Another deeply testing day today: we woke up to strong winds and a complete whiteout and the mood in the tent was pretty sombre as we melted snow to make our breakfast. I then managed to spill my breakfast, mostly over Tarka's sleeping mat, and we set off into the thick white gloom in pretty low spirits (and in my case, with an empty-ish belly).

It was a real pea-souper  - a difficult thing to explain unless you've experienced it. We may as well have been in pitch black darkness, or skiing with pillowcases over our heads. You can't see anything other than white. There's no sky, no ground, no landmarks, and if you're in the lead it can be weirdly disorientating. I was convinced that the compass was wrong and I was skiing to the left the whole time (and half-expected to bump into our last campsite again having completed a perfect circle). At times I lurched and veered off-balance, like a drunk staggering across the deck of a ship in a heavy sea.

Tarka's hat blew off when he was in the lead, and surprising myself with a burst of speed (a relative term as I still had my sledge in tow - it was probably all of 2mph), I chased after it and manage to pin it down with a ski pole, which I felt somewhat redeemed my clumsiness with my breakfast.

We saw nothing at all until the sixth hour of the day, when the sun finally put in an appearance. At first merely behind a slightly lighter strip of cloud, and then finally blazing through and chasing the cloud and fog away, with the contrast and definition returning to the snow surface like a painting being coloured in. It's remarkable how the weather and surface affect our morale; when they were at their worst this morning, Tarka and I were both genuinely struggling to find motivation (and indeed any joy in our task) and I had to dig very deep indeed today to ski our full seven hours. Now, with the sun shining on our tent and hardly a breath of wind, anything feels possible.

One question for today:

Q) What is it like when the horizon is featureless in the same direction day after day? What happens to your sense of direction? What is it like to be fixated on the task at hand when there are no markers on the landscape?

A) Actually having a horizon, and some contrast so you can pick out detail and patterns in the snow surface isn't all that bad compared to being stuck in a whiteout like today's, but it's still a landscape that lacks a huge amount of visual stimulus. Our sense of direction seems very quickly to become linked to the position of the sun in the sky, the time of day and the direction of the wind, and without markers on the landscape, our days are divided (and our progress marked) by time.

We ski for one hour stints, taking it in turns to lead, and then stop for ten minutes to eat and drink, sitting on our sledges by reversing back over them with a ski either side. One interesting side-effect of the lack of scenery (and this could just be me - I've no idea if others have experienced the same) is that I really enjoy seeing colours at the end of the day; our green tent, blue sleeping bags and the red fleece jackets that we sleep in. My dreams on expeditions are - when I can remember them - particularly vivid, and always in full-on technicolour yet if you asked me back in London if my dreams were in colour or black and white, I'd struggle to tell you.