precipitation is increasing over 2" per century) Source: National Oceanic and Atmospheric Administration/National Climatic Data Center Temperature, Precipitation, and Drought Time Series
Key factors that shape Virginia's climate, aside from global weather patterns and latitude, include:
Water heats and cools slower than dry land. Because Hampton Roads is adjacent to the Atlantic Ocean and Chesapeake Bay, it has a more moderate climate than inland cities. Norfolk has less of a temperature range than Bristol, 350 miles west of the Atlantic Ocean. The high temperatures in the summer are higher in Bristol, and the low temperatures in the winter are lower in Bristol.
The minimum temperatures in November-March show how Hampton Roads stays warmer, thanks to the mitigating impact of the water:
the minimum temperature in November-March show how Hampton Roads stays warmer, thanks to the mitigating impact of the nearby water
Source: Library of Congress, "The national atlas of the United States of America," Monthly Minimum Temperature
Virginia has a "temperate" climate. According to the climate classification system developed by Wladimir Köppen and refined by Glen Trewartha, Virginia's climate has a "mild mid-latitude" climate, and is in the subcategory of "Humid subtropical." It is coded "Cfa" on the Köppen classification system because the climate is mild, with no dry season and a hot summer.
Source: United Nations Food and Agricultural Organization (FAO), New gridded maps of Köppen's climate classification
The Food and Agricultural Organization map shows other countries where the average temperature of the coldest month is <18°C and >-3°C, and the average temperature of warmest month exceeds 10°C. The green zones are places that have at least 30 millimeters of precipitation in the driest month, so the vegetation in those places will be comparable to Virginia.
The sub-tropical part is easy to understand - because we get frosts, we're not tropical. However, if you've ever been in Virginia through a summer, you know it's not the heat, it's the humidity that makes the summers "sticky." Warm temperatures with high relative humidity make it difficult for moisture to evaporate from the skin.
Humidity is what people in Tidewater complain about most often in the summer. East of I-95, the Potomac/Rappahannock/York/James rivers and their tributaries drain to the Chesapeake Bay, providing plenty of water to evaporate in the summer sunshine. Before air conditioning appeared in the 1930's, the best way for Virginians to escape the heat was to retreat to mountain resorts.
Highland County's name reflects its elevation. Because cooler air holds less moisture, a human body can cool off easier through evaporation.
When people in Arizona say "it doesn't feel like 100 degrees, it's so dry out here," they are referring to the ability of evaporation to cool off a body. The same cooling effect is available in the mountains of Virginia. Until the 1900's, Tidewater residents traveled to the resorts at Mountain Lake, The Homestead, White Sulfur Springs, and other higher-elevation, lower-humidity vacation spots with ceiling fans.
In contrast, the modern summer vacation destination of many summer vacationers from Richmond is a cottage at Virginia Beach or along a Tidewater river. Much of Virginia Beach's modern popularity reflects the ubiquitous availability of air conditioning in stores, movie theaters, and homes
The New River Valley in Virginia is attracting retirees due in part to its climate. Northern residents who retired and moved from the Snow Belt to Florida are returning to a place with seasons. The retirees may not want to return all the way back to harsh winters, however, so they choose to settle in an area that is both north of Florida and higher in elevation. Those who settle in places like Asheville (North Carolina), Blacksburg, and Roanoke are known as "halfbacks" who made it only halfway back to their old home.
Jimmy Buffet has suggested that we can change our attitude by changing our latitude. The reverse is also true - our personal comfort zone can be based on maintaining the same latitude.
When the state's economic development agents encourage companies from Korea and Japan to open facilities in Virginia, they describe how the company's managers can deal with the cultural differences - and one positive factor is that Virginia has a similar climate to "home."
Latitude alone does not determine the weather patterns or ecology of a place. Follow the original Virginia/Carolina border (the 36° parallel of latitude) eastward across the Atlantic Ocean. England is far north of its old colony of Virginia. Sailors who settled the first two English colonies in North America sailed from Plymouth (north of the 50th parallel of longitude) and London (north of the 51st parallel of longitude) to Jamestown, just north of 3736° latitude.
