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 2024-03-18.

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% 4 7% 6% 3 5% 5% 5% 7% 37%	78651
	Night	14% 9% 4 6% 5% 3 4 4 5% 6% 39%	81189
Transparency	Day	46% 6% 12% 22% 13% 2	78352
	Night	45% 5% 11% 22% 14% 2	81009
Seeing	Day	15% 10% 29% 26% 15% 5%	14792
	Night	17% 9% 31% 30% 12% 1	24853

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	12% 12% 9% 12% 8% 4 5% 4 4 6% 25%	2527
	11th to 20th	Day	12% 12% 5% 10% 10% 4 8% 6% 7% 5% 20%	2530
	21st to end	Day	17% 19% 10% 11% 6% 4 4 3 3 5% 18%	3104
February	1st to 10th	Day	17% 11% 6% 9% 6% 4 6% 3 3 6% 30%	1710
	11th to 20th	Day	14% 10% 3 7% 8% 4 5% 4 5% 6% 35%	1810
	21st to end	Day	15% 11% 6% 10% 6% 4 4 4 5% 6% 32%	1570
March	1st to 10th	Day	15% 13% 5% 7% 4 4 4 3 5% 5% 34%	1900
	11th to 20th	Day	14% 10% 3 6% 6% 3 5% 5% 6% 7% 34%	2023
	21st to end	Day	20% 11% 4 9% 7% 4 5% 5% 5% 6% 25%	2324
April	1st to 10th	Day	15% 9% 5% 7% 6% 4 4 4 4 7% 36%	2160
	11th to 20th	Day	16% 9% 3 6% 5% 3 4 5% 4 6% 38%	2134
	21st to end	Day	20% 7% 3 7% 6% 3 5% 4 4 6% 34%	2160
May	1st to 10th	Day	17% 7% 4 6% 6% 3 5% 5% 6% 9% 32%	2337
	11th to 20th	Day	19% 7% 3 6% 5% 3 5% 5% 5% 7% 35%	2479
	21st to end	Day	14% 6% 3 6% 6% 3 6% 7% 6% 8% 35%	2725
June	1st to 10th	Day	9% 4 2 5% 6% 3 5% 6% 6% 9% 46%	2520
	11th to 20th	Day	8% 4 3 5% 5% 4 6% 5% 6% 7% 47%	2520
	21st to end	Day	11% 5% 3 5% 5% 3 6% 6% 7% 9% 39%	2520
July	1st to 10th	Day	7% 4 2 5% 6% 3 6% 6% 6% 10% 45%	2520
	11th to 20th	Day	9% 3 2 5% 4 3 5% 6% 7% 10% 46%	2520
	21st to end	Day	7% 4 2 5% 6% 4 7% 5% 6% 9% 45%	2743
August	1st to 10th	Day	7% 5% 3 5% 5% 4 6% 6% 7% 11% 42%	2355
	11th to 20th	Day	9% 4 2 6% 6% 3 4 6% 6% 9% 45%	2142
	21st to end	Day	7% 4 2 4 5% 3 5% 5% 6% 8% 50%	2376
September	1st to 10th	Day	12% 5% 2 5% 5% 3 5% 4 4 7% 48%	2160
	11th to 20th	Day	11% 4 2 3 4 2 4 5% 5% 8% 53%	2115
	21st to end	Day	8% 5% 2 5% 4 3 5% 4 6% 9% 48%	1980
October	1st to 10th	Day	12% 6% 2 5% 4 2 4 3 4 7% 49%	1828
	11th to 20th	Day	17% 9% 2 6% 5% 2 4 3 4 6% 42%	1800
	21st to end	Day	20% 9% 3 5% 4 3 4 4 4 6% 39%	1877
November	1st to 10th	Day	11% 7% 2 5% 4 3 3 4 6% 7% 49%	1672
	11th to 20th	Day	17% 8% 4 7% 4 3 4 5% 4 8% 36%	1710
	21st to end	Day	16% 11% 5% 7% 5% 2 4 3 3 5% 39%	1579
December	1st to 10th	Day	14% 12% 5% 7% 6% 3 6% 4 4 6% 34%	1520
	11th to 20th	Day	15% 13% 4 8% 6% 4 4 4 5% 7% 31%	1918
	21st to end	Day	24% 18% 8% 11% 6% 3 3 3 3 4 16%	2783
January	1st to 10th	Night	14% 12% 6% 10% 6% 3 4 4 4 5% 32%	4428
	11th to 20th	Night	12% 17% 6% 9% 7% 4 5% 5% 7% 4 22%	4440
	21st to end	Night	21% 16% 7% 9% 6% 3 4 3 3 4 24%	4560
February	1st to 10th	Night	16% 12% 4 7% 7% 3 4 4 5% 6% 32%	2470
	11th to 20th	Night	14% 9% 4 7% 6% 4 4 4 4 6% 38%	2254
	21st to end	Night	16% 11% 4 7% 5% 3 3 4 4 5% 39%	1884
