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 2024-11-10.

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 6% 7% 38%	101312
	Night	18% 8% 3 6% 4 4 4 4 5% 6% 39%	97853
Transparency	Day	43% 7% 13% 21% 14% 2	104850
	Night	43% 7% 13% 22% 14% 2	102404
Seeing	Day	22% 16% 30% 20% 9% 3	14978
	Night	21% 13% 32% 26% 7% 1	23959

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	14% 7% 2 7% 4 4 4 4 5% 6% 43%	2517
	11th to 20th	Day	16% 8% 4 9% 5% 5% 4 3 5% 5% 34%	2520
	21st to end	Day	21% 10% 4 8% 5% 4 4 3 4 5% 31%	2775
February	1st to 10th	Day	17% 8% 3 7% 5% 5% 5% 5% 6% 7% 34%	1769
	11th to 20th	Day	14% 8% 3 9% 5% 3 4 3 6% 7% 39%	1848
	21st to end	Day	20% 8% 2 8% 5% 4 4 4 5% 6% 35%	1570
March	1st to 10th	Day	24% 12% 3 7% 4 3 3 2 3 5% 32%	3116
	11th to 20th	Day	12% 7% 4 7% 5% 4 6% 4 6% 7% 39%	3246
	21st to end	Day	21% 7% 3 6% 4 4 4 3 5% 5% 38%	4164
April	1st to 10th	Day	19% 7% 3 7% 5% 4 3 3 5% 7% 37%	2280
	11th to 20th	Day	18% 6% 2 5% 5% 4 5% 4 5% 6% 39%	2254
	21st to end	Day	20% 7% 3 6% 4 5% 4 4 5% 6% 38%	2418
May	1st to 10th	Day	25% 8% 3 7% 5% 4 4 4 5% 7% 29%	4106
	11th to 20th	Day	22% 7% 3 7% 4 3 4 3 5% 7% 35%	4294
	21st to end	Day	13% 5% 2 5% 4 4 5% 5% 6% 8% 43%	4837
June	1st to 10th	Day	16% 7% 2 6% 5% 4 5% 4 7% 9% 34%	2660
	11th to 20th	Day	16% 6% 2 5% 4 5% 4 4 7% 9% 37%	2660
	21st to end	Day	15% 6% 2 7% 5% 5% 6% 4 6% 8% 35%	2660
July	1st to 10th	Day	11% 6% 2 7% 5% 6% 5% 4 7% 9% 37%	4440
	11th to 20th	Day	12% 6% 2 5% 4 5% 5% 5% 8% 10% 38%	4440
	21st to end	Day	17% 7% 3 7% 4 4 5% 4 6% 8% 35%	4814
August	1st to 10th	Day	16% 7% 2 7% 5% 6% 5% 4 7% 8% 33%	4273
	11th to 20th	Day	13% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	3831
	21st to end	Day	8% 5% 2 6% 4 5% 6% 5% 7% 9% 43%	4180
September	1st to 10th	Day	13% 6% 2 5% 5% 4 5% 4 5% 6% 44%	2280
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 3 7% 50%	2138
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 8% 40%	2090
October	1st to 10th	Day	22% 6% 2 6% 3 3 4 3 4 5% 39%	2071
	11th to 20th	Day	14% 6% 2 5% 3 4 4 4 6% 7% 45%	1857
	21st to end	Day	21% 7% 3 6% 4 4 4 3 4 6% 38%	1881
November	1st to 10th	Day	13% 7% 2 6% 4 3 4 3 4 6% 49%	1752
	11th to 20th	Day	19% 7% 2 4 3 3 3 3 5% 7% 43%	1710
	21st to end	Day	18% 7% 2 6% 4 4 4 4 4 6% 42%	1663
December	1st to 10th	Day	20% 9% 3 7% 4 4 5% 4 5% 5% 35%	1520
	11th to 20th	Day	23% 6% 3 5% 3 3 3 2 5% 6% 42%	1906
	21st to end	Day	22% 11% 3 7% 4 4 4 3 4 5% 33%	2772
January	1st to 10th	Night	17% 8% 2 5% 3 3 4 3 5% 6% 43%	4429
	11th to 20th	Night	15% 9% 4 10% 5% 4 4 4 5% 5% 36%	4440
	21st to end	Night	22% 13% 3 7% 4 4 4 3 5% 5% 29%	4880
February	1st to 10th	Night	16% 9% 3 6% 4 4 4 4 6% 5% 38%	2409
	11th to 20th	Night	13% 8% 3 7% 5% 5% 5% 4 5% 6% 39%	2218
	21st to end	Night	18% 9% 3 7% 4 4 4 4 4 5% 38%	1884
