Freeze Climatology for 32 Sites in Northern New York, Central and Northern Vermont

METHODOLOGY
Seasonal dates of the first and last freezing temperature (at or below 32^oF) over a 30-year period are analyzed using a percentile rank analysis for each surface observing site. Locations include first-order reporting stations, such as Burlington International Airport, and many long-running cooperative observing stations spread across the North Country. These data are important for agricultural purposes.

How to read the box and whisker graphs: Box and whisker diagrams are employed to view the data. An example of the diagram format is shown at left. The shaded region of each graph (i.e., "the box") shows the middle 50% of the range of first/last freeze dates. The top of the box is the 75th percentile and the bottom is the 25th percentile. The solid line within the box shows the median (or 50th percentile) first/last freeze date for that location. The lines extending upward and downward from the box (i.e., "the whiskers") reach to the 10th and 90th percentile of the data distribution. That is, only 10% of the data lies above and below the ends of the whiskers. Lastly, the "x" indicates the extreme (record) freeze date during the 30-year period of record for that particular location. The intent of the box and whisker format is to give a comprehensive view of the range of first and last freeze dates at individual sites, and facilitate comparison between different locations around WFO Burlington's forecast area (see Banacos 2011 for more info). For additional freeze climatology discussion, see text accompanying each freeze region below.


OVERVIEW (survey of sites within Burlington's forecast area) ST. LAWRENCE VALLEY NORTHERN ADIRONDACKS CHAMPLAIN VALLEY CENTRAL VERMONT NORTHEAST VERMONT

NORTH COUNTRY OVERVIEW
The varied topography of the North Country includes two major mountainous areas (the Adirondacks and Green Mountains), Lake Champlain, river basins both wide and narrow, and smaller, sheltered valleys of varying elevation. Topographic effects contribute largely to the high spatial variability in the dates of first and last freeze, evident in the graphs.

In a typical season across the forecast area, first and last freezes occur in three stages, based on factors related to how well a site is able to radiatively cool on favorably calm, dry, and clear nights during the transition months in the spring and late summer/early fall. This is determined largely by local terrain, but other considerations such as land surface/soil type, and the potential mitigating effect of nocturnal fog - more frequent in some areas - also plays a modulating role. Freeze occurrence in the late summer/early fall typically progresses as follows: from the smaller, sheltered valleys of the Adirondacks and Northeast Vermont, across areas removed from the moderating influence of Lake Champlain, at sites very near Lake Champlain. The last freeze in spring occurs in reverse order. Despite inherent small-scale variability, some generalities can be made from the climatology, especially when divided into regions of similar topography. * Composite Freeze Poster *

SPRING	FALL

ST. LAWRENCE VALLEY
As a relatively wide and flat region (at elevation generally between 200-450 ft.), the St. Lawrence valley tends to have higher surface winds and experiences boundary layer "decoupling" later at night than the adjacent valleys within the Adirondacks. Thus, the potential for downward turbulent mixing of warmer air through the surface/radiative inversion tends to be greater during the night, having the effect of slowing the overall rate of cooling. The climatology shows that this can be sufficient to avoid a freeze with marginal air masses that produce freezes in the Adirondacks or Northeast Vermont. First freeze dates generally stretch from the third week of September into the first week of October, with the earlier freezes occurring away from the immediate area of the river itself (e.g., at Gouverneur and Canton). The unfavorably wet pattern of early October 2005 delayed the first freeze in this region until the night of October 20th/21st. The last freeze of spring will typically occur during early May in locations along the St. Lawrence River, but up to 2.5 weeks later at sites removed from the river and on the fringe of the Northern Adirondacks (e.g., Gouverneur).

