Clouds of the Day - Monday, August 24, 2020

Haze is particulate matter such as smoke, dust or other dry particles in the air. It may also include aerosols which are very small liquid droplets. Haze may obscure both horizontal and vertical visibility. Our Iowa sky is normally a deep blue color in a clear atmosphere however prolonged periods of calm or light winds from the industrial states east of us bring haze our way. Agriculture can also generate dust particles when field work is underway - especially in the spring and fall.

Today a large high pressure area aloft has created stagnant conditions aloft from the central to southwestern United States. Instead of the deep blue sky we are used to, our sky is a milky color. In this case the haze is smoke from wildfires in the western United States.

A haze layer may be readily seen when flying. It is common for a layer of haze to exist below about 15,000 feet under the proper conditions. The height of the layer varies but when an aircraft climbs above the layer the sky suddenly turns blue with the haze visible below. High pressure areas encourage haze. Low pressure stirs the air and mixes out the particles - even using some of them as nuclei for condensation, rain and snow.

The photo above shows the haze very well - including the milky sky. The high pressure in this case is a large area of warm air aloft that is trapping particles in the lower atmosphere. It is also creating very warm humid conditions in the middle of the United States into the southwestern states. The air sinks through a high pressure area causing the air to warm. Warm air aloft suppresses rising motion and therefore storms are also suppressed. Storms are occurring around the northern and southern edges of the high pressure in the central U.S. where the atmosphere is unstable during the later afternoon into the nighttime hours.

Below is the 500 millibar chart from (8-25-2020). This chart depicts the wind and temperature at roughly 18,000 feet. I say “roughly” because the chart is not at a constant height. Instead, it is at a constant pressure of 500 millibars which is near 18,000 feet. In some places the high of the 500 mb pressure is higher and lower in other locations. Winds are strongest where the black lines are closest together. If you know how to read the wind barbs you can see that winds are less than 10 knots (11.5 mph) near the center of the high pressure over Colorado and New Mexico. Winds of 50 knots (58 mph) are measured over Green Bay, WI, Gaylord, MI, and Spokane, WA. It is the stagnant winds and sinking air in the high pressure that trap particulate matter.

The air is flowing from west to east from the Pacific Northwest then along the Canadian border and across the Great Lakes. The flow is turning clockwise around the high center. The wind barbs are pointing in the direction of flow with the flags at the back of the symbols. Hurricane Laura is located just west of Cuba. The red numbers are the 500 mb temperatures in degrees Celsius. Use a temperature conversion calculator on the internet to convert to degrees Fahrenheit if you like. You will find the warmest air aloft is near the center of the high and it is cooler to the south and much cooler to the north.

500 mb Upper Air chart is courtesy of NOAA and the National Weather Service.