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Weather & Climate

Cornell Climate Change – a gateway to initiatives, research, public engagement and more. In the gardening section find our Gardening in a Warming World materials. Also check out tools on Climate Smart Farming.

 

Weather
Growing degree days
Climate
Plant hardiness zone
Average last spring frost date
> Average first fall frost date
Average freeze-free season
Microclimates

Weather

Weather is the state of the atmosphere at a specific time and place. It is the short-term variations of the atmosphere (minutes to weeks). Weather is often referred to in terms of brightness, cloudiness, humidity, precipitation, temperature, visibility, and wind. We commonly talk about the weather in terms of “What will it be like today?”, “How hot is it right now?” and “When will that snow storm hit our town?” All of earth’s weather depends on the interaction of the sun, which heats the earth and causes air to move as winds; and water, which moves between the oceans, skies, and land forming clouds and precipitation. Explore at New York State weather station data. Also check out CoCoRaHS – the citizen network of weather watchers.

Growing degree days

The calculation of growing degree days (GDD) or the “heat” accumulated during each growing season involves a comparison of daily maximum and minimum temperatures to a lower and upper base temperature for growth of an organism. The GDD concept may seem very familiar to you. It’s one way of tracking phenological events — periodic biological phenomena — that are timed by the return and accumulation of warm weather during the growing season, not strictly by calendar date. Japanese beetles, for example, do not just wake up one morning and say, “Ooops. June 15th already. Time to get out there.” Rather, their emergence is determined by the accumulation of growing degree days. They make an earlier appearance when spring is warm and GDDs accumulate early, and appear later when it’s cool. You may already time some of your garden routines by phenological events you observe around you, such as bud break and flowering of lilacs. Being aware of these events as well as growing degree days can help make you a more observant gardener. Phenology can have many uses in gardening and farming. In addition to insect management, phenology can be used to time crop planting, designing flower beds and orchards for sequential bloom or fruit maturity, planning refuge plantings for beneficial insects, and even tracking global warming trends. Find growing degree data at Network for Environment and Weather Applications.

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Climate

Climate is the prevalent long-term weather conditions in a particular place. Generally, climate is considered the weather in a place over a period of 30+ years. Climatic elements include precipitation, temperature, humidity, sunshine and wind velocity, and phenomena such as fog, frost and snow. NOAA’s Climate Normal Data Tools

Plant hardiness zone

Plant hardiness zone information helps you determine which herbaceous perennials and woody trees, shrubs and vines will survive winters where you garden. Look for zone information on plant tags when you purchase perennial plants, or ask the nursery staff if you don’t see it listed. This information is standard in plant catalogs and gardening and landscaping publications, though you may see differences of a zone or so depending on the source. Hardiness zones are based on the average annual minimum temperatures. For example, if you garden in Zone 5a, this suggests you can expect that your low winter temperature may drop as low as -15 F and -20 F. The maps aren’t foolproof, but can provide you with an estimate. Your site may be a zone higher or lower due to inaccuracies in the map and microclimate impacts. For example, if you are near a large body of water, such as one of the Finger Lakes, the water may moderate air temperatures nearby and make your site a zone warmer. If you are in a valley where cold air settles during clear winter nights when radiational cooling occurs, you may be a zone colder. Urban areas are often a zone warmer than surrounding rural areas. A protected spot on the south side of a house or building may actually be a zone warmer than an exposed spot on the north side.

Here is more information to help you consider which plants will survive and thrive where you garden:

    • In general, healthy plants that are well-matched to their site (correct sun exposure, healthy soil, good drainage, suitable pH range, etc.) are more likely to overwinter well.
    • Plants usually survive well if temperatures cool gradually in fall and early winter, allowing the plant to harden off.
    • Do not apply nitrogen fertilizer to plants late in the season as it may cause them to put on a flush of growth that will not harden off properly and stress the plant. If soils and plants are low in potassium, late-season applications may help overwintering.
    • Plants in containers or other highly artificial environments are much less likely to overwinter, compared to the same plants properly planted in the ground.
    • The moderating effects of snow cover on soil temperature may help some marginal plants overwinter. Protecting plants with a thick layer of mulch may also help.
    • Prolonged low temperatures may be more damaging than a single cold night.
    • Marginal evergreens can benefit from shade during winter, and may actually survive better on the north side of buildings.

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Average last spring frost date

Average Last Spring Frost Date

The map shows the average date of the last frost in spring. Use it to help plan when you will transplant plants that are not frost-tolerant. Keep in mind that the dates are guides, and no guarantee that frost will not occur after the last date shown. In fact, chances are that in half the years, you will experience frost after the average last frost date. NOAA’s freeze/frost probability tables

This map does not tell you the best time to plant tomatoes or other frost sensitive crops. Wait until several weeks after the last frost, after the soil has warmed and nighttime temperatures are reliably well above freezing. Also remember that microclimates have a tremendous effect on frost. Some valleys and low-lying areas can be plagued by late frosts well after the last spring frost date on cold nights when there is radiational cooling. Large bodies of water may moderate air temperatures at some sites, reducing the chances of frost.

