Why Does the Sky Turn Dark or Green Before a Storm?

You can often tell a thunderstorm is approaching before the first drop of rain falls. The sky darkens. The air suddenly feels cooler. Leaves begin flipping and branches start moving even though the rain may still be miles away.

And every so often, the sky takes on an eerie green or yellow-green color.

These changes aren't random. They are signs of what is happening inside a thunderstorm—where powerful updrafts and downdrafts are moving enormous amounts of air, water and ice through the atmosphere.

Why Does the Sky Get So Dark Before a Storm?

Storm clouds look dark for a simple reason: they are very thick.

A fair-weather cloud may contain plenty of tiny water droplets, but sunlight can still penetrate and scatter through it. A large thunderstorm cloud—called a cumulonimbus cloud—can extend many miles vertically and contain tremendous amounts of water droplets and ice crystals.

As the cloud becomes thicker and denser, less sunlight reaches its base. From the ground, the bottom of the cloud can appear dark gray, blue-gray or nearly black.

The darker the cloud looks, however, the more severe the storm isn't necessarily going to be. Darkness tells you a lot about the thickness of the cloud and the available sunlight, but it isn't a reliable measure of wind speed, hail size or storm severity.

Why Does the Sky Sometimes Turn Green Before a Storm?

A green sky is much less common—and considerably more memorable.

Scientists have studied green thunderstorms for decades, and the exact optical conditions can vary. The effect appears to involve a combination of the enormous amount of water and ice within a storm and the way sunlight passes through and interacts with the cloud.

The position of the sun matters too. Green skies are often noticed during late-afternoon or evening storms, when sunlight travels through more of the atmosphere and takes on warmer reddish or orange tones. When that light interacts with the blue-gray light associated with a deep, water-filled storm cloud, the resulting sky can sometimes appear green or turquoise.

Does a Green Sky Mean a Tornado Is Coming?

Not necessarily. Green skies have long been associated with severe thunderstorms and tornadoes, but a green sky is not a reliable tornado predictor. Green coloration can occur with storms that never produce a tornado.

It may, however, accompany a very deep thunderstorm containing large amounts of water and ice—which is also the kind of storm capable of producing hail and other severe weather.

So a green sky is worth paying attention to, but it shouldn't be used by itself to decide whether a storm is dangerous. Official weather warnings and radar information are much better indicators. It also helps to understand the difference between a weather watch, warning and advisory.

Why Does the Temperature Suddenly Drop Before a Storm?

One of the most noticeable signs of an approaching thunderstorm is a sudden rush of cool air.

Rain-cooled air inside a thunderstorm is heavier than the warmer air around it. As that cooler air descends toward the ground, it can spread outward ahead of the storm. This spreading boundary is called a gust front, or an outflow boundary.

A gust front can arrive before the rain itself, which is why you might be standing in a dry yard on a hot afternoon and suddenly feel the temperature drop while the wind picks up.

In some cases, the gust front can travel well ahead of the thunderstorm that created it. That separation between a storm and its precipitation is also one reason lightning can occur even when no rain is falling where you are.

Why Does the Wind Suddenly Pick Up?

When descending air from a thunderstorm reaches the ground, it has nowhere to go but sideways. That creates the burst of wind that often announces an approaching storm.

You may see trees begin moving rapidly, dust blowing across a road or leaves swirling through the yard while the sky directly overhead is still relatively clear. Minutes later, the rain arrives.

The wind can also change direction abruptly as the gust front passes. These winds aren't simply the storm "blowing toward you." They are part of the storm's internal circulation—the result of air sinking from the cloud and spreading across the surface.

What Is a Downdraft?

Thunderstorms contain both rising and sinking air.

Warm, moist air rises rapidly inside the storm, forming an updraft. As water droplets and ice particles grow, some begin falling. Their weight, along with cooling caused by evaporation and melting, helps pull air downward.

