Weather & Climate of Virginia

Robert E. Davis Professor of Climatology

Dept. of Environmental Sciences University of Virginia

Email: red3u@virginia.edu

Rivanna Master Naturalists March 17, 2015

Tropic of

Cancer

Arctic Circle

SEASONS

Effect of Solar Angle

Radiation receipt: 1)  sun angle (intensity) 2) day length (duration)

circle of illumination

Top of Atmosphere Sunlight

Equator

Charlottesville

Arctic

34–36

0–44

16–41

Continentality

Mean temperature of warmest minus coldest month

Incoming

Outgoing

General Circulation

of Atmosphere

Ocean Circulation

Jet Stream:

1) Boundary between warm, tropical and

cold, polar air masses

2) Size and strength vary seasonally

3) Linked to midlatitude

storms at surface

4) Linked to storm tracks

5) A component of

(circum)-polar vortex

cold

warm

JULY

IN WINTER, JET STREAM 1) EXPANDS 2) STRENGTHENS

JANUARY

http://www.classzone.com/books/earth_science/terc/content/investigations/es1906/es1906page02.cfm

COLD! COOL

A (Very) Short Course on Weather

Storms are low pressure systems

Types of Fronts

L

Cold Front (vertical cross-section)

Warm Front (vertical cross-section)

Cold Air

Warm Air

Average January Winds

Richmond

(color = speed)

North wind

Southwest wind

Average July Winds

Richmond

(color = speed)

North wind

July Mean Maximum Dew Point Temperature

Common Wind Directions

Surface

Aloft

Mountain Effects

(Elevation: 1000 feet)

(Elevation: 3500 feet)

fronts

thunderstorms

hurricanes

Charlottesville Precipitation

SUMMER

% Rainfall (May-Nov) from Tropical Cyclones

Knight and Davis, 2007

Charlottesville Climatic Water Budget

PE = Potential Evapotranspiration m

illim

eter

s

AE = Actual Evapotranspiration

P = Precipitation

PE P

Surplus

PE

P

Deficit

PHENOLOGY

Study of cyclic and seasonal natural phenomena, especially in relation to climate and plant and animal life

PHRENOLOGY

Study of the and size of the cranium as a (supposed) indication of

character and mental abilities

Grape Phenology

Light, heat accumulation, soil moisture

Organism Growth and Temperature

growth begins

growth at maximum

Growing Degree Days (GDDs)

accumulated degrees over time above a baseline and below a maximum, based on daily mean temperature

April 10: Max = 65, Min = 45, so Mean = 55; 55 – 50 = 5 GDDs April 11: Max = 70, Min = 50, so Mean = 60; 60 – 50 = 10 GDDs

Season total = 15 GDDs

Growing Degree Days (GDDs)

These vary by organism! corn baseline = 50°F

lettuce baseline = 42°F potato baseline = 46°F

Growing Degree Days (GDDs)

As of Sunday (3/15), CHO had accumulated 44 (corn) GDDs—12 below

average for that date

Cautions Some organisms are more light than heat

sensitive, so GDDs will not be as useful

Proper growth depends on adequate soil

moisture as well as heat accumulation

PLANT HARDINESS ZONES

Average Date of Last Spring Frost

Indicator LimitationsChanges in measurement techniques and instruments over time can affect trends. However, these data were carefully reviewed for quality, and values that appeared invalid were not included in the indicator. This indi-cator only includes weather stations that did not have many missing data points.

Data SourcesAll three figures are based on data compiled by the National Oceanic and Atmospheric Administration’s National Climatic Data Center, and these data are available online at: www.ncdc.noaa.gov/oa/ncdc.html. Trends were analyzed by Kunkel (2009).8

Length of Growing SeasonKey Points• The average length of the growing season in the lower 48 states has in-

creased by about two weeks since the beginning of the 20th century. A particularly large and steady increase occurred over the last 30 years (see Figure 1).

