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 2025-05-26.

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	13% 8% 3 7% 5% 4 5% 4 5% 7% 38%	114093
	Night	13% 9% 3 8% 5% 4 4 3 5% 6% 40%	115375
Transparency	Day	43% 6% 12% 23% 14% 2	117660
	Night	44% 5% 11% 23% 15% 2	119951
Seeing	Day	15% 11% 29% 25% 14% 4	16461
	Night	17% 10% 32% 29% 12% 1	26409

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% 14% 6% 5% 5% 3 5% 6% 25%	3617
	11th to 20th	Day	9% 10% 6% 11% 7% 6% 7% 4 6% 6% 27%	3620
	21st to end	Day	15% 15% 8% 10% 6% 5% 4 3 4 5% 26%	4457
February	1st to 10th	Day	17% 11% 5% 10% 6% 6% 4 3 4 6% 29%	1800
	11th to 20th	Day	13% 11% 3 8% 7% 5% 5% 3 5% 6% 34%	1909
	21st to end	Day	14% 12% 5% 10% 5% 4 4 3 5% 6% 34%	1650
March	1st to 10th	Day	19% 12% 3 6% 3 3 3 2 3 4 42%	4480
	11th to 20th	Day	8% 7% 2 6% 5% 5% 5% 4 7% 8% 45%	4905
	21st to end	Day	19% 11% 3 10% 5% 5% 5% 4 5% 6% 28%	5845
April	1st to 10th	Day	15% 12% 4 8% 5% 4 3 3 5% 7% 35%	2400
	11th to 20th	Day	15% 9% 3 7% 5% 5% 5% 4 4 6% 37%	2374
	21st to end	Day	20% 7% 2 7% 5% 5% 4 3 5% 6% 35%	2400
May	1st to 10th	Day	15% 7% 3 7% 5% 4 5% 4 7% 9% 33%	4209
	11th to 20th	Day	16% 7% 2 6% 4 4 4 4 5% 7% 41%	4539
	21st to end	Day	11% 6% 2 5% 4 4 5% 4 6% 8% 44%	4895
June	1st to 10th	Day	9% 5% 2 6% 5% 4 4 4 6% 9% 46%	2660
	11th to 20th	Day	8% 4 3 6% 4 5% 5% 4 6% 8% 47%	2660
	21st to end	Day	10% 6% 3 6% 5% 5% 5% 5% 7% 9% 40%	2660
July	1st to 10th	Day	10% 6% 3 7% 5% 5% 6% 5% 8% 10% 34%	4440
	11th to 20th	Day	7% 4 2 5% 4 5% 5% 4 7% 9% 49%	4440
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 10% 42%	4813
August	1st to 10th	Day	6% 5% 2 6% 4 5% 5% 4 8% 11% 44%	4145
	11th to 20th	Day	10% 5% 2 6% 5% 4 5% 5% 8% 9% 40%	3759
	21st to end	Day	5% 4 2 6% 4 4 4 4 6% 9% 50%	4180
September	1st to 10th	Day	11% 5% 2 5% 4 5% 4 4 5% 7% 50%	2280
	11th to 20th	Day	10% 4 2 4 3 3 3 4 5% 8% 53%	2232
	21st to end	Day	9% 6% 2 6% 4 4 4 3 6% 8% 48%	2090
October	1st to 10th	Day	12% 6% 2 6% 3 3 4 3 5% 6% 51%	1929
	11th to 20th	Day	15% 9% 2 6% 4 3 4 2 4 6% 45%	1900
	21st to end	Day	19% 9% 3 6% 3 4 4 3 4 6% 39%	1981
November	1st to 10th	Day	11% 9% 2 6% 4 3 3 3 6% 6% 48%	1762
	11th to 20th	Day	16% 10% 3 8% 4 4 4 4 4 8% 36%	1800
	21st to end	Day	16% 12% 4 8% 5% 3 4 3 4 5% 37%	1662
December	1st to 10th	Day	8% 9% 4 7% 5% 4 4 3 4 7% 46%	2610
	11th to 20th	Day	14% 15% 6% 11% 6% 5% 4 3 5% 6% 24%	3008
	21st to end	Day	25% 20% 8% 13% 4 4 3 2 3 4 14%	3982
January	1st to 10th	Night	13% 13% 5% 12% 5% 4 4 3 4 5% 33%	6348
	11th to 20th	Night	11% 14% 6% 12% 8% 6% 6% 4 6% 4 24%	6360
	21st to end	Night	16% 14% 5% 9% 5% 4 4 2 3 4 33%	6518
February	1st to 10th	Night	15% 13% 4 8% 6% 4 4 3 5% 6% 32%	2600
	11th to 20th	Night	14% 10% 4 9% 5% 5% 3 3 4 6% 37%	2375
	21st to end	Night	15% 11% 4 8% 4 3 3 2 4 5% 41%	1980
