Weather Forecasts and Climate Information for Landscape Operations

Prepared by:
Katharine B. Perry
Department Extension Leader
Horticultural Science

Published by: North Carolina Cooperative Extension Service

Publication Number: AG-508-8

Last Electronic Revision: March 1996 (JWM)

Including weather and climate information in daily decision making can reduce the impact of landscape practices on water quality and improve water-use efficiency. For example, before irrigation, you should know the probability of rain for the next ftre days and base the irrigation amount and timing on theforecast. You should also consider the forecast before applyingfertilizers or pesticides. If a heavy downpour occurs soon after applyingfertilizer or pesticides, much of the material will run off the target area which keeps it from helping the landscape and may move it into waterways. The climate and microclimate of a landscape, however, should first be considered when plants are chosen.

Understanding Weather

The terms weather and climate are often used interchangeably, but actually have unique meanings. Weather refers to the current state of the atmosphere, such as cloud cover, temperature, relative humidity, wind speed and direction, solar radiation, and dew point. Forecasts are made for future weather. Climate refers to the average or normal weather of a particular location for a specified period of time, usually 30 years.

Basic Weather Principles

Understanding the following basic weather principles, or meteorology, will make weather and climate information more useful to you:

Heat transfer Heat energy in a substance is the energy of motion of the molecules of that substance. Heat may be transferred from one material to another or from one place to another by conduction, convection, or radiation. When the end of a metal rod is warmed from energy originating from the other end, the process of heat transfer along the rod is conduction. Convection is the transfer of heat by the movement of masses of heated liquid or gas. In the atmosphere, warm air from the soil surface becomes less dense, rises, and is replaced by cooler air from above. The mixing of these currents of warmer and cooler air is an example of convection. Radiation is the movement of heat energy from one object to another without a connecting medium.
Energy Exchange All three heat-transfer mechanisms are occurring in our atmosphere. However, one mechanism may dominate at different times. During the day, the sun's radiant energy warms the soil and other solid objects, such as plants. When these objects become warmer than the air, they pass heat to the air by conduction and set up convective currents that warm the lower atmosphere. The surface and plants also radiate heat energy. Water in the atmosphere, some of which can be seen as clouds, and CO2 absorb or reflect some of this radiated energy, trapping it near the earth's surface, which is known as the greenhouse effect. At night the situation reverses. There is no incoming radiant heat to warm the soil and plants. The soil and plants continue to lose heat through radiation and conduction until they are cooler than the surrounding air. The air then conducts heat to the soil and plants, and the lower atmosphere cools. If no water is present in the atmosphere to block the outgoing radiation, the soil, plants, and subsequently, air continue to cool. On a clear night, the heat continues to radiate into space. Temperatures drop significantly, and cool air collects at the surface. The temperature profile in the lower tens to hundreds of feet of atmosphere inverts, that is, the temperature increases with altitude to the top of the air layer. The term for this, inversion, comes from atmospheric conditions being inverse to the normal daytime condition where air temperature decreases with height.
Microclimate Microclimate is the climate of a small area. Factors that alter the incoming solar radiation have the most obvious effect on microclimate, especially temperature. These can be atmospheric factors, such as clouds, or landscape factors, such as location or aspect relative to the sun, shade trees, or hardscape features. Wind also affects microclimate. Windbreaks, either living or non-living increase the moisture-use efficiency of the area downwind. Terrain contributes to microclimatic differences by creating frost pockets or cold spots formed by cold air drainage. Cold, dense air flows by gravity and collects in the lowest parts of an area. This causes temperatures to differ in relatively small areas. Soil color, moisture, and compaction can have a significant effect on microclimate. A dark, moist, compact soil stores more heat during the day than a light, loose, dry soil. Thus, it will transfer more heat to the landscape at night.
Landscape plants themselves have an effect on microclimate. Surfaces covered by plants reflect more solar radiation during the day than those without plants. Plants transpire to keep the temperature below damaging levels. Therefore, the heat stored in plants and heat stored in soil below them is reduced. The publication Landscaping to Protect Water Quality: How to Plan S Design a Wise-Water-Use Landscape (AG-508-2), which is available from your county Cooperative Extension Center, provides more information on this topic.

Low Temperatures

Landscape operators must be concerned with potentially damaging low temperatures. Although the terms frost andfreeze are often interchanged, they describe two distinct phenomena. An advective, or windborne, freeze occurs when a cold-air mass moves into an area, bringing freezing temperatures. Wind speeds are usually more than 5 mph and clouds may be present. The thickness of the cold air layer ranges from 500 to 5,000 feet or more. Options for protecting plants by modifying the environment are very limited under these conditions. A radiationfrost occurs when a clear sky and calm winds (less than 5 mph) allow an inversion to develop and temperatures near the surface drop below freezing. The thickness of the inversion layer varies from 30 to 200 feet. Frost forms on solid objects when the water vapor in the atmosphere changes from its vapor phase to small ice crystals. Frost is not frozen dew!

Most landscape operators are familiar with the United States Department of Agriculture Plant Hardiness Zone Map. It is used by growers to determine when and where to ship plants and by installers to guide plant choices for specific landscapes. In 1990, the United States National Arboretum updated the map to make it more precise. Full-size copies (48 inches by 48 inches) are available for $6.50 each from the Superintendent of Documents, Government Printing Office, P.O. Box 371954, Pittsburgh, PA 15250-7954. Order Miscellaneous Publication 1475, stock number 001-000-04550-4. The Government Printing Office can be reached at (202) 512-1800. A portion of the map including North Carolina is included here (Figure 1).

