Weather Watch

Searching for contrails in the skies of San Luis Obispo

A contrail from a jet seen at sunset in Morro Bay last week.
A contrail from a jet seen at sunset in Morro Bay last week.

Over the past few weeks readers have asked what has happened to the white narrow clouds produced by aircraft traveling up and down the West Coast on their way to Los Angeles, San Francisco or Seattle.

In three words, it’s “low relative humidity.” Here’s what I mean:

These white narrow clouds are called condensation trails or contrails. They can evaporate swiftly if the relative humidity of the surrounding air is low. Conversely, if the relative humidity is high, contrails can last for many hours.

Contrails typically form when the air temperature is at or below minus-40 degrees Fahrenheit or Celsius. At these temperatures, small clouds form when moisture condenses and instantly freezes around the tiny particulates found in jet airliners’ exhaust. These clouds are ice crystals and look similar to wispy strands of cirrus clouds.

Most commercial jet airliners fly at an altitude between 28,000 and 41,000 feet — about 6 miles up in the sky — where temperatures are cold. Over the past few weeks, dew point temperatures have been frigid and relative humidity levels bone-dry.

(The dew point is the temperature at which air can no longer hold all of its water vapor and becomes saturated, causing it to condense into dew or fog.)

So how do we determine the dew point temperature so far up the sky? Every morning, the 30th Weather Squadron from Vandenberg Air Force Base launches weather balloons with a tiny transmitter called a radiosonde attached. As the weather balloon climbs through the atmosphere, its transmitter broadcasts back to the receiving station readings on temperature, dew point temperature, pressure and GPS coordinates for the winds.

The attached Skew T chart is a vertical graph of the atmosphere from Saturday morning’s weather balloon launch from Vandenberg Air Force Base, and it shows some interesting data. The red line to the right is the air temperature in degrees Celsius, the green line to the left is dew point temperature also in degrees Celsius.

At about 925 millibars, or 2,500 feet, the balloon flew through a temperature inversion layer as the red line (air temperature) broke to the right and increased to 18 degrees Celsius or about 64 degrees Fahrenheit and the green line (dew point) moved left to 0 degrees C or 32 degrees F, increasing the dew point spread to 64 degrees F and reducing the relative humidity to near 30 percent.

At 850 millibars, or about 4,800 feet, above the earth the dew point temperature plummeted to minus-25 degrees C. This condition caused the calculated relative humidity level to dropped 7 percent, dry indeed. In fact, it was too dry to produce any contrails.

At 300 millibars, or about 30,000 feet, the air temperature lowered to minus-40 degrees. However, the dew point decreased to minus-52 degrees C (minus-62 degrees F) with a calculated relative humidity level of around 26 percent, still too dry for decent contrail formation.

The weather balloon sounding also revealed 50-knot southwesterly winds in the upper levels of the atmosphere. At this time of the year, upper-level southwesterly flow is usually dry with low relative humidity levels.

These charts can be found on the National Weather Service office in Los Angeles/Oxnard website at on the tab for Upper Air and Soundings.

At other times on the year, if the green line and red line are close to each other, the air is near saturation and the relative humidity levels near 100 percent; if the air temperature is around minus-40 degrees, contrails will surely form and, depending upon the upper-level winds or wind shear, can last for hours. If you look to the sky this week, you probably will see contrails from aircraft as the upper level winds shift out of the northwest, which has higher humidity levels.

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In the United States, the generation of electricity is one of the largest contributors of carbon dioxide. However, in PG&E’s service territory, the generation of electricity utilizing nuclear, hydroelectric, solar, wind, geothermal and biomass has made our state one of the cleanest in the country. In fact, more than 55 percent of the electricity that PG&E delivers to its customers is carbon-free. With each passing year, it’s expected to become cleaner.

John Lindsey is the PG&E Diablo Canyon marine meteorologist and media-relations representative. Email him at or follow him at