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California Weather and Icy Roadways

Ice machines in front of a store covered in snow during a storm.
Ice machines in front of a store covered in snow during a storm.

For people residing or passing through Southern California, snow and ice are not what they typically will encounter unless they drive up into the mountains during the winter time. But amazingly, we experience black ice near the beach in areas like Rancho Palos Verdes, for example. So yeah, ice happens where ill-equipped California drivers least expect it.

Ice on the roadway is one of the most dangerous weather-related situations the California driver will likely face. This more dangerous than the rain and fog driver’s in California face. This danger is because the only comparable driving hazard would be hydroplaning on wet pavement. And this leaves the driver with no control.

Annually, the state of California reports that there are hundreds of serious motor vehicle accidents, with some resulting in fatalities due to the slippery condition of the roadways. In California the total percentage of accidents related to icy conditions on the road number much lower than non-weather related crashes. But even collisions occurring on ice-slicked roads warrant concern.

What is Black Ice?

One of the most dangerous types of icing conditions on the roadway is called black ice. In fact, this is water that that freezes once night falls. And it is often not visible to drivers at night at all. In some cases, the driver may see the ice illuminate from the headlights of the vehicle. But often there is no time to use extra caution driving on this section of the pavement.

Reducing Ice Related Accidents

Having a little knowledge is essential during the colder months of the year. And having access to current weather information can help. That way drivers and their passengers can remain safer. Being prepared for icy conditions is essential when operating a motor vehicle. But drivers must also be ready for the variety of weather conditions. Any of these can result in ice forming on roadways.

A few things to remember during cooler weather are:

  • If there is fog and the temperature is near freezing 32 degrees F, heavy icing is possible on asphalt and concrete.
  • Bridges often freeze before roadways in near-freezing temperatures.
  • Low lying areas and valleys often have icing conditions first, since cold air settles in these areas.
  • When the sky is clear, but there is moisture on the road and the temperature falls below the upper 30’s the streets can become icy.

What Conditions Cause Icing on Roads if it isn’t Raining or Sleeting?

Ice on roadways is not always from rain or freezing rain. The moisture can come from several other weather factors and temperatures around 32F or freezing.

  • Fog settling on the roadway combined with freezing temperatures can cause heavy icing.
  • Frost, which is a collection of frozen moisture from the air that can be seen after sunset, during the night and early morning.
  • Water from snow that has melted during traffic from the warming of the roadway. Then when the pavement cools and temperatures dip during the day or at night causing the melted snow water to freeze.
  • Groundwater seepage, which then freezes on the road due to the air temperature.
  • Freezing rain, which is also called sleet can build up on the road causing icy conditions rather than melting because of freezing air temperatures.

These conditions can occur one at a time. Or they can be a combination, with more than one making the roads hazardous.

Valleys and Low Lying Areas Present Special Dangers?

Valleys and low lying areas often have icy road conditions when other regions may not have any problems. This is because cold air is more substantial and denser than warm air. So this causes it to move down from higher elevations, even if it’s only a minor hill.

Clear nights with relatively few clouds results in colder temperatures. And surfaces like asphalt tend to cool faster than the air above. This air causes low lying areas to often be as much as 2-5 degrees colder than elevated areas a short distance away. City temperatures are warmer than rural areas. But this is usually a double-edged sword. After all, the warmth can melt freezing rain, snow, and rain from creating slick conditions.

When the traffic slows and evening falls, temperatures decrease. So this causes water on the roadway to freeze when the temperatures are low. There are some areas where the air may be colder than other areas. And this is referred to in meteorology terms as thermal signatures. But it is difficult to see at night.

Fog and Ice

Fog occurs when there is moisture in the air and temperatures drop the dew point. This moisture is thick and dense containing large amounts of water. So it is carried by the air current and passes over roadways. And if the temperature is below freezing, it can result in severe icing in a matter of minutes. And that can happen on an otherwise clear night. Cold nights with fog blanketing a roadway has resulted in numerous serious accidents. They were due to roadway icing when temperatures dipped into the mid-’30s.

Frost and Icy Roadways

Frost is moisture that settles on the ground on cool and usually clear nights. It forms with low wind speeds, generally under ten mph. The creation of frost touches on infrared radiation. And these clouds absorb and emit and warmer objects emit more infrared radiation. But on clear nights, the surface or ground emits infrared radiation without the clouds to absorb. So this is lost to space, resulting in the surface and air to cooling rapidly.

The air away from the surface is warmer, so the increasing height of the air is called an inversion. Cloudy night inversion does not occur. This inversion is because the clouds blanket the surface by preventing the quick loss of infrared radiation or heat. Air contains unseen water vapor. That vapor condenses into visible ice crystals. And the amount of water air can provide varies with the temperature.

  • Warmer air contains more water vapor.