March	1st to 10th	Night	17% 11% 4 6% 4 3 3 3 3 5% 41%	5373
	11th to 20th	Night	11% 7% 2 5% 4 4 4 4 4 6% 48%	5085
	21st to end	Night	17% 11% 5% 9% 5% 5% 5% 3 4 5% 29%	5082
April	1st to 10th	Night	14% 10% 2 7% 4 4 3 3 5% 7% 40%	2000
	11th to 20th	Night	16% 9% 3 6% 4 4 4 3 5% 5% 42%	1975
	21st to end	Night	17% 8% 3 6% 5% 4 3 3 6% 7% 38%	2000
May	1st to 10th	Night	18% 8% 3 8% 5% 5% 5% 4 6% 7% 33%	2887
	11th to 20th	Night	14% 6% 2 6% 4 5% 5% 4 6% 8% 40%	2586
	21st to end	Night	8% 6% 2 6% 4 4 4 5% 6% 8% 46%	2788
June	1st to 10th	Night	9% 5% 2 5% 4 4 4 4 6% 8% 51%	1520
	11th to 20th	Night	7% 6% 1 6% 5% 4 5% 3 5% 10% 48%	1520
	21st to end	Night	9% 6% 2 7% 4 4 5% 4 5% 8% 46%	1520
July	1st to 10th	Night	11% 6% 3 6% 4 4 5% 5% 6% 9% 40%	2530
	11th to 20th	Night	4 3 2 4 3 4 3 4 6% 8% 59%	2530
	21st to end	Night	6% 6% 2 6% 5% 5% 5% 5% 7% 9% 46%	2750
August	1st to 10th	Night	5% 4 2 3 4 4 5% 4 7% 9% 54%	2792
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 6% 8% 53%	3070
	21st to end	Night	5% 4 1 5% 4 4 4 3 6% 9% 53%	3476
September	1st to 10th	Night	12% 4 2 5% 3 2 3 3 4 5% 56%	1900
	11th to 20th	Night	9% 5% 1 4 4 3 4 3 6% 7% 54%	1928
	21st to end	Night	9% 4 2 6% 4 4 5% 4 6% 8% 48%	2090
October	1st to 10th	Night	10% 5% 2 5% 4 4 3 3 4 6% 54%	2249
	11th to 20th	Night	13% 6% 2 6% 3 2 4 3 5% 6% 50%	2280
	21st to end	Night	18% 8% 3 6% 5% 4 3 3 5% 6% 40%	2617
November	1st to 10th	Night	12% 7% 2 5% 3 3 3 3 5% 6% 51%	2536
	11th to 20th	Night	13% 8% 3 7% 5% 4 4 3 5% 7% 42%	2600
	21st to end	Night	16% 10% 4 8% 3 4 3 4 4 5% 38%	2735
December	1st to 10th	Night	10% 10% 3 7% 4 3 3 3 4 6% 47%	4580
	11th to 20th	Night	16% 12% 4 10% 5% 4 3 3 4 5% 34%	5199
	21st to end	Night	24% 18% 7% 11% 5% 4 4 3 4 4 16%	6996

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	59% 4 10% 19% 7% 1	3618
	11th to 20th	Day	57% 4 9% 13% 14% 3	3620
	21st to end	Day	61% 3 11% 17% 8% 1	4457
February	1st to 10th	Day	57% 4 14% 16% 7% 2	1800
	11th to 20th	Day	51% 5% 12% 20% 10% 1	1919
	21st to end	Day	51% 5% 10% 22% 9% 2	1650
March	1st to 10th	Day	47% 4 8% 23% 15% 3	4480
	11th to 20th	Day	36% 5% 11% 32% 16%	4905
	21st to end	Day	56% 6% 13% 20% 5% 1	5847
April	1st to 10th	Day	49% 5% 11% 23% 12%	2400
	11th to 20th	Day	47% 5% 9% 24% 12% 2	2374
	21st to end	Day	50% 6% 10% 19% 13% 2	2400
May	1st to 10th	Day	44% 8% 14% 22% 12%	4247
	11th to 20th	Day	43% 6% 11% 30% 10%	4543
	21st to end	Day	36% 8% 10% 22% 22% 2	4908
June	1st to 10th	Day	34% 7% 14% 27% 15% 3	2660
	11th to 20th	Day	34% 7% 11% 23% 22% 3	2660
	21st to end	Day	38% 8% 14% 22% 15% 3	2660
July	1st to 10th	Day	42% 7% 14% 23% 12% 2	4371
	11th to 20th	Day	29% 7% 17% 28% 18% 1	4365
	21st to end	Day	36% 9% 18% 19% 16% 2	4662
August	1st to 10th	Day	32% 7% 18% 30% 12% 1	4130
	11th to 20th	Day	37% 7% 15% 25% 16% 1	3795
	21st to end	Day	30% 6% 13% 30% 17% 3	4179