Forecasting Weather

The National Weather Service issues forecasts of air temperature, sky conditions, and precipitation for 104 forecast zones in North Carolina for the coming three consecutive 12-hour periods. For example, a forecast is issued at 4:30 A.M. for the periods 4:30 A.M. to 4:30 P.M. (called today), 4:30 P.M. to 4:30 A.M. (tonight), and 4:30 A.M. to 4:30 P.M. the next day (tomorrow). Wind speedand wind direction are included for the first two periods but not the third. These forecasts are updated eery six hours. A five-day outlook is issued twice a day, predicting temperatures, sky conditions, and precipitation probabilities on a regional basis. Improvements in understanding the global climate system have changed how the National Weather Service (NWS) provides monthly and seasonal weather outlooks. In January 1995 the NWS began the all-electronic publication entitled Climate Outlook. It is accessible on the Internet home page of the Climate Prediction Center at the address http://ic.fB4.noaa.gov. If you do not use the Internet, you can contact the State Climate Office for this information. The Climate Outlook provides seasonal forecasts at leads of two weeks to about one year ahead. The lead time is the time between issuance of the forecast and the first moment it becomes valid. Forecasts issued include seasonal mean temperature and total precipitation for all of the United States.

The North Carolina Agricultural Weather Program of the Cooperative Extension Service, in cooperation with the Agricultural Weather Service Center of the National Weather Service, provides the following specific products for North Carolina that are useful for landscaping:

eight-day forecast of maximum and minimum air temperature, probability of precipitation, percentage of opaque cloud cover, and wind speed;
zone forecasts of maximum and minimum air temperature, probability of precipitation, sky conditions, and wind speed;
statewide five-day forecast;
statewide weather summary;
five-day forecast of maximum and minimum air temperature;
five-day rain probabilities and outlook;
solar radiation and atmospheric moisture;
five-day forecast of hourly air temperature, dewpoint temperature, wet-bulb temperature, relative humidity, wind speed and direction, and sky conditions;
two-day forecast of dew amount, dew duration, evaporation and lowest relative humidity; and
five-day forecast of hourly air temperature.

This weather information is electronically delivered every day to county Cooperative Extension Centers. Contact your county Center to determine how you can get this information. Private companies, such as The Weather Channel and television and radio stations, provide forecasts for the public. Other companies provide forecasts for specific clients' needs.

Microclimate Monitoring

Although microclimates may vary across a forecast zone, the relationship between the forecast and what actually occurs in the microclimate area is fairly consistent. Therefore, it is helpful to record weather observations in the particular landscape area of interest. You can observe and record actual and forecast temperature, cloud cover, and wind speed. In cloudy, breezy weather, observations are likely to be very close to forecasted values, but under clear, calm conditions, observations may be different. Analysis of past observations can become an essential ingredient in predicting future conditions and modifying the zone forecast for a particular landscape. However, this does not hold true for prediction of rainfall occurrence and amount from thunderstorms.

For more information contact an agent at your County Extension Center and ask for Horticulture Information Leaflet No. 705-A, Frost/Freeze Protection for Horticultural Crops, and consult the references listed at the end of this chapter.

Climate Data

The best source of climate data is the State Climate Office (SCO). The SCO is in the Department of Marine, Earth, and Atmospheric Sciences at North Carolina State University. To contact this office, call (919) 515-3056 or write to the State Climatologist, NCSU, MEAS Department, Box 8208, Raleigh, NC 27695-8208. Personnel at this office can provide daily maximum and minimum temperatures and precipitation for 156 stations in the North Carolina Climate Observer network, which is administered by the National Weather Service. The data are collected, quality controlled, and published by the National Climatic Data Center, Federal Building, Asheville, NC 28801; (704) 271-4800. Because the processes of collection, control, and publishing are time consuming, it takes approximately three to five months for you to receive data from this system. The SCO does, however, have access to a smaller number of stations from which information is available more rapidly, but the quality is not as rigorously controlled.

Data other than daily air temperature and precipitation are more limited. Wind speed and direction and relative humidity are available on a three-hour interval from the SCO for Raleigh-Durham, Greensboro, Asheville, Charlotte, Wilmington, and Cape Hatteras. Daily minutes of sunshine data are also available from these stations. Daily solar-radiation data are available for Asheville, Charlotte, Cherry Point, and Raleigh-Durham. Daily pan-evaporation data can be obtained from the SCO. The data are observed at W. Kerr Scott Reservoir, Chapel Hill, Hofmann Forest, and Aurora. Estimated pan-evaporation "normals" (1951-80) have been developed for Asheville, Raleigh-Durham, Charlotte, and Wilmington, because not enough years of actual data have been collected yet to produce an actual "normal."

In addition, an automated network in North Carolina collects hourly air and soil temperature, baromet ric pressure, solar radiation, near-infrared radiation, photosynthetically active radiation, wind speed and direction, rainfall, and dew point or relative humidity. The stations in this network are located at Experiment Stations of North Carolina State University and the North Carolina Department of Agnculture in Castle Hayne, Clayton, Clinton, Fletcher, Jackson Springs, Kinston, Lewiston, Oxford, Plymouth, Raleigh, Rocky Mount, Salisbury, Waynesville, and Whiteville. The resulting data set has periods of missing data due to breakdowns of the electronics involved. This network is currently being updated with data-acquisition systems that will be more dependable. This data set can be accessed by contacting the SCO.

There are published references for climate data as well. It is very convenient to have these references on hand to access climate data quickly. Some references available from your county Cooperative Extension Center include:

AG-375 - Weather and Climate in North Carolina ($2.50)

AG4ll - Probabilities of Dry Periods in North Carolina ($2.00)

AG-403 - Risk of Frost and Freeze Damagefor North Carolina Fruit Crops (free)

AG-236 - Growing Degree Days in North Carolina ($3.00)

Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

AG-508-8

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