March	1st to 10th	Night	14% 12% 4 6% 5% 3 4 3 4 6% 39%	2280
	11th to 20th	Night	14% 9% 4 6% 5% 3 4 4 5% 7% 39%	2119
	21st to end	Night	21% 10% 4 7% 5% 3 5% 5% 5% 6% 30%	2030
April	1st to 10th	Night	13% 9% 3 7% 5% 3 4 4 6% 7% 40%	1800
	11th to 20th	Night	16% 8% 4 5% 5% 3 4 3 5% 5% 42%	1775
	21st to end	Night	18% 7% 3 6% 6% 3 3 4 6% 7% 38%	1800
May	1st to 10th	Night	15% 7% 3 6% 6% 3 5% 5% 6% 8% 36%	1616
	11th to 20th	Night	17% 6% 3 6% 5% 3 4 4 5% 7% 41%	1416
	21st to end	Night	11% 5% 3 7% 6% 4 4 6% 6% 9% 39%	1551
June	1st to 10th	Night	9% 5% 2 4 4 3 4 4 5% 8% 52%	1440
	11th to 20th	Night	8% 5% 2 5% 6% 3 5% 5% 5% 10% 48%	1440
	21st to end	Night	9% 5% 3 6% 5% 3 6% 5% 4 8% 46%	1440
July	1st to 10th	Night	6% 4 2 5% 4 2 5% 6% 6% 11% 49%	1440
	11th to 20th	Night	5% 3 2 4 4 2 4 5% 6% 8% 57%	1440
	21st to end	Night	4 4 1 4 5% 3 6% 6% 7% 10% 51%	1575
August	1st to 10th	Night	5% 4 2 3 4 3 5% 6% 7% 10% 52%	1581
	11th to 20th	Night	6% 4 2 4 5% 3 5% 4 5% 8% 54%	1758
	21st to end	Night	6% 3 2 4 5% 2 4 3 5% 8% 57%	1980
September	1st to 10th	Night	12% 4 2 5% 4 2 3 4 4 5% 55%	1800
	11th to 20th	Night	10% 5% 1 3 4 2 3 4 6% 7% 55%	1826
	21st to end	Night	9% 4 2 4 5% 3 5% 5% 6% 8% 49%	1980
October	1st to 10th	Night	11% 5% 2 4 5% 3 4 4 4 6% 53%	2130
	11th to 20th	Night	14% 6% 2 5% 4 2 3 4 5% 6% 48%	2160
	21st to end	Night	19% 7% 3 6% 6% 3 4 3 4 6% 40%	2479
November	1st to 10th	Night	11% 7% 3 4 3 2 3 4 4 7% 52%	2406
	11th to 20th	Night	13% 7% 3 6% 6% 3 4 4 5% 7% 43%	2470
	21st to end	Night	17% 9% 5% 6% 4 3 3 4 4 6% 39%	2598
December	1st to 10th	Night	15% 13% 3 7% 5% 3 4 4 5% 7% 36%	2660
	11th to 20th	Night	15% 11% 4 8% 6% 3 4 3 4 6% 36%	3279
	21st to end	Night	23% 17% 7% 10% 7% 3 4 3 4 4 18%	4884

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	61% 5% 11% 18% 6%	2528
	11th to 20th	Day	64% 5% 10% 13% 8%	2530
	21st to end	Day	70% 4 9% 12% 5%	3104
February	1st to 10th	Day	58% 4 13% 17% 7% 2	1710
	11th to 20th	Day	50% 5% 12% 21% 10% 1	1820
	21st to end	Day	53% 5% 10% 22% 8% 2	1570
March	1st to 10th	Day	51% 5% 12% 21% 9% 1	1900
	11th to 20th	Day	48% 6% 14% 22% 9% 1	2023
	21st to end	Day	59% 5% 11% 19% 5% 1	2324
April	1st to 10th	Day	48% 6% 11% 23% 12%	2160
	11th to 20th	Day	47% 5% 9% 24% 12% 2	2134
	21st to end	Day	51% 6% 10% 18% 13% 2	2160
May	1st to 10th	Day	49% 7% 13% 22% 9%	2337
	11th to 20th	Day	48% 6% 12% 21% 12% 1	2483
	21st to end	Day	45% 10% 12% 17% 15% 2	2725
June	1st to 10th	Day	34% 7% 14% 27% 15% 3	2520
	11th to 20th	Day	36% 7% 11% 22% 22% 3	2520
	21st to end	Day	39% 8% 14% 22% 15% 3	2520
July	1st to 10th	Day	32% 8% 15% 26% 14% 4	2451
	11th to 20th	Day	31% 8% 17% 27% 14% 3	2445
	21st to end	Day	34% 9% 14% 27% 15% 2	2591
August	1st to 10th	Day	33% 9% 16% 29% 11% 2	2340
	11th to 20th	Day	33% 7% 14% 27% 17% 1	2142