September	1st to 10th	Day	35% 5% 10% 25% 20% 5%	2280
	11th to 20th	Day	29% 6% 11% 26% 24% 4	2232
	21st to end	Day	34% 6% 10% 27% 21% 2	2090
October	1st to 10th	Day	23% 4 6% 27% 39% 2	3202
	11th to 20th	Day	29% 3 9% 22% 33% 4	3140
	21st to end	Day	41% 4 9% 26% 18% 3	3245
November	1st to 10th	Day	36% 5% 15% 25% 16% 3	1762
	11th to 20th	Day	48% 4 11% 23% 13% 2	1800
	21st to end	Day	50% 5% 12% 23% 9%	1662
December	1st to 10th	Day	40% 3 14% 33% 9%	2610
	11th to 20th	Day	60% 4 11% 18% 6% 1	3005
	21st to end	Day	75% 4 9% 9% 4	3982
January	1st to 10th	Night	55% 4 10% 23% 8% 1	6348
	11th to 20th	Night	61% 3 10% 12% 13% 1	6360
	21st to end	Night	57% 3 9% 19% 12% 1	6518
February	1st to 10th	Night	53% 4 12% 18% 11% 1	2600
	11th to 20th	Night	49% 4 12% 23% 11% 2	2376
	21st to end	Night	47% 3 10% 24% 14% 3	1980
March	1st to 10th	Night	47% 5% 7% 25% 14% 2	5373
	11th to 20th	Night	36% 4 9% 29% 18% 4	5085
	21st to end	Night	55% 6% 10% 20% 7% 1	5082
April	1st to 10th	Night	43% 5% 11% 24% 14% 2	2000
	11th to 20th	Night	43% 5% 8% 25% 15% 4	1975
	21st to end	Night	46% 5% 12% 21% 13% 3	2000
May	1st to 10th	Night	48% 7% 12% 19% 12% 1	2914
	11th to 20th	Night	40% 8% 14% 26% 11% 1	2587
	21st to end	Night	33% 7% 13% 22% 22% 3	2796
June	1st to 10th	Night	31% 6% 14% 25% 19% 5%	1520
	11th to 20th	Night	31% 6% 13% 23% 22% 5%	1520
	21st to end	Night	36% 8% 12% 21% 19% 3	1520
July	1st to 10th	Night	40% 6% 13% 25% 13% 2	2490
	11th to 20th	Night	23% 5% 13% 31% 25% 2	2482
	21st to end	Night	31% 9% 16% 21% 18% 4	2662
August	1st to 10th	Night	24% 7% 18% 31% 18% 2	2783
	11th to 20th	Night	29% 6% 15% 31% 17% 1	3100
	21st to end	Night	26% 7% 14% 33% 16% 4	3476
September	1st to 10th	Night	29% 5% 10% 26% 24% 5%	1900
	11th to 20th	Night	28% 5% 11% 27% 23% 5%	1928
	21st to end	Night	33% 6% 11% 25% 21% 4	2090
October	1st to 10th	Night	23% 3 6% 27% 36% 4	3756
	11th to 20th	Night	25% 3 8% 23% 35% 5%	3779
	21st to end	Night	39% 4 10% 22% 22% 4	4303
November	1st to 10th	Night	34% 6% 11% 26% 18% 4	2536
	11th to 20th	Night	43% 4 12% 24% 16% 2	2600
	21st to end	Night	48% 4 10% 24% 13% 1	2735
December	1st to 10th	Night	40% 3 12% 32% 12%	4580
	11th to 20th	Night	54% 4 11% 21% 9%	5201
	21st to end	Night	73% 4 8% 11% 4	6996

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% 29% 26% 5%	300
	11th to 20th	Day	23% 21% 38% 14% 3	300
	21st to end	Day	21% 22% 34% 19% 5%	330
February	1st to 10th	Day	23% 18% 40% 16% 3	300
	11th to 20th	Day	22% 22% 32% 17% 5% 1	300
	21st to end	Day	22% 15% 48% 14% 1	249
March	1st to 10th	Day	23% 23% 36% 15% 2	300
	11th to 20th	Day	24% 21% 34% 14% 6%	300
	21st to end	Day	31% 16% 32% 17% 4 1	493
April	1st to 10th	Day	23% 15% 34% 23% 5% 1	500
	11th to 20th	Day	21% 17% 33% 17% 11% 2	492
	21st to end	Day	21% 15% 36% 19% 7% 1	500
