Seems simple enough:
int readingCount = 0;
decimal sumOfTemperatures = 0;
socket.eventPool.Subscribe("flight", (x, y) =>
{
sumOfTemperature += y.STR1.temp;
readingCount++;
Console.WriteLine("{0} {1}", y.STR1.temp, y.STR1.alt);
Console.WriteLine("Average temperature: {0}", sumOfTemperature / readingCount);
});