Archive for November, 2011

White-tailed deer tracks

November 7, 2011

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White-tailed deer (Odocoileus virginianus) tracks spotted along the banks of an intermittent stream running through the “Wildlife Sanctuary,” one of seven small parks owned and maintained by the Community Association of Hollin Hills, Fairfax County, Virginia USA.

Photo 1 of 3 was annotated to show the deer’s direction of travel toward the stream (red arrow, pointing forward); Photo 2 of 3 is the original photograph. Grooves in the moist soil (perpendicular to the stream), evident in all photos, may indicate that the deer slid down the stream banks as it crossed the creek. Note: The blue rectangle in Photo 1 highlights human tracks (the impression made by an athletic shoe), probably made before the deer tracks.

Related Resources:

Tech Tips: The preceding photos were post-processed using Apple “Aperture,” a professional-grade tool for organizing and adjusting photos. Apple “Preview” was used to annotate Photo 1 of 3.

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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.


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