March	1st to 10th	Night	28% 9% 2 6% 4 3 3 3 5% 6% 31%	3724
	11th to 20th	Night	13% 8% 3 6% 5% 5% 6% 4 5% 6% 40%	3713
	21st to end	Night	22% 9% 3 6% 3 2 4 4 6% 5% 37%	3466
April	1st to 10th	Night	20% 7% 3 6% 5% 4 4 3 5% 6% 37%	1900
	11th to 20th	Night	21% 7% 2 6% 5% 4 4 4 4 5% 39%	1875
	21st to end	Night	23% 8% 2 6% 3 3 3 3 4 6% 40%	1762
May	1st to 10th	Night	25% 9% 3 7% 4 4 4 3 5% 6% 30%	2834
	11th to 20th	Night	20% 8% 3 9% 5% 5% 5% 5% 5% 6% 29%	2628
	21st to end	Night	17% 5% 2 5% 4 4 4 3 6% 8% 43%	2738
June	1st to 10th	Night	17% 7% 2 7% 5% 4 4 4 4 6% 41%	1520
	11th to 20th	Night	18% 6% 2 6% 4 4 4 3 6% 8% 39%	1520
	21st to end	Night	16% 6% 2 4 5% 5% 4 3 6% 8% 41%	1520
July	1st to 10th	Night	12% 5% 2 7% 5% 5% 6% 5% 7% 9% 38%	2520
	11th to 20th	Night	14% 7% 2 6% 5% 4 4 5% 6% 8% 38%	2520
	21st to end	Night	16% 8% 3 6% 4 4 5% 4 6% 7% 38%	2739
August	1st to 10th	Night	22% 6% 3 7% 5% 5% 4 4 6% 7% 32%	2624
	11th to 20th	Night	13% 7% 3 7% 5% 5% 4 4 6% 8% 38%	2993
	21st to end	Night	10% 4 2 4 3 3 4 4 5% 8% 53%	3476
September	1st to 10th	Night	14% 5% 2 5% 4 4 4 4 5% 6% 46%	1900
	11th to 20th	Night	13% 7% 2 4 3 3 4 3 4 6% 52%	2041
	21st to end	Night	17% 6% 2 6% 4 4 3 3 5% 6% 44%	2090
October	1st to 10th	Night	21% 6% 2 6% 4 4 3 3 4 6% 40%	2105
	11th to 20th	Night	16% 6% 2 6% 4 3 4 4 5% 5% 46%	2323
	21st to end	Night	23% 7% 2 5% 4 3 3 3 4 6% 39%	2717
November	1st to 10th	Night	15% 5% 2 5% 4 3 4 3 5% 5% 50%	2531
	11th to 20th	Night	20% 6% 2 5% 4 3 4 3 4 5% 45%	2470
	21st to end	Night	16% 8% 2 5% 4 4 4 3 4 6% 44%	2512
December	1st to 10th	Night	21% 8% 2 6% 5% 4 3 3 5% 6% 37%	2660
	11th to 20th	Night	21% 8% 2 5% 4 3 3 4 5% 5% 39%	3288
	21st to end	Night	23% 12% 3 7% 4 3 4 3 5% 5% 32%	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	39% 7% 14% 31% 8% 1	2518
	11th to 20th	Day	50% 7% 11% 22% 8% 1	2520
	21st to end	Day	53% 8% 10% 19% 8% 1	2775
February	1st to 10th	Day	47% 7% 15% 18% 12% 1	1769
	11th to 20th	Day	45% 6% 10% 22% 14% 3	1857
	21st to end	Day	49% 6% 11% 19% 13% 2	1570
March	1st to 10th	Day	55% 6% 13% 14% 10% 2	3119
	11th to 20th	Day	38% 12% 16% 23% 10% 2	3246
	21st to end	Day	47% 5% 7% 17% 19% 4	4164
April	1st to 10th	Day	47% 5% 10% 21% 13% 3	2280
	11th to 20th	Day	42% 7% 12% 19% 17% 3	2254
	21st to end	Day	46% 7% 9% 19% 15% 4	2418
May	1st to 10th	Day	53% 9% 13% 13% 11% 1	4120
	11th to 20th	Day	48% 7% 16% 18% 10% 1	4298
	21st to end	Day	35% 9% 13% 24% 17% 2	4837
June	1st to 10th	Day	44% 9% 14% 21% 11% 1	2660
	11th to 20th	Day	42% 9% 14% 20% 13% 1	2660
	21st to end	Day	44% 8% 13% 20% 13% 1	2660
July	1st to 10th	Day	43% 7% 12% 22% 15% 1	4370
	11th to 20th	Day	40% 7% 16% 23% 12% 1	4365
	21st to end	Day	47% 6% 13% 18% 14% 2	4661
August	1st to 10th	Day	48% 7% 17% 20% 8% 1	4259
	11th to 20th	Day	41% 7% 14% 28% 9% 1	3870
	21st to end	Day	34% 7% 14% 28% 15% 2	4180
