It is an overwhelming adventure, a time of wonder with light and thunder, to experience a monsoon thunderstorm wave rolling across the Grand Canyon at sunset. Surrounded by storm cells with heavy rain and far too close lightning strikes of a highly active thunderstorm behind, the multiple echoes of the rolling thunders were deeply impressive. The panoramic view at the North Rim of the Grand Canyon near Cape Royal extends from Vishnu Canyon past Wotan’s thrones into the sunset in Clear Canyon. The eponymous streams of these canyons cascade their muddy-brown rainfall torrents towards the Colorado River, visible on the far left in the rock gap. The South Rim is located on the opposite side of the Grand Canyon, at a distance of 16 km (10 miles).
The monsoon season in Arizona lasts from mid-June to mid-September. However, high temperatures and blue skies mostly characterize the weather during this time. This apparent contradiction is the so-called monsoon break in the southwest of the USA. The highly active monsoon waves develop in irregular cycles in between these times. The combination of dramatic clouds, torrential downpours and lightning creates impressive weather conditions for a short spell.
The spectacle at the Grand Canyon is awe-inspiring when the setting sun illuminates this scenery from below. But how do such weather conditions develop?
The prevailing winds in the southwest of the USA sweep from western directions. The air moistened over the Pacific Ocean mostly rains out on the western slopes of the Sierra Nevada mountains in California, which rise above 4000 m (14,000 ft), so that warm and very dry conditions prevail leeward of the mountains, explaining the arid desert climate of Arizona. During the monsoon season, however, the wind is more frequently directed from southern directions. This allows subtropical humid sea air from the Gulf of California or the Gulf of Mexico to reach the southwestern United States. When this humid air reaches the highlands of the Colorado Plateau, it is forced to rise by the orography and cools as a consequence. The plateau on the North Rim of the Grand Canyon rises to 2683 m (8803 ft). In general, as long as no clouds form, the air temperature decreases by 1°C per 100 m (5.4°F per 1000 ft) when an air mass is lifted. At the North Rim of the Grand Canyon, the ascending air has already cooled down as much as 26.8°C (48°F cooler). Now, warm air can hold significantly more moisture than cold air. In warm unsaturated air, the water molecules contained in it are present as an invisible gas called water vapor. However, due to the constant cooling during uplift, the relative air humidity eventually reaches 100% and the water vapor condenses back into water droplets, forming clouds.
The Grand Canyon incises 1800 m (5800 ft) deep into the Colorado Plateau. Accordingly, the bottom of the canyon, where the Colorado River is flowing, is up to 18°C (32°F) warmer than the North Rim. In addition, the summer sun heats the inner canyon much more efficiently than it does on the plateau. The hot and dry air begins to rise at the bottom of the canyon in the form of thermal air bubbles. When it reaches the rim of the canyon, it collides and mixes turbulently with the cooler and more humid monsoon air. In the updrafts, the air cools down quickly, more and more cloud droplets form and ice up at an altitude of more than 10 km (6 miles) to form storm clouds with anvil-shaped tops. The fast growing ice particles become too heavy to be kept in suspension by the updrafts. As they fall, they melt quickly in the warmer air and heavy rainfall sets in. However, sometimes the air near the ground is so dry that the falling raindrops evaporate before they reach the ground. This creates rainfall streaks that end before reaching the ground, a phenomenon called virga. The friction of all these turbulent air molecules, ice particles and water droplets creates a strong separation of electrically charged particles. We observe the charge equalization as lightning strikes.
Under favorable conditions, strong thunderstorms develop when these air masses collide, whereby a number of individual thunderstorm cells can merge to form a squall line.
It is impossible to predict when or where, or how often thunderstorms will occur. Therefore, if you want to photograph a thunderstorm scenery at the Grand Canyon at sunrise or sunset, you need either a lot of fortune or a great deal of patience and endurance. Monsoon waves, however, are larger scale weather events and can be predicted up to two days in advance. However, even in these cases the exact time and place of the thunderstorm formation is determined by chance, because the extensive monsoon waves consist of individual multi-cell storms. Monsoon waves therefore increase the probability to experience the desired thunderstorm situation. A temporal restriction to sunrise or sunset, on the contrary, limits the probability of such a situation considerably. In this case, the monsoon break lasted for three weeks in August 2018, during which the sky was mostly clear or partly cloudy. The forecast for this intensive monsoon wave came only one day in advance and the weather phenomenon lasted exactly 24 hours.
The chance meeting between the magnificent beauty of the Grand Canyon and the natural forces of the monsoon storm is further enhanced by the incredible age and geological story of the rocks. The yellow sandstones at the rim of the canyon are 250 million years old and were deposited in a shallow sea near the coast. The canyon floors around Wotan’s thrones are already about 500 million years old marine sediments, containing fossils of the first animals ever to crawl across the ocean floors. But beyond these canyons, where the Colorado River flows, the rocks date back to times when bacteria were the only inhabitants of Earth. They are incredible 2000 million years old and consist of alternating layers of sediment and volcanic lavas.
It was an unforgettable experience to witness this magnificent scenery at sunset, being surrounded by active thunderstorm cells with lightning strikes in the immediate vicinity. Such moments of great sublimity revive deeply rooted connections to our home planet. These impressions awaken our instincts and with them the irreversible realization how unique our planet is and how closely we are connected to it.