May	1st to 10th	Day	19% 13% 33% 26% 7% 3	523
	11th to 20th	Day	20% 12% 29% 25% 12% 3	725
	21st to end	Day	14% 8% 31% 25% 15% 7%	760
June	1st to 10th	Day	8% 7% 26% 33% 20% 6%	680
	11th to 20th	Day	6% 6% 30% 33% 22% 4	680
	21st to end	Day	11% 9% 31% 30% 15% 4	680
July	1st to 10th	Day	10% 4 18% 32% 22% 14%	671
	11th to 20th	Day	9% 3 22% 34% 23% 9%	668
	21st to end	Day	7% 3 22% 37% 26% 6%	739
August	1st to 10th	Day	6% 3 21% 37% 24% 9%	676
	11th to 20th	Day	11% 5% 20% 27% 24% 14%	497
	21st to end	Day	6% 4 17% 31% 32% 10%	539
September	1st to 10th	Day	14% 5% 17% 31% 28% 5%	490
	11th to 20th	Day	9% 2 19% 32% 27% 10%	490
	21st to end	Day	9% 11% 26% 25% 26% 3	490
October	1st to 10th	Day	15% 8% 27% 28% 18% 4	329
	11th to 20th	Day	17% 21% 33% 21% 8%	300
	21st to end	Day	31% 14% 30% 20% 5% 1	330
November	1st to 10th	Day	12% 12% 42% 20% 10% 3	300
	11th to 20th	Day	20% 19% 33% 20% 8% 1	300
	21st to end	Day	22% 15% 39% 22% 2	300
December	1st to 10th	Day	14% 20% 42% 18% 5% 2	300
	11th to 20th	Day	18% 18% 37% 22% 5%	300
	21st to end	Day	25% 22% 37% 15% 2	330
January	1st to 10th	Night	16% 17% 39% 22% 6%	1000
	11th to 20th	Night	20% 15% 39% 21% 5%	1000
	21st to end	Night	21% 14% 37% 25% 3	1100
February	1st to 10th	Night	22% 14% 38% 23% 4	1000
	11th to 20th	Night	19% 16% 36% 26% 3	820
	21st to end	Night	21% 17% 40% 19% 3	660
March	1st to 10th	Night	21% 14% 39% 22% 4	800
	11th to 20th	Night	18% 13% 33% 30% 6%	653
	21st to end	Night	24% 12% 36% 21% 6%	660
April	1st to 10th	Night	19% 15% 35% 27% 5%	600
	11th to 20th	Night	20% 13% 33% 25% 8%	593
	21st to end	Night	20% 11% 33% 30% 6%	600
May	1st to 10th	Night	16% 7% 39% 30% 6% 1	600
	11th to 20th	Night	15% 7% 31% 37% 10% 1	591
	21st to end	Night	13% 4 29% 35% 17% 2	634
June	1st to 10th	Night	10% 4 26% 43% 16% 1	570
	11th to 20th	Night	11% 3 27% 40% 18% 1	570
	21st to end	Night	9% 4 30% 40% 17% 1	570
July	1st to 10th	Night	8% 3 15% 38% 33% 4	555
	11th to 20th	Night	6% 1 21% 43% 26% 3	552
	21st to end	Night	6% 1 14% 46% 31% 1	624
August	1st to 10th	Night	6% 2 16% 43% 28% 5%	567
	11th to 20th	Night	6% 2 15% 44% 28% 5%	564
	21st to end	Night	6% 2 16% 39% 35% 2	627
September	1st to 10th	Night	12% 3 18% 37% 27% 3	570
	11th to 20th	Night	11% 2 18% 36% 29% 4	598
	21st to end	Night	11% 7% 25% 34% 22% 2	760
October	1st to 10th	Night	12% 5% 28% 34% 19% 2	760
	11th to 20th	Night	16% 13% 31% 28% 11% 1	760
	21st to end	Night	23% 11% 35% 23% 7%	836
November	1st to 10th	Night	16% 10% 40% 26% 8% 1	780
	11th to 20th	Night	18% 14% 37% 25% 6%	800
	21st to end	Night	21% 12% 38% 24% 4	935
December	1st to 10th	Night	21% 13% 37% 24% 5%	1000
	11th to 20th	Night	21% 13% 34% 26% 5%	1000
	21st to end	Night	28% 15% 35% 21% 2	1100

Page generated at 2025-05-23 08:21:01. Version ac07

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