September	1st to 10th	Day	38% 7% 12% 24% 17% 1	2280
	11th to 20th	Day	36% 5% 10% 25% 22% 2	2138
	21st to end	Day	41% 6% 11% 22% 17% 3	2090
October	1st to 10th	Day	40% 6% 9% 21% 21% 3	3448
	11th to 20th	Day	32% 6% 9% 27% 26% 2	3060
	21st to end	Day	34% 4 13% 29% 16% 3	3081
November	1st to 10th	Day	38% 6% 14% 24% 16% 2	1752
	11th to 20th	Day	40% 6% 13% 21% 18% 2	1710
	21st to end	Day	43% 6% 14% 25% 11% 2	1663
December	1st to 10th	Day	48% 9% 14% 16% 12% 1	1520
	11th to 20th	Day	43% 5% 17% 19% 14% 1	1906
	21st to end	Day	53% 7% 12% 18% 9%	2772
January	1st to 10th	Night	40% 6% 14% 26% 11% 3	4429
	11th to 20th	Night	47% 6% 14% 21% 10% 2	4440
	21st to end	Night	55% 7% 10% 18% 9% 1	4880
February	1st to 10th	Night	45% 7% 14% 21% 11% 2	2409
	11th to 20th	Night	44% 7% 12% 19% 15% 3	2220
	21st to end	Night	47% 6% 10% 22% 11% 4	1884
March	1st to 10th	Night	52% 8% 11% 19% 10% 2	3734
	11th to 20th	Night	39% 10% 16% 21% 12% 2	3714
	21st to end	Night	46% 7% 8% 13% 20% 5%	3466
April	1st to 10th	Night	48% 6% 10% 19% 14% 2	1900
	11th to 20th	Night	46% 7% 9% 19% 18% 1	1875
	21st to end	Night	45% 6% 10% 20% 14% 5%	1762
May	1st to 10th	Night	52% 10% 13% 14% 10% 2	2840
	11th to 20th	Night	52% 10% 14% 14% 8% 2	2629
	21st to end	Night	38% 9% 17% 22% 13% 1	2739
June	1st to 10th	Night	43% 8% 12% 24% 13% 1	1520
	11th to 20th	Night	43% 8% 14% 22% 12% 1	1520
	21st to end	Night	39% 10% 14% 23% 14%	1520
July	1st to 10th	Night	40% 10% 15% 20% 14% 1	2480
	11th to 20th	Night	42% 7% 15% 23% 12% 1	2472
	21st to end	Night	45% 7% 13% 19% 16% 1	2651
August	1st to 10th	Night	51% 8% 12% 19% 9% 1	2614
	11th to 20th	Night	43% 8% 15% 25% 9%	3020
	21st to end	Night	28% 6% 16% 33% 16% 1	3476
September	1st to 10th	Night	39% 7% 11% 25% 18% 1	1900
	11th to 20th	Night	34% 6% 10% 25% 24% 2	2041
	21st to end	Night	41% 6% 9% 23% 19% 3	2090
October	1st to 10th	Night	39% 5% 14% 23% 17% 2	3508
	11th to 20th	Night	28% 5% 10% 28% 28% 1	3859
	21st to end	Night	35% 4 14% 31% 16% 1	4467
November	1st to 10th	Night	36% 7% 12% 23% 20% 3	2531
	11th to 20th	Night	42% 6% 13% 21% 16% 3	2470
	21st to end	Night	41% 6% 16% 25% 10% 1	2512
December	1st to 10th	Night	47% 7% 14% 20% 12%	2660
	11th to 20th	Night	44% 8% 15% 17% 14% 2	3288
	21st to end	Night	51% 7% 12% 21% 8% 1	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	28% 22% 28% 19% 4	180
	11th to 20th	Day	28% 26% 31% 11% 4	180
	21st to end	Day	20% 24% 37% 14% 5% 1	200
February	1st to 10th	Day	31% 26% 31% 11% 1	329
	11th to 20th	Day	29% 24% 32% 13% 2	456
	21st to end	Day	31% 20% 32% 15% 2	382
March	1st to 10th	Day	28% 22% 33% 14% 3	460
	11th to 20th	Day	30% 23% 29% 14% 4	460
	21st to end	Day	29% 23% 30% 11% 4 3	506
April	1st to 10th	Day	30% 22% 28% 14% 6% 1	460
	11th to 20th	Day	27% 21% 30% 14% 6% 1	452
	21st to end	Day	30% 18% 29% 16% 5% 2	460
