AWIS Freeze/Frost Fact Sheets
by Karl Harker
AWIS Weather Services, Inc.
You Be Ready for the Next Killer Freeze?
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You and Your Forecast
Nobody can control the weather and killer freezes are
a fact of every producer's life. A good freeze/frost
forecast delivers the information you need to prepare
for these events. It saves you endless hours of channel
surfing, radio listening, and web scanning. A complete
freeze-frost package tells you when cold events are
coming and how cold it will get. It should also included
the duration of the cold event, the strength of any
inversions, wet bulb temperatures, and wind speeds.
These parameters allow you to decide how to best protect
your crops. Our forecast service gives you the information
you need to protect your operation. AWIS meteorologists
call it straight and don't cry wolf, saving you time
Irrigation Forecast temperatures,
wind speeds, and wet bulb temperatures will help you
apply water and determine its effectiveness. Wind speeds
in excess of 5 mph for tree crops and 10 mph for low
growing crops promote evaporative cooling and limit
the effectiveness of irrigation. Wet
bulb temperature is the best indicator of evaporative
cooling. Irrigation should be applied before wet bulb
temperatures reach critical temperatures. A forecast
of the length of the freeze event helps you determine
if you have sufficient water to protect plants. Insufficient
water may cause devastating losses because of evaporative
cooling. During long cold events, ice accumulation may
also cause damage.
Row Covers and Tree Wraps Forecast
temperatures and cold event durations will tell you
how effective these measures will be. Row covers and
tree wraps work by delaying, but not preventing, heat
loss and are most effective during freezes of short
duration. They are less effective during freezes when
temperatures decrease slowly and remain low for long
periods of time. Eventually, temperatures under wraps
and covers will approach air temperature. The thicker
(more insulating) the material is, the better the protection.
Heaters Forecast inversion
strength and wind speeds are critical for heater use.
Heaters are far more effective when inversions are present.
The hot gases emitted from the top of the stack stir
the air and mix the warm air above with the cool air
below. A much smaller portion of the energy is released
as radiation from the hot metal stack. Because this
heat will reach any solid object in line-of-site, it
provides some protection when wind speeds are higher
and inversions are weak.
Wind Machines and Helicopters
Inversion strength is critical. A temperature inversion
of at least 5 F difference is necessary and an inversion
of 10-15 F makes these cold protection methods very
effective. Wind machines and helicopters should be started
when air temperatures are two or three degrees above
Wet Bulb Temperature
- What It Is and How to Use It
What is Web Bulb Temperature?
The wet bulb temperature is the temperature air cools
to when water is added. We are all familiar with this
phenomenon. You feel colder when you are 40 F and wet,
than when you are at 40F and dry. A sling psychrometer
is used to measure wet bulb temperatures. It consists
of two thermometers, one measures the air temperature
and the other measures web bulb temperature. The bulb
of the web bulb thermometer is covered with a wet cotton
sock. The sling psychrometer is swung in the open until
the wet bulb temperature drops no further. Automated
weather stations report wet bulb temperatures indirectly.
It is usually calculated from air temperature and relative
How Do You Use Wet Bulb Temperature?
Many growers use water to protect crops from freezing
temperatures. It works because when water freezes on
plants it releases small amounts of heat. This heat
keeps plant temperatures at 32 F, even when air temperatures
are colder. What do wet bulb temperatures add to the
picture? Wet bulb temperature is almost always lower
than air temperature. In many cases it is considerably
lower than air temperature. When water is applied to
a crop, plant surface temperature initially drops to
the wet bulb temperature, even when air temperatures
are well above freezing. If the wet bulb temperature
is 25 F, tender buds, growing points, fruits and shoots
will experience 25 F. How long the temperature remains
there depends on the speed of ice formation. This could
range from minutes to hours. This initial 'cold jolt'
can cause damage even when there is good ice formation.
The Bottom Line
Wet bulb temperature should be used to determine when
to turn on and off water used for cold protection. Water
should be turned on before the web bulb temperature
reaches 32 F and turned off when it is about 32 F. The
figure illustrates observed wet and dry bulb temperature
in a cold event in Florida. The arrows indicate when
the water should have been turned on and off. If a grower
had waited until the air temperature reached 32 F, the
plant surfaces would have dropped to 27 F. In this case,
use of the wet bulb temperature extended the protection
period by nine hours and eliminated the 'cold jolt'.
