Weather Blog

Which is more summer-like: June? Or September?

Which is more summer-like: June? Or September?
Which is June and which is September? Or is it both?

Some people might think of June as the first of month of summer. Yes, June is the month of morning clouds and "June gloom" but school's getting out, the solstice is late in the month, rain is much less frequent, and temperatures are rising.

Meanwhile, while September gets the occasional "Indian Summer", it's when we go back to school, Bumbershoot signals the finale of the summer festivals, the nights are getting longer in a hurry and fall begins three weeks in.

So which do you think is the nicer month, weather wise, in Seattle?

It turns out, the months are virtual twins. And we're not talking fraternal twins. We're talking identical twins.

The average high temperature in June: 69.4 degrees. Average high in September: 69.4 degrees. That's spanning Sea-Tac Airport's entire history from 1945-2011.

For rainfall, it's very close as well. It is slightly wetter in September at 1.80" versus 1.50" in June if you count the entire 67-year record. However, if you just go with the latest 30-year period from 1980-2010 (what NOAA uses as the published averages you see online) it flips around -- June's average is 1.56" versus September's 1.45"

But even more amazing, check out this chart comparing how Junes and Septembers have fared when compared individually against each other in the same year

Number of years Average high temp Monthly Rain Days w/ Rain Days 80+
June > September 31 31 24 29
Tie 3 3 10 11
September > June 33 33 33 27

So it seems the speed at which fall arrives late in September is counter-balanced by the delay in which summer arrives in June. (Or, July 5th, as many around here can attest.) September is a little wetter than June as far as rainy days, probably the bulk of that extra September rain comes near the end of the month. But it's not much of an advantage in the total rain gauge.

So next question: Does having one dry and warm cost us in the other month? The last two years have had rather gloomy, cool and wet Junes only to be followed by a rather amazing September. Is that a norm? If summer starts late, do we get a September reprieve? If June is nice are we doomed to an abrupt end to summer in early September?

Is it that Seattle's summer is exactly three months so it's either June or September but not both?

For these calculations, I'm using the entire 1945-2011 averages, not the past 30 year average.

Both Months Cold: 16 Both Months Wet: 13
Both Months Warm: 10 Both Months Dry: 21
Alternating Months 26 Alternating Months 25
Even (Both within 0.5F) 2 Even (Both within 0.10") 2
One month warm/other normal 5 One month wet/other normal 2
One month cold/other normal 8 One month dry/other normal 4

So I'd say there is no real signal here to latch on to. Some years we get a full four months of summer; others it's just two (or one? :) )