	21st to end	Day	30% 8% 12% 27% 20% 4	2376
September	1st to 10th	Day	37% 5% 10% 25% 18% 5%	2160
	11th to 20th	Day	29% 6% 12% 26% 23% 4	2115
	21st to end	Day	32% 7% 10% 27% 21% 2	1980
October	1st to 10th	Day	35% 6% 10% 27% 20% 2	1828
	11th to 20th	Day	45% 4 9% 23% 18% 1	1800
	21st to end	Day	47% 5% 12% 23% 12% 3	1877
November	1st to 10th	Day	35% 5% 15% 25% 17% 3	1672
	11th to 20th	Day	48% 4 11% 23% 13% 1	1710
	21st to end	Day	49% 5% 12% 23% 10%	1579
December	1st to 10th	Day	52% 5% 13% 21% 9% 1	1520
	11th to 20th	Day	53% 5% 10% 24% 7% 1	1915
	21st to end	Day	72% 4 8% 10% 5%	2783
January	1st to 10th	Night	54% 5% 12% 20% 8%	4428
	11th to 20th	Night	62% 3 11% 15% 8% 1	4440
	21st to end	Night	65% 3 10% 16% 6% 1	4560
February	1st to 10th	Night	54% 4 11% 18% 11% 1	2470
	11th to 20th	Night	47% 4 12% 24% 11% 2	2255
	21st to end	Night	48% 3 9% 24% 13% 3	1884
March	1st to 10th	Night	47% 5% 11% 23% 12% 1	2280
	11th to 20th	Night	44% 5% 11% 23% 13% 3	2119
	21st to end	Night	55% 7% 10% 18% 8% 1	2030
April	1st to 10th	Night	43% 5% 12% 24% 14% 3	1800
	11th to 20th	Night	43% 5% 8% 25% 15% 4	1775
	21st to end	Night	47% 5% 12% 20% 13% 4	1800
May	1st to 10th	Night	45% 7% 13% 22% 12% 1	1616
	11th to 20th	Night	43% 5% 12% 21% 17% 1	1417
	21st to end	Night	40% 9% 14% 18% 16% 3	1552
June	1st to 10th	Night	31% 5% 14% 25% 20% 5%	1440
	11th to 20th	Night	32% 6% 13% 22% 21% 5%	1440
	21st to end	Night	36% 8% 12% 21% 20% 3	1440
July	1st to 10th	Night	29% 8% 14% 32% 14% 4	1400
	11th to 20th	Night	24% 7% 13% 31% 22% 3	1392
	21st to end	Night	25% 9% 14% 28% 20% 4	1487
August	1st to 10th	Night	25% 8% 17% 33% 15% 2	1572
	11th to 20th	Night	28% 7% 11% 30% 22% 2	1758
	21st to end	Night	25% 7% 12% 28% 21% 6%	1980
September	1st to 10th	Night	30% 6% 11% 26% 22% 5%	1800
	11th to 20th	Night	28% 5% 11% 28% 23% 5%	1826
	21st to end	Night	31% 6% 12% 25% 22% 4	1980
October	1st to 10th	Night	33% 5% 10% 23% 24% 5%	2130
	11th to 20th	Night	36% 5% 9% 26% 21% 3	2160
	21st to end	Night	46% 5% 10% 21% 17% 2	2479
November	1st to 10th	Night	33% 6% 12% 26% 19% 4	2406
	11th to 20th	Night	42% 4 12% 24% 16% 2	2470
	21st to end	Night	47% 4 10% 25% 13% 1	2598
December	1st to 10th	Night	49% 5% 13% 23% 9% 1	2660
	11th to 20th	Night	51% 4 11% 25% 8% 1	3281
	21st to end	Night	72% 4 7% 12% 5%	4884

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% 19% 29% 26% 6%	270
	11th to 20th	Day	24% 20% 38% 15% 3	270
	21st to end	Day	21% 20% 34% 20% 6%	297
February	1st to 10th	Day	23% 17% 40% 16% 3	270
	11th to 20th	Day	23% 18% 33% 19% 6% 1	270
	21st to end	Day	21% 15% 48% 15% 1	225
March	1st to 10th	Day	23% 24% 34% 16% 2	270
	11th to 20th	Day	24% 20% 35% 14% 7%	261
	21st to end	Day	30% 15% 32% 18% 5% 1	410
April	1st to 10th	Day	21% 13% 35% 25% 5% 1	420
	11th to 20th	Day	18% 16% 34% 17% 12% 2	412