This sinking column of air is a downdraft. When a strong downdraft reaches the ground, it can produce powerful straight-line winds. In severe thunderstorms, those winds can damage trees, roofs and power lines.

And sometimes a downdraft becomes especially concentrated.

What Is a Microburst?

A microburst is a small but intense downdraft that hits the ground and spreads outward rapidly.

Imagine pouring water straight down onto a hard surface. When it hits, the water shoots outward in every direction. Air in a microburst behaves in a somewhat similar way.

Microbursts are typically confined to a relatively small area, but their winds can be extremely strong. Because the wind spreads outward from the point where the downdraft reaches the ground, damage patterns can look different from those caused by a tornado.

Microbursts are also a serious aviation hazard because an aircraft can encounter rapidly changing wind speed and direction over a very short distance.

How Does Hail Form Inside a Thunderstorm?

Hail begins high inside powerful thunderstorms, where temperatures are below freezing.

Strong updrafts carry water droplets upward into these cold regions. Some droplets become supercooled, meaning they remain liquid even though their temperature is below freezing. When they encounter an ice particle, they freeze onto it.

The hailstone grows as it collects additional supercooled water.

Inside a particularly strong thunderstorm, hailstones can be carried through different parts of the cloud while accumulating additional layers of ice. Eventually they become too heavy for the storm's updraft to support—or they move into an area where the upward-moving air is weaker—and they fall toward the ground.

That's why large hail is often associated with powerful thunderstorms: keeping a sizable chunk of ice suspended in a cloud requires a strong updraft.

Why Can Hail Fall When It's Hot Outside?

It can seem strange to see chunks of ice falling on a warm summer afternoon, but the temperature at ground level isn't what matters.

Thunderstorm clouds extend thousands of feet into the atmosphere, where temperatures can be well below freezing even when it's hot at the surface.

A hailstone begins melting as it falls through warmer air. Small hail may melt completely before reaching the ground. Larger hailstones can survive the trip, sometimes arriving while the surface temperature is 80°F or higher.

This is a dramatic example of how precipitation can change depending on conditions between the cloud and the ground. The same atmospheric temperature differences help determine whether precipitation reaches us as rain, snow, sleet or freezing rain.

Why Can One Place Get Hail While a Nearby Yard Gets Only Rain?

Thunderstorms can be surprisingly local.

The strongest updraft, heaviest rain and hail-producing portion of a storm may cover only part of a town—or even part of a neighborhood. Wind inside the storm can also carry hail in one direction while rain falls elsewhere.

That's why one house can receive pounding rain and hail while another a mile or two away gets an ordinary shower.

Rainfall can vary on an even smaller scale. Two rain gauges in the same general area—and sometimes even in different parts of a large property—can record noticeably different totals during a localized summer thunderstorm. Rainfall really can vary substantially over just a few miles.

What Happens as the Storm Finally Arrives?

An approaching thunderstorm is really a moving system of air.

The darkening sky comes from an increasingly deep cloud blocking sunlight. A green tint can result from unusual lighting interacting with a water- and ice-filled storm. The sudden cool breeze may be rain-cooled air spreading outward from a downdraft. Stronger gusts can mark the arrival of the storm's outflow boundary. Hail has been growing far above the ground, supported by powerful rising air.

Weather radar gives us another view of all that activity, allowing meteorologists to follow precipitation and storm movement as it develops. Here's how weather radar knows how much rain is falling.

What looks from the ground like a storm simply "rolling in" is actually a huge amount of air moving vertically and horizontally at the same time.

That is also why thunderstorms can change so quickly. A quiet, hot afternoon can become cool, windy and dark within minutes—and just as quickly, the storm may move on, leaving behind sunshine, dripping trees and a surprisingly different amount of rain from one neighborhood to the next.

Those quick, highly localized storms are also why measuring rainfall during summer thunderstorms can be so interesting—and why the amount in your rain gauge may be very different from what fell just a short distance away.