Why airfares move around so much

A plane does not have one price, it has about twenty

Every flight is cut into fare classes: blocks of seats sold at different prices inside the same economy cabin. The cheapest blocks hold few seats. When they empty, the system moves to the next block up.

That is why prices climb in steps rather than gradually. The jump from $612 to $780 is not an increase, it is the end of a block. The person beside you may have paid $400 and the one in front $1,100, on the same aircraft for the same service.

It also explains why searching for two tickets sometimes costs more per person than one. If three seats remain in the $612 block and you ask for four, the system gives you all four at the next block up. Searching one passenger at a time occasionally reveals a price the four-passenger search was hiding.

Yield management, plainly

Airlines adjust those blocks continuously against the pace of bookings. A flight filling faster than forecast sees its cheap blocks close early. A flight lagging behind reopens them to pull in demand.

The underlying calculation is simple: an empty seat earns nothing, but a seat sold too cheaply too early costs the airline a business traveller who would have paid full fare in the departure week. The whole craft is placing the cursor between those two.

The practical consequence is that fares follow no fixed calendar. The famous "best day to book" does not exist as a rule. What exists is an average filling rhythm per route and per season, and it varies enormously between flights.

No, clearing your cookies changes nothing

The myth persists: the site supposedly noticed you came back and raised the price. Displayed fares come from carrier reservation systems, which answer according to block availability at the moment of the query. They do not know your browsing history.

What people observe is real, but the explanation sits elsewhere. You often searched at a different time of day, or through a different tool querying different sources, or simply after a block emptied in the meantime.

Searching in a private window costs nothing and is reassuring. Just expect to see the same price.

What actually moves the price

Four factors explain most of the spread you will see on a single route.

• The departure date : By far the strongest factor. A Tuesday often costs markedly less than a Friday evening, and shifting by a week can change the fare completely.
• How full that specific flight is : Two flights on the same day, six hours apart, can show very different prices simply because one is filling better.
• Competition on the route : A route served by three carriers stays low. A route where one carrier flies nonstop keeps prices high all year.
• Whatever is happening at the destination : Conferences, festivals, school break, national holidays: local demand rises and the cheap blocks vanish months ahead.

How to judge a price without guessing

Since an amount on its own says nothing, the only useful question is: compared to what? $700 to Barcelona and $700 to Fort Lauderdale tell completely different stories.

That is exactly what Vol Spot displays: the usual price for the route, the day's gap as a percentage, and the days on which we recorded a fare on that route since we started tracking it. A fare in the bottom third of what we have observed across 40 days is information, in a way an isolated number is not.

The rest is risk tolerance. A good price today may fall further, or disappear tomorrow. Nobody can predict which, and treat anyone claiming otherwise with suspicion.