Tayammum (Dry ablution)

Tayammum means wiping one’s face and hands with soil with the intention of purification and preparing oneself for a prayer.
“(A)nd if you are ill or on a journey or one of you comes from the place of relieving himself or you have contacted women [i.e., had sexual intercourse] and find no water, then seek clean earth and wipe over your face and your hands [with it]. Indeed, Allah is ever Pardoning and Forgiving.” [Al Quran 4:43]. This is how Tayammum or dry ablution became permissible.
 How to Perform Tayammum? It starts with expressing the intention (niyat). After mentioning Bismillah (In the name of Allah, the most beneficent, most merciful) strike the soil with hands, and wipes his face and hands up to the wrist, starting with the right hand. Hazrat Ammaar (RA) said: “I once became sexually impure and had no water, so I rolled in the dirt and prayed. This was mentioned to the Prophet (SAW) who said: “This would have been enough for you,” and he struck the earth with his hands, blew in them and then wiped his face and hands with them.” [Al-Bukhaari and Muslim]
This Hadith shows that one strike of the earth is sufficient, and one only wipes the arms to the wrists. It is an act of Sunnah that one who makes Tayammum should blow the dust from his hands first and not make his face dusty or dirty.
When is it permissible?
1 If water is insufficient for performing ablution
2 If one has sustained injuries and use of water is likely to worsen his condition
Hazrat Jaabir (RA) said, “We were on a journey and one of us was injured. Later, he had a wet dream. He asked his companions, ‘Can I perform Tayammum?’ They said, ‘No, not if you have water.’ He performed Ghusl, which caused him to die. When news of this came to the holy Prophet (SAW) he said: “They killed him! May Allah kill them! Should they not have asked if they didn’t know? The remedy for ignorance is to ask. He could have performed Tayammum and not dropped water on his wound, or wrapped it with something, then wiped over the wrapping, and then washed the rest of his body.”
3 If one is ill and cannot use water for medical reasons.
If one is in this condition and thinks that using water will worsen it (he does not have to be absolutely sure, but may base his assumption on past experience or what a knowledgeable person has told him), he may perform Tayammum.
4 If water is cold enough to cause harm to the user. However, if one can heat the water, then he must opt for proper ablution.
5 If water is nearby, but one cannot fetch it due to fear
6 If one is trapped in a situation where saving water for future use becomes necessary.
7 If one fears that the prayer will be over by the time water becomes available.