Williston Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Williston Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	17% 7% 3 6% 4 4 4 4 5% 7% 38%	126847
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 39%	123872
Transparency	Day	44% 6% 12% 21% 14% 2	130367
	Night	44% 6% 12% 21% 14% 2	128439
Seeing	Day	23% 16% 30% 20% 9% 3	16326
	Night	21% 13% 32% 26% 7% 1	25312

By Month

Percentage Cloud Cover NE (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% 7% 4 10% 5% 4 5% 4 5% 6% 37%	3597
	11th to 20th	Day	13% 9% 4 8% 4 4 4 4 5% 6% 40%	3570
	21st to end	Day	16% 8% 3 6% 4 4 4 3 5% 6% 40%	3977
February	1st to 10th	Day	17% 8% 3 6% 5% 5% 5% 5% 6% 7% 33%	1863
	11th to 20th	Day	14% 8% 3 9% 5% 4 4 3 6% 7% 38%	1948
	21st to end	Day	19% 8% 2 8% 4 4 4 4 5% 6% 35%	1650
March	1st to 10th	Day	21% 10% 3 7% 4 3 4 3 4 5% 35%	4476
	11th to 20th	Day	15% 7% 3 6% 5% 4 5% 4 5% 6% 39%	4648
	21st to end	Day	24% 8% 3 6% 4 4 4 3 5% 5% 36%	5968
April	1st to 10th	Day	19% 7% 3 7% 5% 4 3 3 5% 7% 36%	2400
	11th to 20th	Day	18% 7% 2 5% 5% 4 5% 4 5% 6% 39%	2374
	21st to end	Day	20% 7% 3 6% 4 5% 3 4 5% 6% 38%	2545
May	1st to 10th	Day	32% 8% 2 7% 4 4 4 4 5% 7% 25%	5886
	11th to 20th	Day	20% 8% 3 8% 5% 4 4 4 5% 7% 33%	6172
	21st to end	Day	19% 7% 2 6% 4 4 5% 4 6% 8% 34%	6949
June	1st to 10th	Day	16% 7% 2 6% 5% 4 5% 4 7% 10% 34%	2800
	11th to 20th	Day	17% 6% 2 5% 4 5% 4 4 7% 9% 37%	2800
	21st to end	Day	15% 6% 2 7% 5% 5% 5% 4 6% 8% 36%	2800
July	1st to 10th	Day	13% 6% 3 7% 5% 5% 5% 5% 7% 9% 34%	6360
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 5% 8% 11% 35%	6360
	21st to end	Day	15% 6% 2 6% 4 4 4 3 5% 8% 42%	6926
August	1st to 10th	Day	15% 7% 2 7% 5% 6% 5% 4 7% 8% 33%	4408
	11th to 20th	Day	13% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	3954
	21st to end	Day	8% 5% 2 6% 4 5% 6% 5% 7% 9% 43%	4312
September	1st to 10th	Day	13% 5% 2 5% 5% 4 5% 4 5% 7% 44%	2400
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 3 6% 51%	2250
	21st to end	Day	17% 6% 2 6% 4 4 4 4 6% 8% 40%	2200
October	1st to 10th	Day	22% 6% 2 6% 3 3 4 3 4 5% 41%	2180
	11th to 20th	Day	15% 6% 2 5% 3 4 4 4 6% 7% 44%	1954
	21st to end	Day	21% 7% 3 6% 4 4 4 3 4 6% 39%	1980
November	1st to 10th	Day	13% 7% 2 6% 4 4 4 3 4 6% 48%	1851
	11th to 20th	Day	18% 7% 2 4 4 3 3 3 5% 7% 45%	1890
	21st to end	Day	19% 7% 2 6% 4 4 5% 4 4 6% 40%	1837
December	1st to 10th	Day	17% 8% 2 6% 4 4 4 4 5% 6% 40%	2616
	11th to 20th	Day	22% 7% 3 5% 3 3 3 3 4 6% 43%	2986
	21st to end	Day	20% 11% 3 7% 3 3 3 3 4 5% 38%	3960
January	1st to 10th	Night	15% 7% 3 7% 4 4 4 4 6% 6% 41%	6349
	11th to 20th	Night	12% 9% 3 10% 5% 4 4 4 5% 6% 37%	6315
	21st to end	Night	16% 10% 3 6% 4 5% 4 4 5% 6% 37%	6978
February	1st to 10th	Night	16% 9% 3 6% 4 4 4 4 6% 5% 38%	2535
	11th to 20th	Night	13% 8% 3 8% 5% 5% 5% 4 5% 6% 38%	2338
