Tayammum, or dry ablution, is a divinely-ordained concession from Allah, allowing a Muslim to achieve a state of ritual purity to perform acts of worship like Salah when water cannot be used.

Shaykh Ibn 'Uthaymeen explains that a person may perform tayammum in the following circumstances:

1. Water is Non-existent: If a person searches for water where it is expected to be found but cannot find any, they may perform tayammum.

2. Harm from Using Water: If a person is suffering from an illness or injury such that using water would worsen the condition or delay recovery, they may perform tayammum. This is based on experience or the advice of a trustworthy doctor.

3. Water is Inaccessible: If water is present but there is a danger in reaching it, such as from an enemy, a wild animal, or being trapped in a place one cannot leave.

4. Need for Water for Survival: If a person has a limited amount of water that is essential for drinking, for themselves, their family, or their animals, they should save the water for drinking and perform tayammum for purification.

5. Extreme Cold: If the weather is extremely cold, water cannot be heated, and a person fears harm or illness from using the cold water, tayammum is permissible.

The method of tayammum is to strike clean earth or a dusty surface with one's palms once, blow off any excess dust, and then wipe the face and the backs of the hands up to the wrists. Tayammum replaces both Wudu (minor ablution) and Ghusl (major ablution) and is nullified by the same things that nullify Wudu, as well as by the availability and ability to use water.