Summoning Snape at the top of Astronomy tower

Question

Towards the end of HP6 (Half Blood Prince), we see Dumbledore atop the Astronomy Tower, together with Harry. His condition was rather pitiable from drinking the potion, and he asks Harry to call Snape.

Now, why didn't Dumbledore simply use the "patronus" method to call Snape? (We have seen that the Order members sometimes communicate via their patronuses). It would have been much simpler- and there was no guarantee that Potter was going to call Snape. (Harry himself wanted to call Madam Pomfrey).

One possible reason could be that drinking the potion deranged Dumbledore. However, it's stated in the book that the sign of the Death Mark on the top of the Astronomy Tower steeled Dumbledore. He didn't lose his ability to do magic-he undid the protections around Hogwarts, so that they could reach the Astronomy Tower on their broomsticks, and he also put a body-bind curse on Harry.

So, if he didn't lose his power to do magic, or the power to think (his conversations with Draco and other death eaters later show that Dumbledore had a clear head towards the end too), then why did he not use the tried-andtested method of communicating with Snape via his Patronus?

Answer 1

1. Your assumption that he didn't lose his power (as in energy level, not ability) seems to be contradicted by the book: he WAS exausted and severely underpowered:

   'I've been better,' said Dumbledore weakly, though the corners of his mouth twitched. 'That potion... was no health drink...'
   And to Harry's horror, Dumbledore sank on to the ground.
   'Sir – it's okay, sir, you're going to be all right, don't worry –'
   He looked around desperately for help, but there was nobody to be seen and all he could think was that he must somehow get Dumbledore quickly to the hospital wing.
   'We need to get you up to the school, sir... Madam Pomfrey...'
   'No,' said Dumbledore. 'It is... Professor Snape whom I need... but I do not think... I can't walk very far just yet...'

   Note here that Dumbledore barely manages to speak, with pauses. It's pretty clear that he would have trouble doing what is considered to be a difficult and advanced spell (Expecto Patronum) in this condition; when he could barely stand or talk.

   'Right – sir, listen – I'm going to knock on a door, find a place you can stay – then I can run and get Madam –'
   'Severus,' said Dumbledore clearly. 'I need Severus...'
   'All right then, Snape – but I'm going to have to leave you for a moment so I can –'

   Seeing the Dark Mark made him reach for his reserves, however, which implies why he managed the last-gasp magic like castle defenses and body-binding Harry:

   'We need to return to the castle at once,' said Dumbledore. 'Rosmerta,' and though he staggered a little, he seemed wholly in command of the situation, 'we need transport – brooms –'

2. A second reason would be that he was simply conserving his energy on stuff he DID have to do (which acting like a messenger wouldn't be).... for he then asked Rosmerta to communicate as well:

   'Rosmerta, please send a message to the Ministry,' said Dumbledore, as he mounted the broom nearest him. 'It might be that nobody within Hogwarts has yet realised anything is wrong... Harry, put on your Invisibility Cloak.'

3. A third reason - for sending Harry while on the Astronomy tower - was to protect Harry. Dumbledore knew that Death Eaters would be coming for him because of Malfoy's work:

   'Go and wake Severus,' said Dumbledore faintly but clearly. 'Tell him what has happened and bring him to me. Do nothing else, speak to nobody else and do not remove your Cloak. I shall wait here.'
   'But –'
   'You swore to obey me, Harry – go!'
   Harry hurried over to the door leading to the spiral staircase, but his hand had only just closed upon the iron ring of the door when he heard running footsteps on the other side. He looked round at Dumbledore, who gestured to him to retreat. Harry backed away, drawing his wand as he did so.

   The door burst open and somebody erupted through it and shouted: 'Expelliarmus!'

   Harry's body became instantly rigid and immobile, and he felt himself fall back against the Tower wall, propped like an unsteady statue, unable to move or speak. He could not understand how it had happened – Expelliarmus was not a Freezing Charm –

   Then, by the light of the Mark, he saw Dumbledore's wand flying in an arc over the edge of the ramparts and understood... Dumbledore had wordlessly immobilised Harry, and the second he had taken to perform the spell had cost him the chance of defending himself.