	21st to end	Night	18% 8% 3 8% 4 4 4 4 4 6% 38%	1980
March	1st to 10th	Night	23% 7% 2 6% 4 4 4 3 5% 5% 37%	5364
	11th to 20th	Night	17% 8% 3 6% 4 4 5% 4 5% 6% 39%	5311
	21st to end	Night	24% 9% 3 6% 3 3 4 3 5% 5% 36%	4962
April	1st to 10th	Night	20% 7% 3 6% 5% 4 5% 3 5% 6% 36%	2000
	11th to 20th	Night	21% 7% 2 6% 5% 4 4 4 4 6% 38%	1975
	21st to end	Night	22% 8% 2 6% 3 3 3 3 4 6% 40%	1854
May	1st to 10th	Night	33% 8% 3 6% 3 3 4 3 5% 6% 26%	4054
	11th to 20th	Night	20% 9% 3 9% 6% 5% 5% 4 5% 6% 30%	3764
	21st to end	Night	22% 7% 3 7% 5% 4 4 3 5% 7% 33%	3926
June	1st to 10th	Night	17% 7% 3 7% 5% 4 4 4 4 6% 40%	1600
	11th to 20th	Night	18% 6% 2 6% 4 4 4 3 6% 8% 38%	1600
	21st to end	Night	16% 6% 2 4 5% 5% 4 3 6% 8% 41%	1600
July	1st to 10th	Night	13% 6% 2 7% 4 4 5% 4 7% 9% 39%	3600
	11th to 20th	Night	13% 7% 3 7% 5% 4 5% 5% 7% 8% 36%	3600
	21st to end	Night	12% 6% 3 5% 3 4 4 3 5% 7% 48%	3926
August	1st to 10th	Night	22% 6% 3 7% 5% 5% 4 4 5% 7% 33%	2708
	11th to 20th	Night	13% 7% 3 7% 5% 5% 4 4 7% 8% 38%	3090
	21st to end	Night	10% 4 2 4 3 3 4 4 5% 8% 54%	3586
September	1st to 10th	Night	14% 5% 2 5% 4 4 4 4 5% 6% 46%	2000
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 4 6% 53%	2149
	21st to end	Night	17% 6% 3 6% 4 4 3 3 5% 6% 44%	2200
October	1st to 10th	Night	20% 6% 2 5% 4 4 3 3 4 6% 42%	2216
	11th to 20th	Night	17% 6% 2 5% 3 3 4 4 5% 5% 46%	2445
	21st to end	Night	22% 7% 2 6% 4 3 3 3 4 6% 39%	2860
November	1st to 10th	Night	15% 6% 2 5% 4 3 4 3 5% 6% 49%	2667
	11th to 20th	Night	18% 6% 2 5% 4 3 4 3 4 5% 46%	2730
	21st to end	Night	16% 8% 2 5% 4 4 4 3 4 6% 43%	2777
December	1st to 10th	Night	19% 8% 2 6% 4 4 4 3 5% 6% 40%	4609
	11th to 20th	Night	25% 8% 2 5% 3 3 3 3 4 4 40%	5208
	21st to end	Night	20% 11% 3 7% 4 3 3 3 4 5% 38%	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	44% 7% 10% 27% 11% 1	3598
	11th to 20th	Day	44% 6% 11% 20% 15% 5%	3600
	21st to end	Day	44% 6% 10% 22% 17% 1	3977
February	1st to 10th	Day	46% 7% 15% 18% 12% 1	1863
	11th to 20th	Day	46% 6% 10% 22% 13% 3	1957
	21st to end	Day	48% 6% 11% 19% 14% 1	1650
March	1st to 10th	Day	50% 6% 12% 14% 10% 7%	4479
	11th to 20th	Day	41% 9% 16% 24% 9% 2	4648
	21st to end	Day	50% 5% 6% 17% 18% 5%	5968
April	1st to 10th	Day	48% 5% 10% 20% 13% 3	2400
	11th to 20th	Day	43% 7% 12% 19% 17% 2	2374
	21st to end	Day	46% 7% 9% 19% 16% 4	2545
May	1st to 10th	Day	58% 7% 13% 12% 9% 1	5897
	11th to 20th	Day	51% 6% 14% 18% 11% 1	6176
	21st to end	Day	46% 7% 12% 22% 12% 1	6949
June	1st to 10th	Day	44% 8% 14% 21% 11% 1	2800
	11th to 20th	Day	43% 9% 14% 20% 13% 1	2800
	21st to end	Day	44% 8% 12% 20% 14% 1	2800
July	1st to 10th	Day	46% 7% 12% 21% 14% 1	6290
	11th to 20th	Day	41% 7% 17% 23% 11% 1	6285
	21st to end	Day	41% 5% 12% 25% 16% 1	6728
August	1st to 10th	Day	47% 7% 16% 20% 9% 1	4394
	11th to 20th	Day	41% 7% 14% 28% 9% 1	3993
