Torrential Rain Turns Ordinary Roads Into Dangerous Places
The second image shows a city roadway under a heavy downpour. Cars travel along a wet surface, their headlights and taillights reflected in the water. Tall buildings fade into a gray background as rain reduces visibility.
This is a familiar scene in many cities, but it can be extremely hazardous.
Heavy rainfall affects transportation in several ways. Water accumulates on road surfaces, tires lose some of their ability to maintain contact with the pavement, and drivers may have difficulty seeing vehicles, lane markings, pedestrians, or traffic signals.
Even experienced drivers can struggle when rain becomes intense enough to overwhelm windshield wipers and obscure the road ahead.
The problem becomes more serious when rainfall continues faster than drainage systems can carry water away.
Low-lying streets, underpasses, intersections, and roads near streams may flood rapidly. Water can hide potholes, damaged pavement, fallen branches, and other obstacles.
In areas with steep terrain, intense rainfall can also trigger dangerous runoff, erosion, and landslides.
How Heavy Rain Causes Flash Flooding
Flash flooding occurs when water rises rapidly after intense rainfall, sudden runoff, or other events that overwhelm the local drainage system.
Unlike some forms of river flooding, flash floods can develop quickly, sometimes leaving little time for people to react.
Several factors influence the risk.
Urban areas often contain extensive paved surfaces, including roads, sidewalks, parking lots, and rooftops. These surfaces prevent much of the rainwater from soaking into the ground.
As a result, water runs rapidly into drainage channels and low-lying locations.
If rainfall exceeds the capacity of the drainage system, streets may become flooded.
In rural areas, saturated soil, steep slopes, and narrow valleys can increase the speed at which water flows toward streams and roads.
A storm that produces intense rain over a relatively short period can therefore create dangerous conditions even if the surrounding region does not experience a prolonged period of rainfall.
The amount of rain falling in one location is not the only consideration. Where the rain falls, how quickly it accumulates, and how effectively the landscape drains are also important.
The Hidden Danger of Flooded Roads
One of the most dangerous mistakes people make during severe rain is assuming that a flooded road is safe because the water appears shallow.
Moving water can exert substantial force on a vehicle. The depth may be difficult to judge, and the road surface beneath the water may have been damaged or washed away.
A car can lose traction or become buoyant in sufficiently deep water. Even a relatively small amount of moving water can create hazardous conditions, particularly for smaller vehicles.
Floodwater may also contain sewage, chemicals, sharp debris, or other contaminants.
Drivers should never attempt to cross a flooded roadway when the depth, current, or condition of the road is uncertain.
The safest choice is to turn around and find another route.
This principle is especially important at night, when the edges of a flooded road may be difficult to see.
Why Driving During a Severe Storm Requires Extra Caution
Heavy rain reduces the amount of time available to recognize and respond to hazards.
When visibility deteriorates, drivers should slow down, increase following distance, and use their headlights appropriately.
Sudden braking and sharp steering movements can increase the risk of losing control on wet pavement.
Cruise control should be avoided when driving in heavy rain because drivers need to maintain direct control of acceleration and speed.
If conditions become too dangerous to continue, a driver should seek a safe place away from traffic and flood-prone areas, provided it is possible to stop without creating another hazard.
Stopping beneath trees, near unstable slopes, or in low areas that may flood can introduce additional risks.
If authorities advise people to avoid travel, that instruction should be taken seriously.
A delayed trip is far less costly than becoming trapped in floodwater or being involved in a collision.
Giant Hailstones: When Ice Falls From a Thunderstorm
The third photograph shows large, rounded hailstones scattered among blades of grass.
Their white surfaces contrast sharply with the green and brown vegetation, making their size immediately noticeable.
Hail can be one of the most destructive hazards associated with severe thunderstorms.
Unlike ordinary raindrops, hailstones are solid pieces of ice that develop inside strong thunderstorm updrafts.
When conditions are favorable, these updrafts can keep ice particles suspended within a storm long enough for them to grow.
Water droplets may freeze onto the surface of an ice particle as it moves through regions containing supercooled liquid water. Repeated growth can produce a hailstone that becomes large enough to fall to the ground.
The exact process depends on the storm's temperature structure, the strength and duration of its updraft, and the availability of liquid water.
Not every storm has the conditions required to produce large hail.
When large hail does develop, however, it can cause considerable damage within a short period.
Why Hailstones Can Become So Large
A thunderstorm's updraft is a major factor in hail growth.
Strong upward-moving air can support hailstones that would otherwise fall toward the ground. As the stones circulate through the storm, they may encounter additional supercooled water and continue growing.
Eventually, a hailstone becomes too heavy for the surrounding updraft to support, or it moves into an area where the upward airflow is insufficient.
It then falls toward the ground.
The largest hailstones can cause serious injuries to people caught outdoors. They can also crack windows, damage roofs, destroy crops, and dent vehicles.
Even hailstones that are considerably smaller than the largest recorded examples can damage sensitive plants and exposed equipment.
The risk depends on the stones' size, their speed, and the vulnerability of whatever they strike.
Why Hail Is Dangerous for Farmers
For agricultural communities, hail can represent a major financial threat.
Crops may be damaged during critical stages of growth. Leaves can be shredded, stems broken, and fruit bruised or knocked from plants.
The effects vary according to the crop, the size and density of the hailstones, and the timing of the storm.