The early English colonists did not sail due west to Virginia. If they had tried sailing due west in a straight line, they would have ended up in Canada, north of the mouth of the St. Lawrence River in Labrador and Newfoundland - where the Vikings first settled in North America.
The English colonists first sailed south towards the Equator, then they turned west.
The 36° parallel, the southern border of Virginia after Carolina was chartered initially, crosses through the Straights of Gibraltar, south of Spain where modern Clementine oranges are harvested each September. The parallel goes across Malta, south of Italy, Greece and Turkey, and through Tehran in Iran and Mosul in Iraq. It touches the northern border of Tibet, crosses China between Shanghai and Beijing, then goes across South Korea before reaching Tokyo. In California, that parallel is just south of Silicon Valley.
What's at latitude 36° north, longitude 45° east?
Source: ESR!, ArcGIS Online
Virginia gets frosts and snowstorms from the cold polar air masses, especially in January. The shortest days of winter are in December, but the coldest days come weeks later in January. There is a lag time. The landmass of Virginia keeps cooling even after the December 21/22 winter solstice, when days start to get longer. For several more weeks after the winter solstice, Virginia radiates heat faster than the short winter days can re-heat the land... one reason February can feel like the longest month of the year, despite having the shortest number of days.
Hampton Roads has a longer growing season, with 240 days between the last freeze in Spring and the first freeze in Fall, thanks to the moderating impact of large water bodies in the region
Source: Library of Congress, "The national atlas of the United States of America," Freeze-Free Period
Climate shapes vegetation. Colonists in Eastern North America were disappointed when they tried to grow crops common to Western Europe at 36° latitude. Citrus and silkworms thrived at that latitude as Western Europe, but the Virginia winters were too harsh due to the winter frosts. When Europeans reached Southern California, they found a parallel region with a "Mediterranean" climate similar to Spain - and where orange and olive groves are successful.
Thomas Jefferson recorded in his Garden Book that the vegetation in May, 1774 was growing as if the winter was over until a late frost arrived:3:
Tables of statistics, such as the Historical Climate Summaries for Virginia, can be useful even though hard to read. The monthly climate summary for Fredericksburg, using data from 1930-1997, shows how precipitation (including snow) and temperature normally vary between November and February:
Graphs to visualize the data can show key points easily. A brief glance at the graphic below shows that the temperature in Abingdon rises and falls in a predictable pattern each year, while the precipitation pattern is not so consistent:
The climate normals over the last 30 years will vary from the averages established by records over the last century, back to 1895 in some cases. A comparison of today's with the climate of the Pleistocene 18,000 years ago (when Pennsylvania was half-buried by ice and there was no Chesapeake Bay), or at the end of the Paleozoic 250 million years ago (when massive coal deposits formed), shows than climate change over geologic time has been dramatic.
The 30-year normals reflect just the patterns of the last 30 years. Climate changes, though not as rapidly as the weather. The averages of the last 30 years may include warmer temperatures, higher moisture, etc. than averages calculated using all the data collected for over 100 years since 1895, but the patterns are consistent.
average normal rainfall, 1971-1990
average normal minimum temperature, 1971-1990
average normal maximum temperature, 1971-1990
There's a pattern to the temperature differences in Virginia; not every place has the same weather, and not every place in Virginia has the same climate. On the map below, compare the temperatures in Bath/Highland counties to Richmond/Norfolk. Think about the vegetation in the mountains vs. Coastal Plain - should the plants be different because the climate is different? (Hint: plants have a growing season, between the last frost of the Spring and the first frost of the Fall...)
The cool summer temperatures show why the rich Virginians went to the mountain springs and resorts during August in the 1800's before the invention of air conditioning - and why they did not go to the mountains in the winter before skiing became a sport.
Bath and Highland counties, and cities of Richmond and Norfolk
Thomas Jefferson recorded temperature throughout the day when writing the Declaration of Independence in Philadelphia, and for many years at Monticello
Source: Library of Congress, Weather Record, 1776-1818