Why is it so wet?

Yesterday felt quite “sticky”. Wet and a bit uncomfortable. Several people emailed me about it and social media was full of complaints (see sample below).

The best measure of uncomfortable humidity in the air is Dewpoint temperature, often referred to simply as dewpoint.  

The amount of water vapor that air can hold increases rapidly with temperature. If you start with a sample of air that has a certain amount of water vapor and cool it, the air will eventually become saturated (100% relative humidity). The temperature at which this occurs is called the dew point.

The higher the humidity in the air, the less you need to cool the air to reach saturation.

Therefore, the dew point is a good measure of the humidity of the air. A higher dew point means there is more water vapor in the air.

When the air is saturated, you can’t cool yourself by sweating—evaporating water from your skin. And without that cooling mechanism, you’ll feel warm when the temperature is high. That’s the “sticky” feeling.

Most people feel quite comfortable when the dew point is below 60F.

They feel a bit “sticky” at dew points of 60-64. Not happy with dew points of 65-70. And definitely uncomfortable above 70.

The highest dew point I’ve ever experienced was 83F in Washington DC. It was horrible.

So what was the dew point in the surrounding area yesterday?

Below is a map of dew points at 5pm. Every location shown is above 60F and many are in the high 60s. Some are even in the 70s! (yellow).

If you look closely, the highest dew points (70 and above, yellow) are on the western slopes of the Cascades and near Portland.

Around Northwest Seattle and Kirkland at the same time, many locations had dew points above 60 degrees. Disgusting.

The past few days have seen the highest dew points of the year in our area. The University of Washington dew point chart shows the recent increase and much lower levels last week.

The “Secret” to Getting High Dew Points in Western Washington and Oregon

Believe it or not, it’s hard to get high dew points around here. Why is that?

Most of our air comes from the Pacific Ocean, making it difficult to achieve high dew points.It is difficult for the air to absorb much water vapor as it flies over the Pacific Ocean.

Shocked? The reason is because the Pacific Ocean is cool (about 50F year round in our area) and therefore the air cannot absorb as much moisture from above.

Occasionally, such as during the most intense heat waves, winds blow eastward (from the east) across the region. This air starts out warm over the interior and becomes warmer as it sinks west of the Cascades.

The air is so dry because the area east of the Cascades is quite arid.

To summarize, here is a diagram of the situations described so far. If the air is coming from the west OR east, it will be difficult to achieve a high dew point.

So how can we get air with higher humidity and therefore higher dew points in western Washington?

I will describe this “trick”.

If air passes over moist vegetation in western Washington, western BC, and western Oregon under warm conditions, moisture can accumulate, causing dew points to rise to 60 degrees.

And the likelihood of a high dew point increases if it has rained recently.

And it DID rain a few days ago…mostly on July 27th. Here are the totals for the past week. It was very heavy, especially in Northwest Washington and around Portland.

We can use model simulations to calculate the three-dimensional trajectory of air over time…in this case, the three-dimensional path of air approaching the surface. Looking at the airflow approaching SeaTac yesterday afternoon, it came from northwest Washington (see below).

The air that blows into Portland comes from the surrounding humid terrain.

I’ve looked at a lot of data on such trajectories……all agree that…the super humid air flow is not from the ocean or the east, but has passed through the wet, wooded west side of our area.

Of course, not everyone is happy with this.