The Decker-Grebner-Van Zandt Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Decker-Grebner-Van Zandt Observatory clear sky chart from 2005-11-06 to 2026-09-29.

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	12% 8% 3 8% 5% 5% 5% 4 6% 7% 38%	149552
	Night	13% 9% 3 8% 5% 4 4 3 5% 6% 40%	142190
Transparency	Day	43% 5% 12% 24% 14% 2	153073
	Night	43% 5% 11% 23% 15% 2	146747
Seeing	Day	16% 11% 29% 25% 14% 4	18423
	Night	17% 10% 32% 29% 11% 1	28147

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	11% 12% 6% 13% 6% 5% 5% 4 6% 7% 26%	4691
	11th to 20th	Day	7% 10% 5% 11% 8% 6% 6% 4 6% 6% 29%	4710
	21st to end	Day	12% 14% 7% 10% 6% 5% 5% 4 5% 5% 28%	5767
February	1st to 10th	Day	16% 11% 5% 10% 6% 6% 4 3 4 5% 29%	1890
	11th to 20th	Day	13% 11% 3 8% 7% 5% 5% 3 5% 6% 34%	2007
	21st to end	Day	13% 12% 5% 11% 5% 4 3 3 5% 5% 33%	1730
March	1st to 10th	Day	19% 12% 3 6% 3 3 3 2 3 4 41%	4580
	11th to 20th	Day	8% 7% 2 6% 5% 5% 5% 4 7% 8% 45%	5013
	21st to end	Day	19% 11% 3 10% 5% 5% 5% 3 5% 6% 28%	5974
April	1st to 10th	Day	16% 12% 4 8% 5% 4 4 3 4 6% 35%	2520
	11th to 20th	Day	15% 9% 3 7% 5% 4 5% 4 5% 6% 37%	2494
	21st to end	Day	19% 8% 2 7% 5% 5% 4 3 5% 6% 35%	2520
May	1st to 10th	Day	15% 7% 3 7% 5% 4 5% 4 6% 8% 36%	7612
	11th to 20th	Day	14% 7% 3 6% 4 4 5% 4 5% 7% 43%	8222
	21st to end	Day	12% 7% 3 8% 5% 5% 5% 4 6% 8% 36%	9047
June	1st to 10th	Day	9% 5% 2 6% 5% 4 5% 4 6% 9% 45%	2940
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 4 6% 8% 46%	2940
	21st to end	Day	11% 6% 3 6% 5% 5% 5% 5% 7% 9% 40%	2940
July	1st to 10th	Day	8% 5% 2 6% 5% 4 5% 5% 7% 10% 42%	8280
	11th to 20th	Day	8% 5% 2 6% 4 5% 4 4 7% 9% 46%	8280
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 9% 38%	8990
August	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 4 7% 10% 41%	6070
	11th to 20th	Day	12% 7% 3 8% 5% 5% 6% 4 7% 8% 35%	5531
	21st to end	Day	4 3 2 6% 4 4 4 4 6% 9% 53%	6080
September	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 4 7% 51%	2520
	11th to 20th	Day	11% 5% 2 5% 3 3 3 4 5% 8% 52%	2466
	21st to end	Day	9% 6% 2 6% 4 4 4 3 6% 8% 47%	2288
October	1st to 10th	Day	12% 6% 2 6% 3 3 4 3 5% 6% 51%	2030
	11th to 20th	Day	15% 9% 2 6% 4 3 4 2 4 6% 44%	2000
	21st to end	Day	19% 9% 3 6% 3 4 4 3 4 6% 40%	2085
November	1st to 10th	Day	10% 8% 2 6% 4 3 3 3 6% 7% 48%	1852
	11th to 20th	Day	16% 10% 3 8% 3 4 4 3 4 8% 36%	1890
	21st to end	Day	16% 12% 5% 8% 4 3 4 3 3 5% 37%	1745
December	1st to 10th	Day	8% 9% 4 8% 5% 4 4 3 4 7% 46%	2690
	11th to 20th	Day	14% 15% 6% 11% 6% 5% 4 3 5% 6% 24%	3088
	21st to end	Day	25% 20% 8% 13% 4 4 3 2 3 4 14%	4070
January	1st to 10th	Night	13% 14% 5% 11% 5% 4 4 3 4 5% 34%	8232
	11th to 20th	Night	9% 12% 5% 12% 8% 6% 6% 4 6% 5% 28%	8280
	21st to end	Night	14% 13% 5% 9% 5% 4 4 3 4 4 35%	8444
February	1st to 10th	Night	14% 12% 4 8% 6% 4 4 3 5% 6% 33%	2730
