Tuesday, December 19, 2006

How Big Is the Diurnal Cycle This Time of Year?
And a Warm Front Passage!

This Thursday is the Winter Solstice, the shortest day of the year for much of the Northern Hemisphere. Today, Helsinki experienced about 5 hours and 47 minutes of day, with the sun rising at 9:24 a.m. and setting at 3:11 p.m.

For the last few days, we've had totally clear or partly clear skies. Today was clear all day. Here is the 24-hour temperature trace for Kaisaniemi (near city center) ending at about 9 p.m. (21 on the horizontal axis). Notice the 1 degree C rise in temperature followed by 2 degree C decrease in the early afternoon (between 12-15). This was the effect of daytime heating, superimposed on a larger-scale temperature decline due to radiative cooling dominating the anemic daytime heating.

Notice the rapid rise in temperature in the last hour (5 deg C or 9 deg F). We have a warm front approaching our area. Note the temperatures in this surface map
to the right.

The third image is a plot of the stability from the top to bottom of the masted stations in the Helsinki Testbed. [As the ex-expats mention in their comment, the top height of the masts vary from mast to mast, so although the stability is normalized by the mast height, the values may not be measured over a consistently deep layer. Buyer beware!]

If the values are large and positive as they are over most stations on the land, that indicates strong static stability, indicative of the warm front moving in. Stations over the sea have negative static stability (unstable conditions).


At 12:01 AM, Blogger Ex-Expats said...

Read the small print on the Testbed page:
"Note: stability is calculated by using formula S=(T1-T0)/(z1-z0), i.e. the temperature difference
between the mast top and the base divided by the difference of the corresponding altitudes.
Positive values indicate surface inversion i.e. very stable conditions."
At least they try to make the values comparable (does not work if the gradient is not the same though the entire layer).

At 10:58 AM, Blogger David Schultz said...

The ex-expats are right. I have corrected the blog entry. I'll also provide a link to your site. Kiitos
from the New Guy in Town. ;-)



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