And if the air is cooled to the dew point, the moisture cannot be held as an invisible gas in the air. This cooling forces the water vapor to condense into drops or ice crystals falling from the sky. The ground surface is cooled to near freezing temperatures from night temperatures.

And the air reaching the dew point results in frost. This frost, when the sun is out, remains unseen and melts rapidly from surfaces. All of this is from the rise in temperature or increased infrared radiation.

The more vapor there is in the air, the more frost will form. So you will see it in areas with swamps, lakes, and ponds. They will have a higher density of vapor in the air and result in a thicker blanket of frost.

  • Frost is also more prevalent in lower lying areas than in higher elevations where wind speeds are usually lower.
  • Generally, frost does not accumulate more than 1/16 of an inch.
  • And while that seems like a minute amount of the ice crystals, it can create a dangerous situation on roadways where it has formed.

This thin coating of ice can make it harder to stop. Also, other driving functions remain more difficult. Frost is responsible for numerous motor vehicle accidents due to the thin coating of ice on roadways it creates.

Drivers should be alert to roadway conditions when the sky is basically cloud free and the night air temperatures are in the mid-’30s.

Freezing Rain

This is rain that has frozen due to the air temperatures or nearly frozen. Also, this may be referred to as freezing rain, sleet or drizzle. The way this occurs is because of a layer of air. Now it is below freezing near the surface. But warmer air is above freezing higher in the atmosphere.

As the rain falls because the air higher is warm and passes through the temperatures that are below freezing. Then it freezes upon hitting the surface. This rain turns into a block of glazing ice on the surfaces. This glazing happens because of surfaces being cooled. And it can make tree limbs heavy enough to snap and power lines.

Areas of California can experience freezing rain. This freezing happens when the air temperatures are right and helped by low level cold. This warm rain is from the Pacific. This results in rain freezing on contact with the surface. So this makes higher elevations treacherous because of the warm Pacific weather systems.

These are coming into the coast where warm rain falls. So the air temperatures are in the area of 32 degrees. Hence, it causes freezing rain. And in higher mountain locations of the state, it often changes over to snow. High winds can result in temperatures plummeting and result in freezing surface areas.

Snow and Ground Water Seepage

Groundwater in lower lying areas and melted snow in the upper elevations of California can freeze as the temperatures drop. During the day roadways heat up due to traffic and the warmer daytime temperatures.

This causes water seepage or melted snow to keep from freezing on the asphalt. Once the traffic slows and the night temperatures begin, this standing water or wet asphalt can freeze. So this results in dangerous driving conditions for motorists.

Roadways made of pavement also known as blacktop roads heat up by absorbing heat from the sun. And this melts any ice. But as soon as dusk arrives, there is no more extended heat absorption. So the road quickly cools. And if the temperatures are freezing, the seepage or melted snow freezes. This is particularly the case if there are few clouds in the sky. But this is because cloud cover acts as an insulator during cold weather.

This condition often occurs on roads that have had the snow plowed in higher elevations. And it takes place in lower elevations where there is a water source that spreads. On cold nights without cloud cover, the cold night air causes evaporation of heat from the roadway. And then with the temperatures near freezing, the wet areas turn to ice. This remains true even in low areas where cold air tends to pool.


Slush is melted snow that generally affects the higher elevations in the state of California. And this is caused when snow begins to fall rapidly. Then the road surface is warm due to the sun and traffic. But when the temperatures drop either quickly or because of the lack of warm the partially melted snow freezes and are called slush.

This is a mixture of snow and ice. And while it is much wetter than snow alone, it is also more hazardous for drivers. Plus, it can turn to solid ice in the right temperatures.

Bridges Freeze Before Roadways

Bridges often freeze before roadways become icy for some reasons. First, it may depend on the material the bridge is made from as they usually are not covered with asphalt. But might be concrete. Concrete does not hold the same warmth as asphalt because of sun radiation and traffic.

So this means the colder the temperature, the faster the bridge will become dangerous. Bridges also do not have ground beneath them. So this leaves no room for more cold air to affect the temperature of the bridge, even if it is covered with pavement.

Another factor why bridges may freeze quicker than roadways is because they might not absorb the same amount of heat. Plus, freezing rain or snow may not completely melt. Or if it does melt, the surface does not dry because the water may have no place to drain. When night temperatures or rapidly falling temperatures occur, the surface cools fast. So this makes a perfect condition for the liquid to freeze.

Air Temperature and Surface Temperature

Air temperature is generally determined using a thermometer about five feet from the ground. And often it remains in an enclosed shelter. This is how official weather reports are decided for daily and nightly temperatures. But roadway and other surface temperatures are far different than the temperature taken above the surface.

And this is since cold is more massive from moisture contained in the air. The ground temperature can be between 2 and 5 degrees cooler than air temperature. This is why frost can occur even when the air temperature is between 35 and 37 degrees. And this is a temperature that is above freezing.