You've probably heard that when La
Nina conditions prevail winters are typically drier
and warmer than normal. Does that mean that you won't
have a killer freeze? Most certainly not! Freeze events
can and often do occur during La Nina winters.
Consider this, La Nina winters normally
result in colder and wetter than normal conditions over
the Northern Plains states. That increases the possibility
of a deeper than normal snow pack. A brief change in
the upper atmosphere can easily bring down a shot of
arctic air that has built up over the snowy/icy Northern
Plains. That means a sudden temperature drop, a classic
killer-freeze interrupts our nice warm, dry winter.
More importantly, freezes that occur
during La Nina winters are often capable of inflicting
more crop damage than similar freeze events occurring
in other years. Crops don't harden-off in mild winters
making them more susceptible to freeze damage.
Drier than normal soils in La Nina
years also increase the risk of freezes for ground crops.
Dry soils radiate heat more rapidly, thus resulting
in colder temperatures at the ground surface when compared
with wet soils. Just a few degrees difference around
32 F can make or break a crop.
A wise grower will keep these facts
in mind as he prepares for a La Nina winter. Enjoy the
weather, but keep your eye on it! Or better yet, get
help from a professional forecasting service.
Adjust the Forecast for your Terrain
Even a great forecast
may need some adjustment for the particulars of your
location. This adjustment can mean the difference
between cold damage and no damage. All Ag producers
concerned with cold damage do this instinctively.
Here are a few tips that might fine-tune your technique.
Minimum temperatures can vary by as
much as 15 degrees at night over small distances. Some
of the key factors that cause these differences are
topography, soil conditions, and wind.
On nights with no wind and clear skies,
heavy cold air will roll or drain to valleys, creek/river
bottoms, and small depressions surrounded by higher
terrain. This can drop the temperature in low-lying
areas by several degrees. The temperature change is
most pronounced on nights when there is very little
wind and skies are clear.
Have you ever noticed frost on the lee sides of wind
breaks after cold nights with gentle winds? Colder temperatures
can prevail in areas downwind of trees, hills and buildings.
Lighter winds on the lee side of windbreaks make heat
radiation more effective. Less mixing of warm air above
and cold air below occurs. This effect is most pronounced
near the ground. This does not occur on extremely windy,
cold nights when the coldest temperatures are found
at the highest elevations.
Soil moisture can also influence minimum
temperatures. The water in wet soils helps retain heat
and will keep temperatures warmer. Conversely, dry soils
radiate heat more readily resulting in colder temperatures.
Tweaking the forecast for your unique situation is a
valuable skill for Ag producers. Knowing which areas
run hot and which run cold can help you distribute your
Getting a good freeze/frost forecast
is half the battle, using it correctly is the other
half. All freeze/frost forecasts do not result in the
same operational actions for every grower. Now is a
good time to review some of the factors that affect
cold protection actions.
Know your crop. Protection effectiveness
is a combination of the threshold tolerance of the crop
and the effectiveness of the cold protection method
in use. Most foliage crops have higher critical temperatures
than fruit crops. It can be too late for one crop before
another is in danger.
Understand the limitations of the
protective measures. For instance, the density and material
of crop covers and tree wraps result in varying degrees
of protection. Read the labels and know what you are
working with. A three-degree protection cover is of
limited value if the temperature falls seven degrees
below the crop threshold value.
Watch out for strong winds. Wind machines,
helicopters, and heaters will be less effective in strong
winds. These measures rely heavily on the presence of
inversions (warm air above) to protect plants. They
essentially mix warm air above with the cooler air at
the surface, raising the temperature at the surface.
You will get some protection from heaters, but much
less than on a still night. If other measures are available
(water or covers) you might want to use them. Strong
winds can also hinder the laying of crop covers, increasing
the labor and time required to place covers.
Watch the wet bulb
temperatures and dew points if you are using water
to protect. A drop in the dew point temperature results
in evaporative cooling. The water must be turned on
before the wet bulb temperature reaches the threshold
temperature, even if air temperature is well above it.
This can extend the time during which water is applied
by several hours.
Keep an eye on the duration of the
cold event. Extended periods of icing can cause significant
damage with branches and leaves breaking under the weight
of the ice. Wind will aggravate this problem.