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Average first fall frost date

Average First Fall Frost Date

The map shows the average date of the first frost in fall. Use it to help plan when to plant late-season or fall crops, and when you can expect frost-sensitive plants to finally succumb to freezing temperatures if not covered with sufficient protection. Keep in mind that the dates are guides, and no guarantee that frost will not occur before the first date shown. Also remember that microclimates have a tremendous effect on frost. Some valleys and low-lying areas can be plagued by early frosts well before the first frost date on cold nights when there is radiational cooling. Large bodies of water may moderate air temperatures at some sites, reducing the chances of frost. NOAA’s freeze/frost probability tables

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Average freeze-free season

Frost Free Season Map

The map shows the average freeze free period between the last frost in spring and the first frost in fall. Use it to help decide which varieties or cultivars of frost-sensitive crops to grow. Keep in mind that the dates are guides, and no guarantee that your frost free period will always fall in the range indicated by the map. Also remember that microclimates have a tremendous effect on frost. Some valleys and low-lying areas can be plagued by late frosts and early frosts on cold nights when there is radiational cooling. Large bodies of water may moderate air temperatures at some sites, reducing the chances of frost. NOAA’s freeze/frost probability tables

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Microclimates

We garden in microclimates, not hardiness zones. A microclimate is the climate of a small area that is different from the area around it. It may be warmer or colder, wetter or drier, or more or less prone to frosts. Microclimates may be quite small – a protected courtyard next to a building, for example, that is warmer than an exposed field nearby. Or a microclimate may be extensive – a band extending several miles inland from a large body of water that moderates temperatures. When you use hardiness zone and frost maps, you need to be aware that your microclimate may make where you garden very different from the information found on the maps. If you are in a cold valley, your minimum winter temperatures may be lower than what the map indicates. As a result, you may actually be in a hardiness zone that is colder than that shown on the map, and some marginal plants may not survive your winters. Cold valleys may also be prone to late spring frosts and early fall frosts, making your growing season shorter than indicated by the frost-free season map. If you put out tender plants too early, you might lose them. If you plant long-season heat-loving plants, they may not mature before fall frost.

Large-scale microclimates

Some microclimates are much larger, extending for miles because of the effects of:

      • Large bodies of water, such as the Great Lakes, the Finger Lakes, Lake Champlain, Long Island Sound and the Atlantic Ocean, tend to moderate air temperatures of adjacent inland areas. Low temperatures in winter are not as extreme, and these areas are less prone to late spring and early fall frosts. Smaller bodies of water also have the same effect, usually to a lesser extent.
      • Urban areas tend to have less extreme low temperatures than the surrounding countryside. Buildings and paved surfaces absorb heat during the day, then radiate it back into the air at night, reducing the chances of frost and moderating low temperatures during winter. Buildings also offer protection from wind in many places. Urban areas may be a full Hardiness Zone warmer than rural areas just a few miles away. These warming effects carry over into summer, as well. Urban microclimates can trap heat, creating a scorching environment that can damage plants.
      • Topography has a profound effect on microclimates. Cold air is heavier than warm air. So on cold winter nights or nights when frost threatens, the cold air flows downhill and collects in low spots — just like water flows down hill and collects in puddles. On winter nights, some valleys may be 10 degrees or more colder than neighboring slopes. These valleys may also be more prone to frost. Hilltops may not suffer as much from frost or cold temperatures on nights with radiational cooling. But if they are exposed, winter winds can often wreak havoc. Winds dry out plants, and are particularly hard on evergreens, which cannot replace moisture lost through their needles or leaves when the ground is frozen. The slopes between cold valleys and windy hilltops can have different microclimates depending on their aspect (which direction they slope). North-facing slopes are slow to warm up in spring because they receive less direct sun, compared with south-facing slopes. But gardening on a south-facing slope can be a mixed blessing, especially when early spring warmth causes plants (fruit trees in particular) to begin flowering prematurely, only to have the blossoms killed by a sudden frost.

Microclimates in your yard

There is little that you can do to affect these large-scale microclimates, other than to be aware of them and let them guide your plant selection and timing. But you can find very similar microclimate effects at work even in the smallest yard, and you can take advantage of them and even modify them to a certain extent.

      • Your house and other buildings create many microclimates around your yard. Just like urban areas, your house absorbs heat during the day and radiates it back at night. If your prevailing winds are from the northwest, this creates a warmer, more sheltered microclimate on the south and east sides of your house. While the north side of your house may receive harsh winds and no sun during the winter, keep in mind that in summer – when the sun rises north of east and sets north of west – these areas can be baked by heat and dried out by the same prevailing winds. Keep in mind, too, that when wind hits your house, it creates turbulence and higher wind speeds along the wall and as the wind goes around the corners of the building. These areas may not be good places to plant broad-leaved evergreens or other plants that can be easily dried out by winds. Bark on young trees planted on the south or southwest sides of buildings are more prone to cracking in winter.
      • Balconies and rooftops — because they are above ground level — may escape frosts that kill tender plants at ground level on nights with radiational cooling. But cold, drying winds may be an even bigger factor depending on the location, orientation and exposure of the balcony or rooftop.
      • Fences, walls and large rocks can protect plants from wind and radiate heat, creating sheltered spots. Sometimes, if fences block cold air drainage through your property, the cold air can puddle behind them causing very localized frost damage on near-freezing nights.
      • Raised beds and terraces — like hillside slopes — can warm and drain earlier in spring, especially if they are oriented toward the south.
      • Paved surfaces – such as patios, driveways and sidewalks – can absorb heat and reradiate it at night, moderating night-time temperatures. Such impervious areas can’t absorb water, and may create wet spots if the water that flows off of them is concentrated in one area. Watch for similar wet areas where water flows off roofs or out of downspouts. Buildings can also create “rain shadows” on the lee side of a house if rains are accompanied by winds.
      • Trees can also prevent rain from reaching the ground. That, coupled with competition for water and nutrients from the tree’s roots, may make it difficult to grow less-competitive plants around the base of larger trees.
      • Soil types can also affect frost. Heavy clay soils can act much like paved surfaces, moderating the temperature near ground level. Lighter soils that have many air pockets in them can act as an insulating layer on top of warmer subsoils, trapping that heat below ground and are hence more prone to frosts at ground level.

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