	11th to 20th	Night	14% 10% 4 9% 5% 5% 4 3 4 6% 37%	2497
	21st to end	Night	15% 11% 4 8% 4 3 3 3 4 5% 41%	2076
March	1st to 10th	Night	17% 12% 4 6% 4 3 3 3 3 5% 41%	5493
	11th to 20th	Night	11% 8% 3 5% 4 4 4 3 4 6% 48%	5198
	21st to end	Night	17% 11% 5% 9% 5% 5% 5% 4 5% 5% 29%	5195
April	1st to 10th	Night	15% 10% 2 7% 4 4 4 3 5% 7% 39%	2100
	11th to 20th	Night	16% 8% 3 6% 4 4 3 3 5% 6% 42%	2075
	21st to end	Night	17% 8% 3 6% 5% 4 3 3 6% 7% 38%	2100
May	1st to 10th	Night	17% 8% 3 7% 5% 4 4 4 5% 6% 36%	5224
	11th to 20th	Night	11% 5% 2 6% 4 5% 5% 4 7% 9% 42%	4670
	21st to end	Night	12% 8% 3 7% 4 4 4 4 6% 8% 39%	5139
June	1st to 10th	Night	9% 5% 2 5% 4 4 4 4 5% 7% 51%	1680
	11th to 20th	Night	8% 6% 2 6% 5% 4 5% 3 5% 10% 48%	1680
	21st to end	Night	9% 6% 2 7% 4 4 5% 4 5% 8% 46%	1680
July	1st to 10th	Night	8% 5% 2 6% 4 4 5% 5% 7% 11% 45%	4708
	11th to 20th	Night	6% 4 2 6% 4 4 4 4 6% 8% 53%	4707
	21st to end	Night	9% 6% 2 7% 4 5% 5% 4 7% 9% 43%	5117
August	1st to 10th	Night	10% 5% 2 5% 4 4 5% 4 7% 8% 48%	4083
	11th to 20th	Night	8% 5% 2 6% 5% 5% 5% 4 7% 8% 45%	4516
	21st to end	Night	4 3 1 5% 4 4 4 3 6% 9% 56%	5046
September	1st to 10th	Night	11% 4 2 5% 3 3 4 3 5% 5% 56%	2100
	11th to 20th	Night	9% 5% 2 4 4 3 4 3 5% 7% 54%	2131
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 6% 8% 47%	2289
October	1st to 10th	Night	10% 5% 2 5% 4 4 3 3 4 6% 55%	2368
	11th to 20th	Night	13% 6% 2 6% 3 3 4 3 5% 6% 49%	2400
	21st to end	Night	18% 8% 3 6% 5% 4 3 3 5% 6% 40%	2755
November	1st to 10th	Night	12% 7% 2 5% 3 3 3 3 5% 6% 51%	2666
	11th to 20th	Night	13% 8% 3 7% 5% 4 4 3 5% 7% 42%	2730
	21st to end	Night	17% 10% 4 8% 4 4 3 4 4 5% 38%	2872
December	1st to 10th	Night	10% 11% 3 7% 4 3 3 3 4 6% 46%	4720
	11th to 20th	Night	16% 12% 4 10% 5% 4 3 3 4 5% 33%	5339
	21st to end	Night	24% 18% 7% 11% 6% 4 4 3 4 4 16%	7150

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	57% 4 13% 19% 6% 1	4692
	11th to 20th	Day	55% 4 8% 17% 13% 3	4710
	21st to end	Day	59% 3 11% 19% 7% 1	5767
February	1st to 10th	Day	57% 4 14% 17% 7% 2	1890
	11th to 20th	Day	51% 5% 12% 21% 10% 2	2017
	21st to end	Day	52% 5% 10% 22% 9% 2	1730
March	1st to 10th	Day	48% 4 8% 23% 15% 2	4580
	11th to 20th	Day	36% 5% 11% 32% 16%	5013
	21st to end	Day	55% 5% 13% 20% 5% 1	5976
April	1st to 10th	Day	50% 5% 11% 22% 11%	2520
	11th to 20th	Day	47% 5% 10% 24% 12% 2	2494
	21st to end	Day	49% 6% 10% 20% 13% 2	2520
May	1st to 10th	Day	44% 6% 11% 22% 16% 1	7648
	11th to 20th	Day	41% 5% 11% 26% 14% 3	8226
	21st to end	Day	45% 6% 10% 20% 17% 2	9061
June	1st to 10th	Day	35% 7% 13% 27% 15% 3	2940
	11th to 20th	Day	35% 7% 11% 24% 21% 3	2940
	21st to end	Day	38% 8% 14% 22% 15% 2	2940
July	1st to 10th	Day	36% 6% 15% 26% 16% 1	8211
	11th to 20th	Day	33% 6% 15% 28% 17% 1	8205
	21st to end	Day	40% 7% 17% 24% 11% 1	8794
August	1st to 10th	Day	36% 6% 15% 29% 12% 1	6055