	21st to end	Day	34% 7% 14% 28% 15% 1	4312
September	1st to 10th	Day	38% 7% 12% 25% 17% 1	2400
	11th to 20th	Day	35% 5% 10% 25% 23% 2	2250
	21st to end	Day	41% 6% 11% 22% 17% 3	2200
October	1st to 10th	Day	39% 6% 9% 21% 22% 3	3557
	11th to 20th	Day	32% 6% 9% 27% 25% 2	3157
	21st to end	Day	34% 4 13% 29% 16% 3	3180
November	1st to 10th	Day	39% 6% 13% 24% 16% 2	1851
	11th to 20th	Day	39% 6% 12% 21% 19% 3	1890
	21st to end	Day	45% 5% 13% 25% 10% 2	1837
December	1st to 10th	Day	43% 6% 15% 26% 8% 1	2616
	11th to 20th	Day	42% 4 15% 19% 17% 2	2986
	21st to end	Day	49% 5% 9% 19% 14% 4	3960
January	1st to 10th	Night	42% 6% 11% 27% 13% 2	6349
	11th to 20th	Night	46% 6% 14% 19% 12% 4	6360
	21st to end	Night	46% 6% 11% 16% 17% 4	6978
February	1st to 10th	Night	46% 7% 13% 22% 11% 2	2535
	11th to 20th	Night	45% 7% 12% 19% 14% 3	2340
	21st to end	Night	47% 6% 10% 22% 11% 4	1980
March	1st to 10th	Night	47% 7% 12% 16% 15% 4	5374
	11th to 20th	Night	43% 8% 18% 19% 10% 2	5312
	21st to end	Night	49% 6% 6% 13% 22% 4	4962
April	1st to 10th	Night	49% 6% 10% 19% 14% 2	2000
	11th to 20th	Night	47% 6% 9% 18% 18% 1	1975
	21st to end	Night	45% 6% 10% 19% 15% 5%	1854
May	1st to 10th	Night	57% 8% 14% 11% 9% 1	4060
	11th to 20th	Night	54% 8% 12% 15% 9% 2	3765
	21st to end	Night	50% 7% 14% 18% 10% 1	3927
June	1st to 10th	Night	44% 8% 12% 23% 13% 1	1600
	11th to 20th	Night	44% 8% 14% 22% 11% 1	1600
	21st to end	Night	39% 9% 14% 23% 14% 1	1600
July	1st to 10th	Night	41% 8% 14% 21% 15%	3560
	11th to 20th	Night	44% 7% 17% 20% 12% 1	3552
	21st to end	Night	36% 6% 13% 28% 16% 1	3809
August	1st to 10th	Night	50% 8% 12% 20% 9% 1	2698
	11th to 20th	Night	43% 8% 15% 25% 10%	3117
	21st to end	Night	29% 6% 15% 33% 16% 1	3586
September	1st to 10th	Night	38% 7% 11% 26% 18% 1	2000
	11th to 20th	Night	33% 6% 10% 25% 24% 2	2149
	21st to end	Night	41% 6% 10% 23% 18% 3	2200
October	1st to 10th	Night	38% 5% 14% 23% 17% 2	3619
	11th to 20th	Night	29% 5% 10% 28% 27% 1	3981
	21st to end	Night	35% 4 14% 31% 16% 1	4610
November	1st to 10th	Night	36% 7% 12% 24% 19% 3	2667
	11th to 20th	Night	41% 6% 12% 21% 18% 3	2730
	21st to end	Night	42% 6% 15% 26% 10% 1	2777
December	1st to 10th	Night	45% 6% 12% 28% 9%	4609
	11th to 20th	Night	47% 6% 14% 15% 17% 2	5208
	21st to end	Night	48% 6% 10% 20% 13% 5%	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	28% 25% 27% 17% 4	200
	11th to 20th	Day	28% 26% 32% 12% 4	200
	21st to end	Day	22% 23% 36% 14% 4	223
February	1st to 10th	Day	31% 26% 31% 11% 1	363
	11th to 20th	Day	31% 23% 31% 13% 1	496
	21st to end	Day	29% 21% 34% 14% 2	414
March	1st to 10th	Day	29% 23% 32% 13% 3	500
	11th to 20th	Day	30% 22% 29% 13% 4	500
	21st to end	Day	29% 24% 29% 11% 4 3	550
April	1st to 10th	Day	32% 22% 27% 13% 6% 1	500
	11th to 20th	Day	29% 21% 30% 13% 6% 1	492
