Desert Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Desert Center clear sky chart from 2003-08-01 to 2005-11-07.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	74%	8671
	Night	5% 73%	8500
Transparency	Day	11% 26% 56%	7610
	Night	5% 15% 28% 48%	7469
Seeing	Day	11% 26% 38% 21%	591
	Night	5% 16% 32% 39% 6%	2993

By Month

Percentage Cloud Cover
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month		Occurrence of each Forecast	Data Points
January	Day	6% 7% 61%	565
	Night	5% 5% 7% 54%	787
February	Day	13% 5% 6% 5% 45%	561
	Night	14% 6% 8% 49%	684
March	Day	5% 75%	661
	Night	6% 73%	701
April	Day	82%	724
	Night	80%	596
May	Day	5% 82%	806
	Night	5% 79%	558
June	Day	95%	780
	Night	98%	537
July	Day	75%	795
	Night	6% 78%	558
August	Day	5% 5% 6% 63%	1130
	Night	5% 5% 5% 65%	913
September	Day	83%	982
	Night	88%	964
October	Day	6% 81%	650
	Night	79%	714
November	Day	7% 66%	468
	Night	5% 74%	698
December	Day	6% 67%	549
	Night	8% 6% 65%	790

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	8% 29% 46% 12%	407
	Night	8% 8% 24% 36% 21%	547
February	Day	10% 11% 33% 27% 17%	330
	Night	9% 17% 30% 25% 17%	434
March	Day	11% 17% 29% 40%	562
	Night	9% 25% 23% 39%	584
April	Day	12% 30% 54%	639
	Night	17% 24% 55%	525
May	Day	9% 27% 62%	748
	Night	14% 25% 57%	498
June	Day	17% 80%	768
	Night	36% 58%	529
July	Day	6% 20% 69%	681
	Night	7% 25% 63%	497
August	Day	10% 25% 57%	868
	Night	13% 31% 49%	696
September	Day	13% 84%	907
	Night	12% 83%	900
October	Day	10% 28% 58%	849
	Night	14% 27% 52%	948
November	Day	27% 33% 30%	430
	Night	8% 25% 31% 31%	690
December	Day	7% 9% 38% 40%	421
	Night	5% 14% 42% 33%	621

Seeing (when cloud cover <= 80%)
Color Meanings: Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month		Occurrence of each Forecast	Data Points
January	Day	31% 37% 18% 12%	16
	Night	9% 25% 35% 27%	244
February	Day	20% 33% 29% 14%	48
	Night	10% 31% 33% 21%	200
March	Day	12% 24% 32% 27%	58
	Night	7% 20% 34% 34%	241
April	Day	21% 28% 32% 17%	56
	Night	8% 27% 36% 24%	225
May	Day	10% 36% 36% 14%	57
	Night	6% 23% 36% 28%	185
June	Day	5% 33% 41% 20%	60
	Night	12% 31% 51%	180
July	Day	19% 58% 17%	62
	Night	27% 57% 10%	184
August	Day	13% 11% 49% 24%	89
	Night	22% 58% 14%	340
September	Day	6% 24% 39% 27%	87
	Night	8% 26% 54% 7%	356
October	Day	10% 34% 20% 29% 5%	58
	Night	16% 28% 42% 8%	367
November	Night	10% 15% 44% 26%	246
December	Night	10% 20% 39% 26%	225

Page generated at 2008-09-01 23:07:44. Version ab11