	21st to end	Day	21% 14% 35% 20% 8% 1	420
May	1st to 10th	Day	20% 14% 32% 25% 6% 3	441
	11th to 20th	Day	21% 11% 29% 25% 11% 2	625
	21st to end	Day	14% 7% 30% 25% 15% 8%	679
June	1st to 10th	Day	8% 6% 25% 33% 21% 7%	630
	11th to 20th	Day	6% 6% 29% 32% 23% 4	630
	21st to end	Day	11% 8% 30% 31% 16% 5%	630
July	1st to 10th	Day	9% 3 16% 32% 24% 15%	621
	11th to 20th	Day	10% 3 21% 34% 24% 9%	618
	21st to end	Day	7% 3 22% 37% 26% 5%	684
August	1st to 10th	Day	6% 3 19% 37% 25% 10%	626
	11th to 20th	Day	10% 4 18% 27% 26% 15%	456
	21st to end	Day	6% 4 18% 30% 32% 11%	495
September	1st to 10th	Day	15% 5% 16% 30% 29% 5%	450
	11th to 20th	Day	10% 2 20% 32% 26% 11%	450
	21st to end	Day	6% 10% 27% 26% 28% 4	450
October	1st to 10th	Day	15% 8% 29% 28% 16% 4	298
	11th to 20th	Day	17% 22% 34% 19% 7%	270
	21st to end	Day	33% 13% 30% 20% 4 1	297
November	1st to 10th	Day	10% 12% 44% 20% 11% 3	270
	11th to 20th	Day	19% 19% 33% 20% 8% 1	270
	21st to end	Day	21% 14% 39% 23% 2	270
December	1st to 10th	Day	14% 19% 40% 19% 5% 3	270
	11th to 20th	Day	16% 15% 39% 24% 6%	270
	21st to end	Day	22% 23% 39% 14% 2	297
January	1st to 10th	Night	16% 16% 40% 21% 6%	950
	11th to 20th	Night	20% 14% 40% 21% 5%	950
	21st to end	Night	21% 13% 36% 26% 3	1045
February	1st to 10th	Night	22% 12% 38% 24% 4	950
	11th to 20th	Night	18% 15% 36% 27% 3	779
	21st to end	Night	22% 17% 39% 18% 3	628
March	1st to 10th	Night	21% 13% 39% 22% 4	760
	11th to 20th	Night	19% 11% 33% 31% 7%	615
	21st to end	Night	24% 11% 36% 23% 6%	594
April	1st to 10th	Night	18% 14% 36% 27% 6%	540
	11th to 20th	Night	20% 11% 33% 26% 9%	533
	21st to end	Night	21% 10% 33% 30% 7%	540
May	1st to 10th	Night	16% 7% 39% 30% 6% 1	540
	11th to 20th	Night	16% 6% 31% 37% 10% 1	531
	21st to end	Night	13% 4 28% 36% 18% 2	584
June	1st to 10th	Night	10% 4 24% 44% 17% 1	540
	11th to 20th	Night	11% 3 25% 40% 19% 1	540
	21st to end	Night	9% 4 29% 41% 18% 1	540
July	1st to 10th	Night	8% 3 14% 38% 34% 4	525
	11th to 20th	Night	6% 1 20% 44% 25% 2	522
	21st to end	Night	5% 1 14% 47% 31% 1	591
August	1st to 10th	Night	6% 2 15% 43% 29% 5%	537
	11th to 20th	Night	5% 2 13% 44% 30% 5%	534
	21st to end	Night	7% 2 16% 38% 36% 2	594
September	1st to 10th	Night	13% 3 18% 37% 26% 3	540
	11th to 20th	Night	11% 2 18% 36% 28% 4	566
	21st to end	Night	10% 7% 25% 34% 23% 2	720
October	1st to 10th	Night	13% 5% 29% 33% 19% 2	720
	11th to 20th	Night	17% 13% 32% 28% 11% 1	720
	21st to end	Night	24% 11% 35% 24% 7%	792
November	1st to 10th	Night	15% 10% 40% 26% 9% 1	740
	11th to 20th	Night	17% 14% 37% 26% 6%	760
	21st to end	Night	21% 11% 38% 24% 5%	888
December	1st to 10th	Night	21% 13% 37% 24% 5%	950
	11th to 20th	Night	21% 12% 35% 27% 5%	950
	21st to end	Night	26% 15% 35% 22% 2	1045

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