During the day, the road temperature can often be warmer than the air temperature. But this is due to cars and trucks warming the surface, along with the sun’s radiation. Once the night temperature falls, and traffic decreases the warmth of the surface decreases. And on nights that have few clouds, the surface temperature cools rapidly. Icing conditions can occur when the surface temperature hits 32 degrees. The air temperature may be as high as 37 degrees.

Temperatures and Slippery Ice Conditions

Temperature changes during icy conditions can mean the ice changes and can become more slippery. This warming is because of the way ice is formed on surfaces according to science. Ice has a thin layer of water on the outside even when the temperatures are below freezing. This layer of water makes the ice slippery. And it is thickest when the temperature nears freezing. And then the ice thins when the temperature well below 32 degrees.

This creates a condition where the ice is the most slippery when the temperatures are between 26 and 32 degrees. When the temperature reaches single digits, and below 0 degrees the ice becomes less slippery. So caution should be used by motorists when the air temperature is between 32 and 26 degrees. And this is since the ice on roadways will be the most dangerous.

What are Some Other Effects?

  • Trees and Objects

Trees near roadways can cause shading of the surface as well as hills and other objects. And this can result in the potential icing of roads. Trees that overhang on roadways or objects that cover the street can block the loss of infrared heading from the sun.

Often when frost is seen on the grass and other surfaces including roads a driveway may contain frost. But the car might not if it was parked underneath a carport. This is because the roof of the carport would have reduced the amount of heat loss or air containing moisture.

The other side of this is roadways that do become icy and have tree cover may remain icy much later into the day. This is because the tree cover is shielding the pavement from absorbing infrared heat from the sun. So until the sun moves to a location where the roadway can absorb the heat, it remains icy.

Because drivers may not consider this situation. And this might last later into the day or even all day. This is where motorists can hit unexpected areas of ice protected by shade. So this leads to car accidents that are often fatal.

Night Temperatures and Icing

Nights that are clear and cold can result in the surface losing heat quickly. And this generally occurs within the first three or four hours after sunset. The temperatures can even begin falling before sunset on surfaces such as roadway areas that have shade from trees and other objects.

In colder temperatures, this can create icing conditions on these parts of the roadway earlier in the day. And the temperature of the surface will continue to drop more rapidly than other areas if there are no clouds or fog. Temperatures will fall slowly during the night and be the lowest near sunrise or a short time after sunrise.

Nights that have fog will have fewer ice conditions on surfaces than clear nights. This is because the mist acts as a thermal blanket. But the nights that have icing remain the largest threat between the hours of 5:00 a.m. and 8:00 a.m.

The threat of Roadway Ice Tips

During months of cool or cold weather, motorists may be safer using some of the information above.

  • Clear Sky: When the sky is clear when the weather is cold, especially at night, the night temperatures can rapidly fall near or after sunset. Monitoring local weather channels can help to determine clear skies and nighttime temperatures.
  • Official weather temperatures are observed by thermometers that are approximately 5 feet from the surface. So this means surface temperatures can be several degrees colder. If the air temperature is 37 degrees, the surface temperature can be 32 degrees or freezing. This means moisture in the air or rain can freeze on contact with the surface or within a short amount of time.
  • Some websites have real-time weather and surface temperature information for the state of California. So motorists can use them to determine if their location or one they will travel too has issues. At least now you can see if there is rain, frost, snow or ice on the roadways. Using the temperatures, cloud, fog or clear skies motorists can also determine possible driving conditions.
  • Fog is something motorists should pay close attention to during days of cold weather. When the temperature is below the mid-’30s, and there is fog. Mist can travel over roadway surfaces and leave large amounts of ice quickly. This is especially important in areas where there are wet or swampy areas and river valleys.
  • Roadway areas that may be shaded from the sun on nights when the roads become icy may have ice until later in the morning.  And in some cases, ice may remain on this area of the roadway all day when temperatures remain in the 30’s.
  • Unforeseen objects: During conditions when ice may be a factor on the roadway caution should be used. After all, the potential for unforeseen objects such as animals means less stopping time. This combination has the potential for fatal accidents.

As can be seen, ice sheets can form on the road surfaces in many parts of the Golden State. Reaction times and control over vehicles can be severely compromised during icy conditions. California drivers not accustomed to icy road conditions are at particular risk of losing control. So they are cautioned to reduce vehicles speeds when traversing areas where frost has developed.

Michael Ehline

Michael Ehline sufferred a horrific rollover accident when he was young. Dealing with his injury lawyers, and the medical industrial complex made Mike want to study law. So after his tour in the U.S. Marines, and running several small businesses, Ehline went for it. Check out some of his Los Angeles Car Accident Lawyer Verdicts and Settlements. Now Ehline is a highly successful, ambitious car crash lawyer. He operates, which is an accident and injury law practice based in Los Angeles, CA.

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