Vog in paradise

Vog? What is that? Well, fog is tiny suspended droplets in the air, while vog are suspended particulates from volcanoes. This volcanic air pollution casts a hazy scene near the Hawaiian Islands as sulfur dioxide from the volcanoes mix with oxygen and water vapor in the atmosphere to form tiny sulfate particles. These particles can reflect the sunlight, making the extent of vog detectable by satellites, such as in this example from 2008.

Image from MODIS on 2 December 2008 showing the milky haze around the Hawaiian Islands indicating vog. Image courtesy of NASA’s Earth Observatory: http://earthobservatory.nasa.gov/IOTD/view.php?id=36089

While the vog in this example was an extreme case, from the ground, these particulates can create a hazy view. This picture was taken by Stephen Green on a plane near the Kona airport on the Big Island of Hawaii, showing an example of the haze.

Stephen Green (Imaginscape Photography), Kona, Hawaii, 1 Apr 2015

Notice how the haze is trapped in a shallow layer near the ground. This happens because of what’s called an inversion, where the temperature increases with height instead of typical decreasing. This “trade inversion” provides a cap to the vertical growth of clouds, which is why the cumulus clouds in this photo remain shallow in this layer. This stable scenario forms when winds are weak, so the vog persists in this shallow layer of stagnant air near the surface. Balloons launched twice a day from locations around the U.S., including Hawaii, carry instruments into the atmosphere that measure the vertical profile of temperature, moisture, pressure, and winds. An example of these measurements from Hilo, on the day this photo was taken (1 April 2015), shows the existence of this temperature inversion, with dry air above it and moist, relatively calm conditions below.

A study by Guanxia et al. [Guangxia Cao, Thomas W. Giambelluca, Duane E. Stevens, and Thomas A. Schroeder, 2007: Inversion Variability in the Hawaiian Trade Wind Regime. J. Climate, 20, 1145–1160. http://journals.ametsoc.org/doi/full/10.1175/JCLI4033.1] used these observations from Hilo and from another location on the island to determine how often this trade wind inversion occurred. They found that the inversion occurs approximately 82% of the time at each station. The following figure from their paper also shows the height and strength (determined by temperature) of the inversion varies based on time of the year.

Annual cycles of the (a) inversion base height and (b) inversion strength at Hilo and Līhu‘e, Hawai‘i, based on data from 1979–2003. (Figure 6 from Guangxia et al. 2007)

Here is another picture from Stephen of an obstructed view of the sky due to vog. In this example, haze from the Pu’u O’o eruption limited the view of lenticular clouds near Mauna Kea on the big island on 8 February 2015.

Stephen Green (Imaginscape Photography), Hawaii, Feb 2015

These inversions aren’t present all of the time as weather systems can move through and eliminate the stable layer, provide moisture, and remove the vog particulates. In these cases, the view on the big island is clearly stunning.

Stephen Green (Imaginscape Photography), Hawaii, March 2015

The upside to this inversion is that the vog and clouds are trapped in the lower part of the atmosphere, leaving a crystal clear view of the sky above. The Mauna Kea observatory is truly a sight to behold and we’ve had the fortune to gaze at the stars from that location on one of these clear nights.

Check out more of Stephen’s pictures on his Facebook page: https://www.facebook.com/stephengreenimages?fref=ts

Stacked lenticular east of the Cascades

Jerry Tangren
Location: East Wenatchee, Washington
Date: 29 April 2013

When stable air flows over mountain ranges, the flow can oscillate with stacks of lens-shaped clouds forming in the rising areas of the motion. The image below shows a simplified diagram of this.

Note that there’s also turbulent air closer to the surface. These gusty downslope winds are not uncommon in this scenario and in fact, Jerry Tangren, who sent the beautiful lenticular photo posted above, noted that area was under a high wind warning that day. East Wenatchee is located just to the east of the Cascades in Washington State, creating a prime location to view these beautiful wave clouds as stable flow comes in at mid-levels off the Pacific.

 

Wave clouds over Colorado

Jessica Sue Mills McGee
Location: Westminster, CO
Date: 28 June 2014

When strong stable air flows perpendicular to mountain ranges, an oscillation is created downstream of the mountains. These peaks and valleys in the air are trapped within the stable layer, and wherever there’s enough moisture, clouds can form. These clouds commonly appear in the mid-levels and are lens-shaped, appearing like flying saucers at times. They are called Altocumulus lenticularis and a particularly good example of this cloud type can be seen in the top right photo above. In the other photos, you can see the wave clouds over Westminster, located to the east of the Rocky Mountains. The strong zonal (west-to-east) airflow across the mountains created this beautiful sight at sunset while lower levels of the atmosphere remained dry.

Here is a diagram from the COMET program that helps show what we mean by oscillating air in a stable environment.

A mixture of clouds in NW Taiwan

Ming-Hwa Tang
Location: Miaoli City, Taiwan
Date: 17 June 2014

What a fun mixture of clouds over northwest Taiwan on this late spring afternoon! High above, the faint, nearly transparent clouds are Cirrus clouds made up of ice crystals. Moving down to the mid-levels are Altocumulus clouds. Note that they are slightly lens-shaped. This indicates the lenticularis variety, or more commonly referred to as Lenticular clouds. They form when relatively stable air flows over mountains, which in this case are part of the Central Mountain Range oriented N-S across this country. Heating of these mountains peaks, however, allows the sub-tropical moist air to rise to produce the shallow cumulus clouds over the high terrain. The presence of the more stable lenticular clouds aloft suggest that these cumulus clouds will be limited in vertical growth.