	11th to 20th	Day	44% 7% 15% 23% 12%	5567
	21st to end	Day	27% 5% 14% 33% 19% 2	6079
September	1st to 10th	Day	34% 5% 9% 27% 20% 4	2520
	11th to 20th	Day	30% 6% 12% 25% 23% 4	2466
	21st to end	Day	35% 6% 10% 26% 21% 2	2288
October	1st to 10th	Day	23% 4 6% 26% 39% 2	3303
	11th to 20th	Day	29% 3 9% 22% 33% 4	3240
	21st to end	Day	41% 4 8% 26% 18% 3	3349
November	1st to 10th	Day	36% 5% 14% 26% 16% 3	1852
	11th to 20th	Day	48% 4 11% 24% 13% 2	1890
	21st to end	Day	50% 5% 12% 23% 9%	1745
December	1st to 10th	Day	41% 3 14% 32% 9% 1	2690
	11th to 20th	Day	60% 4 11% 18% 6% 1	3085
	21st to end	Day	75% 4 9% 9% 4	4070
January	1st to 10th	Night	54% 4 13% 22% 7%	8233
	11th to 20th	Night	58% 3 10% 15% 14% 1	8280
	21st to end	Night	54% 3 11% 21% 11% 1	8444
February	1st to 10th	Night	53% 4 12% 20% 11% 1	2730
	11th to 20th	Night	48% 4 12% 24% 11% 2	2498
	21st to end	Night	47% 3 10% 25% 13% 2	2076
March	1st to 10th	Night	47% 5% 7% 25% 14% 2	5493
	11th to 20th	Night	37% 4 9% 28% 18% 4	5198
	21st to end	Night	55% 6% 10% 20% 8% 1	5195
April	1st to 10th	Night	44% 5% 11% 24% 14% 2	2100
	11th to 20th	Night	43% 5% 9% 24% 15% 4	2075
	21st to end	Night	45% 5% 11% 22% 14% 3	2100
May	1st to 10th	Night	47% 6% 9% 20% 16% 2	5251
	11th to 20th	Night	37% 6% 14% 23% 16% 4	4671
	21st to end	Night	42% 5% 10% 22% 19% 2	5149
June	1st to 10th	Night	32% 5% 14% 25% 20% 5%	1680
	11th to 20th	Night	32% 6% 13% 23% 22% 4	1680
	21st to end	Night	36% 7% 12% 23% 18% 3	1680
July	1st to 10th	Night	33% 6% 15% 28% 17% 2	4670
	11th to 20th	Night	29% 6% 13% 29% 22% 2	4662
	21st to end	Night	36% 8% 15% 25% 14% 2	5002
August	1st to 10th	Night	32% 6% 16% 28% 17% 1	4074
	11th to 20th	Night	36% 7% 19% 26% 12% 1	4546
	21st to end	Night	24% 6% 14% 37% 17% 3	5046
September	1st to 10th	Night	29% 5% 10% 28% 24% 5%	2100
	11th to 20th	Night	29% 5% 11% 27% 23% 5%	2131
	21st to end	Night	34% 6% 12% 24% 22% 4	2289
October	1st to 10th	Night	23% 3 6% 27% 37% 4	3875
	11th to 20th	Night	26% 3 8% 23% 35% 5%	3899
	21st to end	Night	39% 4 10% 22% 22% 4	4441
November	1st to 10th	Night	34% 6% 11% 27% 19% 4	2666
	11th to 20th	Night	43% 4 12% 24% 16% 2	2730
	21st to end	Night	49% 3 10% 24% 13% 1	2872
December	1st to 10th	Night	41% 3 12% 31% 12% 1	4720
	11th to 20th	Night	54% 4 11% 21% 9% 1	5341
	21st to end	Night	73% 4 8% 11% 4	7150

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	21% 18% 30% 25% 5%	330
	11th to 20th	Day	23% 22% 38% 14% 3	330
	21st to end	Day	21% 22% 35% 17% 5%	363
February	1st to 10th	Day	22% 18% 40% 16% 3 1	330
	11th to 20th	Day	23% 22% 33% 17% 5% 1	330
	21st to end	Day	21% 16% 49% 13% 1	273
March	1st to 10th	Day	25% 24% 35% 15% 2	330
	11th to 20th	Day	25% 20% 35% 14% 5%	330
	21st to end	Day	30% 17% 33% 16% 4 1	534
April	1st to 10th	Day	25% 15% 32% 22% 4 1	540
	11th to 20th	Day	21% 18% 33% 16% 10% 2	532
