Warm But Not Hot

 During the first two weeks of May, there is often a short warm period that surges to 80°F or more, followed by a serious cool down.

This year will be no different.

Below are the forecast temperatures for Seattle.  Warming to 80F on Monday, followed by cooling into the upper 60s, with no major heat through May 11.

To illustrate this typical temperature pattern, below is a plot of temperatures in Seattle for April 1- May 31, 2024.  Observed temperatures are in blue and record highs in red.  Record lows in blue.

A major warm-up around May 9, followed by cooler temperatures the rest of the month.  Classic.  

You will notice that May brings the end of any frost threat.....good for gardeners to keep in mind.

So why do we often see a spike in temperatures in early May followed by cooling?

First, the sun has become strong in May, as strong as it will be in August.  Solar radiation at noon is summer-like, and days have become longer.  

Below is the solar radiation reaching Seattle since January 2025.  By May 1, we are really cooking.

To get warm temperatures, even in summer, we need a period of offshore flow, since onshore flow off the chilly Pacific will not allow us to get out of the 60s.

To produce the upcoming warming, this weekend will bring offshore-directed (easterly) winds, as illustrated by the winds, temperatures, and heights (pressures) around 5000 ft (850 hPa pressure) on Saturday morning (below).

Why do we typically have fewer really warm days later in the month?  

Because later in May brings extensive low-cloud areas over the eastern Pacific as high-pressure builds offshore.  This high pressure pushes the cloudy/cool air into western Washington. 

We call this "June Gloom," but it typically starts to move in during mid-May.

Here is an example of June Gloom cloudiness from last May.  Enough to make me want to grab a sweater.

Contrail Fest over Eastern Washington

The satellite imagery over eastern Washington this morning looked like someone had gone crazy with a white crayon,  drawing many white lines, most extending roughly east-west.

Here is a closer view.   The white lines are contrails created by aircraft.

Look very closely, and you will see some black lines as well:  the shadows are produced by the contrails stopping the sun's light from reaching the surface.

Contrails are formed by the combustion of jet fuel in aircraft engines.  This combustion produces water vapor that condenses into water droplets, which eventually freeze into crystals in the cold temperatures aloft.   Combustion particles aid the condensation.

Contrails are thicker when the air is already close to saturation.  Thus, contrails are particularly obvious when the air aloft is already moist.   

Look closely at the satellite imagery above, and you will see a thin veil of cirrostratus clouds, indicating air already at saturation.  Adding more water from the jet engines simply makes the clouds thicker.

Or we can look at the temperatures (red lines) and dew points (green lines) from the radiosonde (balloon-born weather observations) at Forks, on the Washington Coast (below).  Near the level at aircraft fly (marked trop on the figure), temperature and dew point are nearly the same, indicating a moist atmosphere near saturation! (the Y-axis is height in terms of pressure; 500 is about 18,000 ft, 250 is about 35,000 ft).

Temperature (°C)

Climate Change and Contrails

Contrails both cool that atmosphere (by reflecting solar radiation to space) and warm it (by emitting infrared radiation down to the ground).   

A number of careful studies have found that the warming is dominant.  

Thus, contrails contribute to global warming.  Much, much less than CO2, but still a contribution.

One way to reduce this warming by contrails is to alter flight paths to avoid cirrostratus clouds or regions where the upper troposphere (where planes fly) is near saturation with water vapor.   Some folks have suggested this be done.

In any case, the effects of contrails are relatively small, much, much smaller than the aggregate effects of global CO2 increases.