SPRING	FALL

NORTHERN ADIRONDACKS
A region of complex terrain consisting of high peaks (many in excess of 4000 ft., with Mt. Marcy at 5344 ft.) and narrow, steep valleys. Valley floors in the region are of relatively high elevation, above 1000 ft. The Lake Placid cooperative observer site is at an elevation of 1940 ft. These sheltered, "elevated valley" sites tend to decouple quickly and radiate effectively at night when weather conditions are favorable. This results in relatively early first (late last) freeze dates, rivaling sheltered valleys of Northeast Vermont. Also, similar to Northeast Vermont, rare mid-summer freezes have been recorded during the period (e.g., 7/2/92 at Lake Placid, 7/10/83 at Tupper Lake, and 7/28/01 at the KSLK ASOS). The distribution of first freezes displays a slight skewness toward earlier dates at 4 of the 5 analyzed stations, suggesting the effectiveness of the radiative cooling in the region. The ASOS at Saranac Lake - a noted "cold spot" in the Burlington forecast area - has a median first freeze of September 7th, based on the past 11 years of data from the site. In this region, the first freeze is rarely delayed into October, and last freezes can occur into early June.

SPRING	FALL

CHAMPLAIN VALLEY
This region has the longest growing season owing largely to the moderating influence of Lake Champlain, and also its low elevation. The long north-south valley orientation, and steep terrain at its limits, tends to result in higher surface winds compared to smaller adjacent valleys. As a result, the boundary layer tends to decouple more slowly on clear nights. The time of median first freeze is progressively delayed at sites closer to the lake (and particularly at South Hero, situated on the Champlain Islands). The relatively warm lake also limits the strength of the nocturnal inversion, allowing for downward turbulent mixing of warmer air through the nocturnal inversion, slowing the cooling rate. This effect is also evident during the spring. However, because the lake temperatures are closer to freezing in the spring, the differences are less pronounced, with more overlap in the data between Burlington and South Hero for the last freeze as opposed to the first. First freeze during the fall typically will occur from the last few days of September into the first two weeks of October. At South Hero, the first freeze is typically closer to mid October. In the spring, the last freeze at South Hero is typical between the last week of April and the first week of May. Elsewhere, early to mid May is normal.

SPRING	FALL

CENTRAL VERMONT
This region is more arbitrarily defined, but includes most of Vermont, not appreciably influenced by Lake Champlain (Northeast Vermont is considered separately). Terrain is highly variable, with several mountain ranges, steep river valleys, and a few sheltered valleys. Central Vermont is generally a more moderate freeze region than Northeast Vermont, with first freezes typical between September 18th and October 7th. Some earlier freezes are possible in isolated/sheltered valley locations. The wet weather pattern of October 2005 delayed the first freeze until the night of October 20th/21st uniformly across this region. The last freeze of spring is typical between the second and third week of May, but does extend into early June in some years.

SPRING	FALL

NORTHEAST VERMONT
Northeast Vermont displays high spatial variability between stations situated in smaller, more sheltered valleys (e.g., West Burke) as compared to larger river basins (e.g., St. Johnsbury). Sheltered locations have recorded rare freezes during the summer months, such as occurred at Canaan, Island Pond, and West Burke on July 2nd, 1992. The first freeze can occur as late as mid October in Newport and St. Johnsbury. First freeze typically occurs in two stages across this region, first in the sheltered valleys between the last few days of August and mid-September, and then in the larger basins during the last week of September into the first week of October. The propensity for nocturnal/valley fog in the larger river basins may also contribute to the delayed freezes in portions of the Upper Connecticut and Passumpsic River basins. The last freeze typically occurs between the second and third week of May at Newport and St. Johnsbury, and from the last few days of May into early June in the smaller valleys.

SPRING	FALL

Questions pertaining to this resource may be referred to peter.banacos@noaa.gov.

REFERENCE

Banacos, P. C., 2011: Box and whisker plots for local climate datasets: Interpretation and creation using Excel 2007/2010. NWS Eastern Region Technical Attachment No. 2011-01, National Weather Service, Eastern Region Headquarters, Bohemia, NY, 20 pp.