• The length of the growing season has increased more rapidly in the West than in the East. In the West, the length of the growing season has increased at an average rate of about 20 days per century since 1900, compared with a rate of about six days per century in the East (see Figure 2).

• The final spring frost is now occurring earlier than at any point since 1900, and the first fall frosts are arriving later. Since 1985, the last spring frost has arrived an average of about four days earlier than the long-term average, and the first fall frost has arrived about three days later (see Figure 3).

Figure 3. Timing of Last Spring Frost and First Fall Frost in the Lower 48 States, 1900–2002This figure shows the timing of the last spring frost and the first fall frost in the lower 48 states compared with a long-term average. Positive values indicate that the frost occurred later in the year, and negative values indicate that the frost occurred earlier in the year. The trend lines were smoothed using an 11-year moving average. Choosing a different long-term average for comparison would not change the shape of the trends.

Year

Devi

atio

n fro

m a

vera

ge (d

ays)

-6

-4

-2

0

2

4

6

Spring frost

Long-term average

Fall frost

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Data source: Kunkel, 20097

61

(Kunkel, 2009)

Average Date of Last Spring and First Fall Frost Continental U.S.

Late

r E

arlie

r

(Kunkel, 2009)

Length of Growing Season, 1900–2002 Continental U.S.

Length of Growing SeasonBackgroundThe length of the growing season in any given region represents the number of days when plant growth takes place. The growing season often determines which crops can be grown in an area, as some crops require long growing seasons, while others mature rapidly. Growing season length is limited by many different fac-tors. Depending on the region and the climate, the growing season is influenced by air temperatures, frost days, rainfall, or daylight hours.

Changes in the length of the growing season can have both positive and negative effects. Moderate warming can benefit crop and pasture yields in mid- to high-latitude regions, yet even slight warming decreases yields in seasonally dry and low-latitude regions.4 A longer growing season could allow farmers to diversify crops or have multiple harvests from the same plot. However, it could also limit the types of crops grown, encourage invasive species or weed growth, or strain water supplies. A longer growing season could also disrupt the function and structure of a region’s ecosystems, and could, for example, alter the range and types of animal species in the area.

About the IndicatorThis indicator looks at the length of the growing season in the lower 48 states, as well as trends in the timing of spring and fall frosts. For this indicator, the length of the growing season is defined as the period of time between the last frost of spring and the first frost of fall, when the air temperature drops below the freezing point of 32ºF.

Trends in the growing season were cal-culated using temperature data from 794 weather stations throughout the lower 48 states. These data were obtained from the National Oceanic and Atmospheric Administration’s National Climatic Data Center. Growing season length and the timing of spring and fall frosts were aver-aged spatially, then compared with a long-term average to determine the deviation from “normal” in any given year.

This indicator measures the length of the growing season in the lower 48 states.

Figure 1. Length of Growing Season in the Lower 48 States, 1900–2002This figure shows the length of the growing season in the lower 48 states compared with a long-term average. For each year, the line represents the number of days shorter or longer than average. The trend line was smoothed using an 11-year moving average. Choosing a different long-term average for comparison would not change the shape of the trend.

Year

-10

-5

0

5

10

15

Devi

atio

n fro

m a

vera

ge (d

ays)

Long-term average

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Data source: Kunkel, 20095

Figure 2. Length of Growing Season in the Lower 48 States, 1900–2002: West Versus EastThis figure shows the length of the growing season in the western and eastern United States compared with a long-term average. The trend line was smoothed using an 11-year moving average. Choosing a different long-term average for comparison would not change the shape of the trends.

Year

Devi

atio

n fro

m a

vera

ge (d

ays)

-15

-10

-5

0

5

10

15

20

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

West

East

Long-term average

Data source: Kunkel, 20096

60

Weather & Climate of Virginia

Robert E. Davis Professor of Climatology

Dept. of Environmental Sciences University of Virginia

Email: red3u@virginia.edu

Rivanna Master Naturalists March 17, 2015