May	1st to 10th	Day	33% 19% 25% 14% 7% 1	463
	11th to 20th	Day	22% 17% 39% 15% 6% 1	487
	21st to end	Day	19% 13% 31% 23% 12% 3	529
June	1st to 10th	Day	17% 15% 33% 22% 10% 2	490
	11th to 20th	Day	17% 14% 34% 24% 8% 3	490
	21st to end	Day	14% 17% 33% 23% 11% 3	490
July	1st to 10th	Day	16% 8% 30% 29% 14% 3	485
	11th to 20th	Day	13% 8% 29% 31% 16% 3	484
	21st to end	Day	16% 8% 33% 28% 12% 2	532
August	1st to 10th	Day	12% 9% 27% 35% 13% 4	486
	11th to 20th	Day	14% 8% 32% 25% 16% 5%	479
	21st to end	Day	10% 12% 27% 28% 18% 6%	539
September	1st to 10th	Day	20% 9% 25% 25% 14% 8%	490
	11th to 20th	Day	18% 10% 25% 20% 14% 12%	490
	21st to end	Day	20% 12% 29% 23% 12% 5%	490
October	1st to 10th	Day	27% 12% 23% 20% 12% 7%	490
	11th to 20th	Day	18% 16% 28% 24% 10% 3	467
	21st to end	Day	29% 14% 25% 23% 9% 2	429
November	1st to 10th	Day	21% 15% 30% 24% 8% 2	392
	11th to 20th	Day	28% 17% 31% 18% 5% 1	370
	21st to end	Day	27% 16% 30% 20% 6% 1	323
December	1st to 10th	Day	24% 28% 31% 13% 4	180
	11th to 20th	Day	33% 22% 32% 12% 2	180
	21st to end	Day	24% 24% 31% 16% 3 1	198
January	1st to 10th	Night	21% 15% 41% 20% 3	760
	11th to 20th	Night	27% 17% 33% 20% 3	760
	21st to end	Night	19% 16% 40% 23% 3	836
February	1st to 10th	Night	22% 16% 39% 22% 1	760
	11th to 20th	Night	22% 20% 41% 16% 1	743
	21st to end	Night	25% 19% 34% 19% 2	628
March	1st to 10th	Night	24% 18% 37% 19% 2	760
	11th to 20th	Night	21% 21% 35% 20% 3	760
	21st to end	Night	23% 18% 35% 19% 5%	836
April	1st to 10th	Night	24% 18% 35% 20% 3	760
	11th to 20th	Night	24% 18% 32% 21% 5%	751
	21st to end	Night	25% 18% 31% 22% 3	622
May	1st to 10th	Night	26% 14% 34% 21% 4	570
	11th to 20th	Night	21% 13% 39% 22% 5%	455
	21st to end	Night	20% 8% 31% 33% 8%	411
June	1st to 10th	Night	19% 8% 29% 36% 8%	380
	11th to 20th	Night	19% 7% 32% 33% 8%	380
	21st to end	Night	17% 5% 31% 35% 12% 1	380
July	1st to 10th	Night	14% 3 29% 41% 14% 1	370
	11th to 20th	Night	17% 3 23% 42% 16%	368
	21st to end	Night	11% 6% 25% 45% 13%	416
August	1st to 10th	Night	12% 2 29% 45% 11% 1	444
	11th to 20th	Night	14% 5% 24% 39% 17% 2	682
	21st to end	Night	14% 5% 22% 39% 19% 2	836
September	1st to 10th	Night	16% 4 28% 36% 14% 2	760
	11th to 20th	Night	15% 7% 23% 32% 21% 3	760
	21st to end	Night	19% 9% 25% 31% 14% 2	760
October	1st to 10th	Night	25% 8% 24% 30% 11% 2	760
	11th to 20th	Night	18% 12% 31% 29% 10% 1	760
	21st to end	Night	26% 12% 32% 24% 6%	836
November	1st to 10th	Night	18% 13% 33% 28% 8% 1	779
	11th to 20th	Night	23% 15% 32% 21% 7% 1	760
	21st to end	Night	22% 15% 32% 25% 5% 1	760
December	1st to 10th	Night	25% 14% 39% 18% 5%	760
	11th to 20th	Night	25% 15% 34% 23% 3	760
	21st to end	Night	26% 15% 36% 20% 2	836

Page generated at 2024-11-07 06:55:48. Version ac07

Copyright © 2024 Attilla Danko.