The most direct way to characterize macroclimate is to consider how each of its major components – temperature, precipitation, and light have behaved “historically” at least over the past 5 to 10 years. The National Weather Service and other organizations collect raw climatological data and make summaries available to the public regarding high, low and average temperatures, rainfall, number of frost free days, date of first and last frost, etc. Keep in mind that conditions at your particular site may vary from regional averages based on factors affecting microclimate. Nonetheless, regional climate summaries from selected weather stations near you can be useful in the process of crop selection. (1)

1. Hardiness Zone
Hardiness Zones are often one of the first elements evaluated when growing crops, and can often be used to generalize the basic climate for an area.

Defined as the “average annual minimum temperature” for an area, hardiness zones are delineated bands of 10˚F minimum temperatures (often subdivided into 5˚F bands). Plants have a cold threshold, which if surpassed will kill the plant. Hardiness zones provide guidelines for successful plant production. Many plants also have maximum temperature zones, which if surpassed could also kill the plant or decrease production. Each species or variety should have a statement such as: Grows best in zones 4-8.

The USDA National Arboretum houses the official Plant Hardiness Zone Map, and most plants have established growing zones.

2. Frost Free Days
Frost Free Days refers to the average number of days your area has between last frost in the spring and first frost in the fall. FFDs are important since many plants do not really begin to grow until after the last frost and can be killed, or prevented from ripening, after the first frost.

Here you can find first frost, last frost and average frost free days maps for New York.

Keep in mind that these are averages and that frosts can occur earlier in the fall and later in the spring than what is stated.

Use the worksheet at the top of this page to enter your Frost Free Days for your location.

3. Growing Degree Days (CU Entomolgy and Gardening Web Sites)

Growing degree-days (GDDs) are a measure of heat accumulation during a growing season. Because many developmental events of insects and plants are dependent on the accumulation of specific quantities of heat, we can predict when these events should occur during a growing season regardless of differences in temperatures from year to year. This method is much better than using a calendar for predicting such events because of the large differences in temperatures that occur from one year to the next.

How are growing degree-days used?
By comparing the accumulated GDDs during a growing season with tabulated values for plant growth or pests of interest, we can better predict stages of plants and insects each year.

This is very helpful in pest management because it allows us to use our time more efficiently. For example, if we know that a particular pest should appear after 1000 GDDs and we are currently at 500 GDDs, we can spend our time on things other than scouting for that pest. GDD values have been tabulated for many landscape pests.

How are growing degree-days measured?
Growing degree days accumulate anytime the average temperature for the day is more than 50 F. For example, if the high for the day was 70 F and the low was 40 F, the average temperature was 55, and so 5 GDDs accumulated.

GDD measurements start March 1, and the GDDs for each day are added to the previous total. (When the average temperature for the day is below 50 F, it is ignored. It is not subtracted from the total.)

For those who prefer formulas, it looks like this:

Max Temp. + Min. Temp.
50 = Daily GDD

50 F is used as the base for these GDDs because this is about the temperature that woody plants in the Northeast begin growth. Scientists may use other base temperatures for GDDs in other regions or for other purposes, for example to predict the growth of heat-loving corn plants.

There are ways to measure GDDs automatically. A number of companies manufacture weather stations that can measure GDDs for you, which has the advantage of measuring heat accumulation in your immediate vicinity. These units can be purchased for under $300. A number of newspapers and other media outlets post GDDs, and of course there are a few web sites that relay the information:

4. Rainfall
Plants need three things to grow: Sunlight, Soil, and Water. We will discuss soils and sunlight later on, but water, or more accurately available water, is often what will make or break a farm any given year. Without water, the plants just won't grow.

Many of us rely on rainfall as a primary irrigation source, either as moisture direct to our plants or to refill shallow water supply sources. And, while rainfall can be somewhat predictable, we can go from floods to drought in a relatively short amount of time. Average rainfall calculations can let us know if we need to be considering additional irrigation or retention ponds to make our farm successful. Plants and animals have minimum water requirements that can be calculated for the year. Rainfall can go a long way to meeting that requirement.

Look here for a table of average precipitation for sites across the US

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