	21st to end	Day	21% 15% 36% 20% 7% 1	540
May	1st to 10th	Day	18% 16% 33% 24% 6% 3	564
	11th to 20th	Day	21% 13% 30% 24% 11% 2	775
	21st to end	Day	14% 9% 32% 25% 14% 7%	844
June	1st to 10th	Day	9% 8% 27% 32% 19% 6%	780
	11th to 20th	Day	7% 8% 31% 31% 20% 3	780
	21st to end	Day	12% 8% 32% 31% 14% 4	780
July	1st to 10th	Day	9% 4 18% 34% 22% 13%	771
	11th to 20th	Day	10% 3 22% 33% 23% 9%	768
	21st to end	Day	9% 4 22% 36% 24% 5%	849
August	1st to 10th	Day	6% 4 22% 36% 23% 9%	776
	11th to 20th	Day	11% 6% 20% 28% 23% 13%	579
	21st to end	Day	6% 4 19% 32% 29% 10%	627
September	1st to 10th	Day	14% 5% 18% 32% 26% 5%	570
	11th to 20th	Day	10% 2 18% 32% 27% 10%	570
	21st to end	Day	10% 10% 26% 24% 25% 5%	562
October	1st to 10th	Day	14% 8% 27% 29% 18% 4	360
	11th to 20th	Day	18% 20% 32% 22% 8%	330
	21st to end	Day	31% 14% 32% 19% 4 1	363
November	1st to 10th	Day	13% 13% 42% 20% 10% 3	330
	11th to 20th	Day	19% 20% 33% 19% 7% 1	330
	21st to end	Day	24% 15% 37% 22% 2	330
December	1st to 10th	Day	14% 19% 42% 18% 5% 2	330
	11th to 20th	Day	19% 17% 38% 21% 5%	330
	21st to end	Day	26% 21% 37% 14% 2	363
January	1st to 10th	Night	17% 17% 38% 22% 6%	1050
	11th to 20th	Night	19% 16% 39% 20% 5%	1050
	21st to end	Night	21% 15% 37% 24% 3	1155
February	1st to 10th	Night	21% 14% 38% 23% 4	1050
	11th to 20th	Night	19% 16% 36% 26% 3	862
	21st to end	Night	21% 17% 40% 18% 3	692
March	1st to 10th	Night	22% 14% 39% 21% 4	840
	11th to 20th	Night	20% 13% 32% 30% 6%	686
	21st to end	Night	24% 13% 38% 21% 5%	693
April	1st to 10th	Night	20% 15% 35% 25% 5%	630
	11th to 20th	Night	21% 13% 33% 25% 8%	623
	21st to end	Night	20% 11% 33% 30% 6%	630
May	1st to 10th	Night	16% 8% 39% 30% 6% 1	630
	11th to 20th	Night	15% 8% 32% 35% 9%	621
	21st to end	Night	13% 4 30% 35% 16% 1	683
June	1st to 10th	Night	10% 4 27% 42% 16% 1	630
	11th to 20th	Night	12% 4 27% 39% 17% 1	630
	21st to end	Night	10% 3 31% 39% 17% 1	630
July	1st to 10th	Night	8% 3 15% 39% 31% 4	615
	11th to 20th	Night	7% 1 20% 44% 26% 2	612
	21st to end	Night	7% 2 14% 46% 30% 1	690
August	1st to 10th	Night	6% 2 18% 41% 28% 4	627
	11th to 20th	Night	7% 2 14% 44% 28% 5%	624
	21st to end	Night	6% 2 17% 40% 35% 1	693
September	1st to 10th	Night	11% 3 19% 37% 27% 3	630
	11th to 20th	Night	11% 3 18% 36% 28% 3	661
	21st to end	Night	11% 7% 24% 34% 22% 2	833
October	1st to 10th	Night	12% 5% 29% 35% 18% 2	800
	11th to 20th	Night	16% 12% 32% 28% 11% 1	800
	21st to end	Night	24% 11% 36% 23% 7%	880
November	1st to 10th	Night	16% 11% 39% 26% 8% 1	820
	11th to 20th	Night	18% 15% 37% 24% 6%	840
	21st to end	Night	22% 13% 37% 24% 4	982
December	1st to 10th	Night	21% 14% 37% 23% 5%	1050
	11th to 20th	Night	22% 14% 34% 25% 5%	1050
	21st to end	Night	28% 15% 35% 20% 2	1155

Page generated at 2026-09-26 03:18:39. Version ac07

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