Posts Tagged ‘weather’

Diptics: Freezing rain event

December 29, 2011

I went for a short photowalk to photograph a freezing rain event that occurred overnight January 17-18, 2011, in Alexandria, Virginia USA. The following three- and four-panel diptychs were created using Apple Aperture and Diptic app for Apple iOS mobile devices.

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Here is a link to 18JAN2011_freezing-rain, one of my Google Picasa Web Albums, featuring all of the photos I shot soon after the weather event ended.

Teacher Tips: What is freezing rain? For a graphic explanation, see “How winter storms bring rain, ice and snow,” an interactive online article from the USA TODAY Weather Book by meteorologist Jack Williams. See also, “Snowflakes – A Thematic Approach (A Flurry of Interdisciplinary Ideas for Teachers)” perfect for enriching/extending everyday instruction during the winter season.

Photos © Copyright 2011 Walter Sanford. All rights reserved. www.wsanford.com

Frozen dew — it’s just a phase I’m going through!

November 4, 2011

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I observed frozen dew on the outside of the windshield of my 2007 Honda Civic four-door sedan, at ~8:30 a.m. on Wednesday, 02 November 2011. There was dew (liquid) on the driver’s side door. In fact, I remember my hand got wet when I opened the car door. I thought the windshield had dew on it, that is, until I turned on the windshield wipers … surprise, the windshield was covered with ice (solid)! In the preceding photo gallery (shown above), you can see frozen water droplets as well as rivulets where liquid water had been flowing down the windshield before freezing solid.

What is frozen dew? The National Weather Service Glossary defines “frozen dew” as follows:

When liquid dew changes into tiny beads of ice. This occurs when dew forms and temperatures later drop below freezing.

First, dew formed when the air temperature reached the dew point temperature and water vapor (gas) in the atmosphere condensed to become dew (liquid). Next, some surfaces (e.g., my automobile windshield) reached a temperature below the freezing point temperature of water substance (0°C, 32°F). Finally, the dew froze into ice (solid). Voila, frozen dew! Frozen dew occurs only a few times a year, usually during spring and fall.

Is frost the same as frozen dew? In a word, no! Remember, frost NEVER exists in the liquid phase — frost forms when water vapor (gas) changes phase to ice (solid) in a process called deposition.

In-Depth Analysis

The following data table shows select weather observations at Ronald Reagan Washington National Airport (DCA) on Wednesday, 02 November 2011. (Note: All temperatures expressed in Fahrenheit degrees. Dew Point Depression = Temperature – Dew Point Temperature. Weather data provided courtesy Brandon Peloquin, Senior Forecaster, National Weather Service Baltimore/Washington Forecast Office, Sterling, Virginia.)

Time (UTC) Time (EDT) Temperature Dew Point Temperature Dew Point Depression
04:00 12 midnight 46 42 4
05:00 1:00 a.m. 45 41 4
06:00 2:00 a.m. 43 40 3
07:00 3:00 a.m. N/A N/A N/A
08:00 4:00 a.m. 43 39 4
09:00 5:00 a.m. 42 39 3
10:00 6:00 a.m. 42 38 4
11:00 7:00 a.m. 42 38 4
12:00 8:00 a.m. 42 38 4

Notice that the temperature never was equal to or lower than the dew point temperature at any time overnight. Further, the temperature never was below the freezing point temperature. Nonetheless, dew formed on some surfaces of my car and frozen dew formed on other surfaces. A rule of thumb commonly used by meteorologists says that condensation and/or deposition may occur when the dew point depression is either equal to or less than five (5) Fahrenheit degrees, due to radiative cooling on cloudless nights (like Tuesday night/Wednesday morning).

Related Resource:Freezing Rain Event,” (a post on my WordPress blog) featuring a few more photos of my car covered in ice that formed by a different type of physical change.

Light Snow on Cherry Blossoms

March 27, 2011

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One week after the March Equinox, sometimes called the Spring Equinox (in the Northern Hemisphere), it snowed in Washington, D.C. Although snow in late March is unusual in Washington, D.C., the latest measurable snowfall was recorded on 28 April 1898.

I went for a short photowalk to photograph snow on the cherry blossoms in the community where I live: light snow was falling when the photowalk began; the sky was clearing rapidly when the photowalk ended. According to the National Park Service, cherry blossoms around the Tidal Basin will peak on Tuesday, 29 March 2011.

Freezing rain event diptych

March 13, 2011

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I went for a short photowalk to photograph a freezing rain event that occurred overnight January 17-18, 2011 in the Washington, D.C. metropolitan region. The diptych (shown above) was created using Diptic app.

Related Resources:


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