Winter in Estonia was hard for Spanish Air Force Officer Francisco Elias Entrialgo. Back home at this time of year it's a sunny 25 degrees Celsius, which stands in stark contrast to conditions at the Estonian Ämari air base, where the buds on the trees are struggling to open in temperatures of around eight degrees.

In a hangar, Estonian and Spanish soldiers are working together to pack the last lot of boxes earmarked for transportation. "Mission accomplished", says Entrialgo with a smile. He, his 120 comrades and four Eurofighters belonging to the Spanish Air force are clearing out to make space for the Brits poised to take assume responsibility for air surveillance over the Baltic Sea.

An advance party of British soldiers has already arrived and four Eurofighters are due in the coming days. It is all part of a NATO Air Policing mission that has so far also seen Danish and German pilots at the controls.

"It is our job to observer Russian aircraft that have not identified themselves correctly," deputy commander of the air base, Estonian Lieutenant Colonel Ülar Lohmus said. "Russian planes often don't have their tracking systems on or might not even be equipped with transponders." In those instances, NATO planes get as close as they can and take photos of them or if they are violating Estonian air space over the Gulf of Finland, push them back.

"Aggressive and unpredictable"

Air policing over the Baltic Sea is a shared responsibility

It has not happened as much recently as it did in the early days of the mission, which began in 2014. The mission is clearly working, yet Russian air force planes still intentionally provoke the NATO defense teams.

"Our pilots can even wave at the Russian pilots," Entrialgo said. There were ten such cases in the four months the Spanish squadron was there. The planes get within a couple of hundred meters of each other, which is a dangerous situation given that both the NATO aircraft and Russian jets are often armed.

Estonian Prime Minister Taavi Roivas describes his Russian neighbor's behavior as "aggressive and unpredictable." Speaking at a press conference, he said Estonians were "both grateful and proud to be taken seriously as NATO partners."

He added that NATO Air Policing in Estonia proves the alliance has not forgotten its outermost eastern members. "It feels good to be under the NATO umbrella in these times of crisis."

An air force in the making

The country's own air force is not yet one to be reckoned with. With some two percent of the GDP going on national defense, Estonia is ahead of the other Baltic states, but it is not enough for fighter jets and a well-oiled air force.

Britain takes over this week, then the job will fall to Germany again in September

It provides its NATO allies with the air base. The hangars at Ämari accommodate a couple of Estonian training planes, helicopters, two Antonovs and even an ancient biplane. Having other members of the alliance stationed locally is a plus for the country's own troops.

"We have learned a lot from the NATO soldiers that have been here so far," Lohmus says in reference to the Danish, German and Spanish forces who have taken part in the mission to date.

"Cold week" in spring

It is quiet on the tower of the air base at the moment. Estonian pilots drink coffee and eat cake as they watch the airfield on their computers. It is a "cold week", in which other NATO quick reaction forces have taken over the armed surveillance mission. The skies over the Baltic have been monitored from Lithuania since 2004.

A Spanish jet has just landed, and reports back to the tower with the news of no major incidents. For Francisco Elias Entrialgo's troops and his Spanish comrades it was the last sortie from the Estonian base. On 5 May, their British counterparts will take over, until September, when German pilots return.

And all things being equal, even Entrialogo and his troops will be back. Maybe then they will get lucky with the weather.