Snow without Clouds

Snow crystals, called diamond dust, in the form of ice needles can fall from the sky even when clouds are not present.  At temperatures below minus 20 degrees, ice needles can form as long as six-sided deposition nuclei are present for the water vapor to freeze on.  Ice needles form when water vapor changes to ice, skipping the liquid phase.  This is called deposition.

A six-sided deposition nuclei's shape exposes it more directly to water vapor in the atmosphere allowing it to convert the water vapor to ice more easily. 

San Jose Fall Temperatures

I have family coming to San Jose in the last week of September and the first week of October.  What is the average high and low temperature for that time of year? 

The average high is near 80 degrees with an overnight low near 55 degrees.  The chances of rain are less than 10 percent.

Bay vs. Midwest Humidity

Q: Given the fog and its proximity to so much water, why doesn’t the Bay Area feel as humid as the Midwest? 

A:  The Midwest heat index is higher due to warmer temperatures, average July high of 66 vs. 90 degrees for San Francisco vs. St. Louis.  Also, the San Francisco Bay wind makes it feel less oppressive. 

Fronts and Forecasting

A front's vertical thickness and slope are also factors in forecasting. The thicker the wedge of air and the steeper the slope, the stronger the rising motion it produces. Vigorous rising motion helps create showers and thunderstorms. This same rising motion is created by topography, such as mountains.

Difference between weak and strong fronts.

Q:  What is meant by a "weak front" or a "strong front”?

A:  Meteorologists typically use two criteria in determining whether a front is weak or strong.  The greater the pressure and temperature change from one side of the front to the other, the stronger the front.

Tioga Pass

Hi Mike,

My husband and I are traveling to Yosemite in mid-May. We would like to continue on to June Lake. What are the chances that the Tioga Pass will be open by the third week of May? At this point it is probably anyone's guess. Right?

Nancy Lund, CTC, DS
Peak Travel Group
408-286-2633vm310
408-499-3229 Cell
nancytravels@sbcglobal.net

A: The National Park Service (NPS) begins clearing the Tioga Road around April 15th. Depending on snow depth, this usually takes between one and two months. Check this link for updates, Tioga Pass Info.