	21st to end	Day	30% 18% 29% 16% 5% 2	500
May	1st to 10th	Day	35% 19% 25% 14% 7% 1	503
	11th to 20th	Day	22% 17% 39% 15% 6% 1	527
	21st to end	Day	20% 12% 32% 23% 11% 2	573
June	1st to 10th	Day	18% 14% 32% 23% 11% 2	530
	11th to 20th	Day	18% 14% 33% 24% 8% 3	530
	21st to end	Day	15% 16% 33% 23% 11% 3	530
July	1st to 10th	Day	16% 8% 31% 29% 13% 3	525
	11th to 20th	Day	13% 8% 29% 31% 16% 3	524
	21st to end	Day	15% 7% 33% 29% 13% 2	576
August	1st to 10th	Day	11% 8% 26% 35% 14% 6%	526
	11th to 20th	Day	14% 7% 30% 25% 18% 6%	519
	21st to end	Day	9% 12% 28% 28% 17% 5%	583
September	1st to 10th	Day	19% 9% 25% 25% 14% 8%	530
	11th to 20th	Day	17% 10% 24% 22% 16% 12%	530
	21st to end	Day	20% 11% 28% 24% 12% 5%	530
October	1st to 10th	Day	26% 12% 22% 20% 12% 7%	530
	11th to 20th	Day	20% 16% 28% 24% 10% 2	504
	21st to end	Day	29% 13% 26% 22% 9% 2	462
November	1st to 10th	Day	22% 15% 30% 24% 8% 2	425
	11th to 20th	Day	27% 17% 31% 18% 6% 1	430
	21st to end	Day	30% 16% 30% 18% 5% 1	377
December	1st to 10th	Day	23% 28% 30% 13% 6%	204
	11th to 20th	Day	33% 23% 30% 12% 2	200
	21st to end	Day	24% 23% 32% 16% 5% 1	220
January	1st to 10th	Night	22% 17% 40% 19% 3	800
	11th to 20th	Night	26% 17% 33% 21% 3	800
	21st to end	Night	19% 15% 40% 24% 3	880
February	1st to 10th	Night	22% 16% 39% 22% 1	800
	11th to 20th	Night	24% 19% 40% 16% 1	783
	21st to end	Night	24% 19% 35% 20% 2	660
March	1st to 10th	Night	25% 17% 37% 19% 2	800
	11th to 20th	Night	21% 21% 35% 21% 3	800
	21st to end	Night	24% 17% 35% 19% 5%	880
April	1st to 10th	Night	26% 17% 34% 20% 3	800
	11th to 20th	Night	25% 19% 31% 20% 5%	791
	21st to end	Night	25% 19% 31% 22% 3	654
May	1st to 10th	Night	28% 14% 33% 21% 4	600
	11th to 20th	Night	21% 13% 39% 22% 5%	479
	21st to end	Night	21% 7% 31% 33% 8%	433
June	1st to 10th	Night	19% 8% 28% 37% 8%	400
	11th to 20th	Night	20% 8% 32% 32% 8%	400
	21st to end	Night	16% 5% 31% 35% 13% 1	400
July	1st to 10th	Night	14% 3 29% 41% 13% 1	390
	11th to 20th	Night	17% 3 23% 41% 16%	388
	21st to end	Night	11% 5% 25% 46% 12%	438
August	1st to 10th	Night	12% 2 28% 45% 12% 1	468
	11th to 20th	Night	13% 5% 23% 39% 18% 2	719
	21st to end	Night	14% 5% 23% 38% 18% 2	880
September	1st to 10th	Night	16% 5% 28% 36% 14% 2	800
	11th to 20th	Night	15% 7% 23% 33% 21% 3	800
	21st to end	Night	19% 9% 25% 32% 14% 2	800
October	1st to 10th	Night	25% 7% 24% 29% 13% 2	800
	11th to 20th	Night	19% 12% 30% 29% 9% 1	800
	21st to end	Night	26% 12% 31% 24% 6%	880
November	1st to 10th	Night	19% 13% 32% 27% 8% 1	820
	11th to 20th	Night	22% 15% 32% 22% 7% 1	840
	21st to end	Night	22% 16% 32% 26% 4	840
December	1st to 10th	Night	25% 13% 38% 18% 5%	809
	11th to 20th	Night	26% 15% 33% 23% 3	800
	21st to end	Night	26% 15% 36% 20% 3	880

Page generated at 2025-11-30 01:00:52. Version ac07

Copyright © 2025 Attilla Danko.