Riyadh airport rocked by Houthi missile strikes – how big is the group's arsenal and why are Saudi Arabia's air defenses struggling?

Saudi Arabia has been rocked by a barrage of ballistic missiles, cruise missiles, and drones in recent days -- attacks carried by Houthis, the armed group that controls parts of Yemen. The King Khalid International Airport in the Saudi capital Riyadh has been hit multiple times by the Houthis since they launched a military campaign against the kingdom in September.

On Saturday, an attack on the airport killed at least 13 people and injured hundreds, marking the deadliest attack on Saudi Arabia in over a decade. This was followed by another attack on the airport the next day, which killed one person and wounded nearly a dozen. But, how are Houthi missiles ripping through Saudi Arabia's advanced air defenses?

What's in the arsenal of the Houthis?

The Houthis' weaponry has seen a massive boost since the group seized control of the Yemeni capital Sanaa from the government over 12 years ago, in 2014. And the series of attacks on the Riyadh airport over the weekend show that the group is capable of hitting particular and sensitive targets.

The Houthis have used a mix of weapons in their attacks, bringing in drones and missiles of varying speeds to puzzle the Saudi air defenses. The militant group's arsenal includes medium-range ballistic missiles such as the Palestine-2, and a two-stage solid fuel missile which is believed to have a range of 2,000 km with hypersonic powers.

Toufan is another of their advanced weapons, and is thought to have a range of up to 1,950 km. The missiles have shown the capability to reach as far as Israel, located some 2,000 km from Yemen.

The Houthis also have a cache of short-range missiles, including their Burkan and Badr series, which have been used in cross-border strikes into Saudi Arabia. Experts say that the Houthis have been receiving technological help from Iran and its Iranian Revolutionary Guard Corps (IRGC). But the exact size of Houthis’ missile collection remains unknown, raising questions about the group's ability to continue attacks on Saudi Arabia.

The limitations of Saudi's defense

Saudi Arabia boasts of one of the best-equipped militaries in the world, with advanced and multilayered air defense systems. The kingdom's warplanes have pounded Houthi positions across Yemen in recent days, attempting to diminish the Iran-backed group's advances against the Yemeni government army.

Authorities in Saudi Arabia said their forces shot down several projectiles last week, including some aimed at the capital. But the projectile that hit Riyadh’s King Khalid airport was missed. Nawaf Obaid, a senior fellow at King's College War Studies Department in London, told Al Jazeera that an occasional breach of air defense is expected, given Saudi Arabia's size and its border with Yemen.

"As much as you can have a multi-layered air defence system, one will always come through," Obaid told the publication.

Besides, Saudi Arabia and many other regional and global allies of the United States have struggled with limited stockpiles of Patriot and THAAD munitions. That's because the weapons were swiftly exhausted by the US and its allies during the initial months of the war with Iran.