Know the expected daytime temperatures
when cold events extend over several nights. Crop covers
need to be removed during the day in many cases to avoid
damage resulting from heat. Warm days under wrap may
also cause plants to break dormancy, making them even
more sensitive to the cold night. Make sure adequate
labor and supplies are available in cases where long
periods of damaging weather conditions are expected.
If your forecaster is not giving you
the above information, by all means, get another one.
Air temperature is just not enough.
Point Temperatures to Forecast Frost
Frost forms when moisture is adequate
and freezing temperatures occur. Dew point temperature
can be very useful to determine if frost will form.
Simply put, dew point is the temperature at which the
air is saturated with water. The higher the dew point,
the more water in the air. Dew point is always less
than the air temperature, unless the relative humidity
is 100%, then the dew point is equal to the temperature.
As the air temperature cools on a
winter night and approaches the dew point, the moisture
in the air will condense out of the air and on to solid
objects. The surface temperature of an object must be
freezing in order for frost to form. Objects that radiate
energy more efficiently tend to frost up first. Thin
objects (leaves, small branches, shoots, etc.) frost
quicker than thick objects. The ground, particularly
when moist, does a relatively good job of holding heat,
so frost damage tends to occur at the top of vegetation.
Frost can form when dew point temperatures
and air temperatures are in the upper 30s. Patchy and
scattered frost can occur in areas that typically run
cold, low areas where cold air accumulates, the lee
side of wind blocks, etc. In addition, dew point and
temperature are measured at the standard height of five
feet. It can be colder at ground level.
Prime dew point temperatures for widespread
frost formation are in the upper 20s to lower 30s. When
this occurs with freezing air temperatures, everything
is in place for potentially heavy frost formation.
Frost formation is less likely with
dew point temperatures below the mid 20s. There just
isn't enough moisture in the air for significant frost
formation, even if temperatures are below freezing.
The length of time conditions are
favorable for frost formation determines the intensity
of frost. The more time the dew point and air temperature
hover around or below 30 degrees, the heavier the frost.
Dew point is not the entire story.
Other factors also affect frost formation. Wind speeds
above 5 MPH slow the radiational cooling of objects,
inhibiting frost formation. Cloud cover reduces the
effectiveness of radiational cooling. The thicker the
cloud cover, the slower the cooling. Soils containing
abundant water increase the risk for frost formation
by putting more water into the equation, particularly
in cases where the dew point temperature is in the mid
20s to mid 30s range. This may counteract the fact that
wet soils retain heat better.
Forecast dew point temperature is
a key factor in anticipating frost formation. While
it does not necessarily give a conclusive answer, it
will give you a significant 'heads-up' call. If you
aren't getting dew point temperature from your current
weather provider, you should be.
Inversion strength can have a large
impact on the effectiveness of your cold protection
methods. Heaters, wind machines and helicopters are
all more effective when low level inversions are strong.
Light winds and clear skies favor
the development of low level inversions. These same
factors encourage radiational cooling. As the earth's
surface loses heat, the air close to the ground cools
faster than the air above. Under strong radiational
cooling conditions, temperatures at ground level can
be a full 10 degrees cooler than the air 100 to 200
ft. above. Occasionally, temperature differences of
up to 15 degrees can occur.
The height of the inversion is affected
by topography, inversion strength and other factors.
Determining the height is important because the warmest
air is at the top of the inversion. There are few locations
with adequate air temperature measurements to determine
the height of the inversion. The easiest way to determine
this is to start a smoky fire and watch where the upward
moving smoke levels off. The smoke levels off at the
top of the inversion - the point where the temperature
is warmest. Normally the top of the inversion is between
100 and 200 ft, but it can be lower. Be sure to follow
all local regulations on open burning.
Heaters work more efficiently in the
presence of strong inversions. The warm air rising from
the heater mixes the air. Convection cells are formed,
pushing the warm air above down to the ground.
Wind machines and helicopters are
frequently used to bring the warmer air down closer
to ground level. The closer the wind machine or helicopter
is to the top of the inversion, the more successful
the attempt will be to warm the air near ground level.
Only a portion of the inversion strength
can be realized. However it only takes a few degrees
to save or lose a crop!