Williston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Williston 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	8% 7% 3 7% 5% 4 5% 4 6% 8% 43%	105248
	Night	7% 7% 3 7% 5% 5% 5% 4 6% 8% 43%	102590
Transparency	Day	39% 3 15% 29% 13% 1	104993
	Night	40% 3 15% 28% 12% 2	102435
Seeing	Day	12% 9% 31% 30% 16% 3	16154
	Night	11% 7% 32% 37% 12% 1	26124

By Month

Percentage Cloud Cover SW (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	3 9% 4 9% 8% 10% 8% 6% 6% 8% 28%	2538
	11th to 20th	Day	3 7% 4 14% 10% 8% 11% 8% 9% 7% 21%	2540
	21st to end	Day	4 10% 4 9% 7% 6% 5% 5% 6% 7% 39%	2794
February	1st to 10th	Day	6% 13% 5% 13% 8% 6% 5% 4 6% 6% 28%	1585
	11th to 20th	Day	7% 10% 6% 11% 7% 6% 5% 4 7% 7% 28%	1716
	21st to end	Day	5% 11% 4 11% 7% 4 6% 5% 6% 8% 32%	1570
March	1st to 10th	Day	7% 12% 5% 14% 7% 4 4 4 4 5% 35%	3130
	11th to 20th	Day	7% 10% 3 7% 5% 5% 5% 4 7% 11% 36%	3267
	21st to end	Day	13% 16% 4 11% 6% 5% 4 3 5% 6% 26%	4165
April	1st to 10th	Day	7% 9% 3 7% 6% 5% 5% 4 6% 9% 39%	2280
	11th to 20th	Day	13% 13% 4 9% 5% 5% 4 3 5% 7% 32%	2273
	21st to end	Day	14% 9% 2 7% 5% 5% 5% 4 6% 7% 37%	2498
May	1st to 10th	Day	16% 9% 2 6% 4 4 4 4 5% 7% 38%	4430
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 4 7% 8% 41%	4388
	21st to end	Day	13% 6% 2 6% 4 4 4 4 7% 9% 41%	4825
June	1st to 10th	Day	13% 5% 2 4 4 4 5% 5% 6% 9% 43%	2736
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 4 7% 8% 42%	2850
	21st to end	Day	7% 4 2 6% 4 4 4 5% 6% 8% 50%	2748
July	1st to 10th	Day	4 3 2 6% 5% 5% 5% 4 7% 9% 50%	4430
	11th to 20th	Day	4 2 1 4 3 3 4 3 7% 8% 61%	4430
	21st to end	Day	3 3 1 4 3 4 4 4 7% 10% 58%	4802
August	1st to 10th	Day	10% 4 2 5% 3 3 3 3 5% 6% 56%	4411
	11th to 20th	Day	6% 4 2 6% 5% 4 4 4 6% 10% 50%	4245
	21st to end	Day	7% 4 2 4 3 3 4 4 5% 8% 57%	4180
September	1st to 10th	Day	9% 5% 2 5% 4 3 4 3 5% 7% 54%	2280
	11th to 20th	Day	11% 4 2 5% 4 4 4 4 5% 7% 51%	2223
	21st to end	Day	14% 6% 2 4 3 2 3 3 4 6% 53%	2090
October	1st to 10th	Day	9% 4 2 5% 3 4 3 2 4 6% 58%	3237
	11th to 20th	Day	7% 6% 3 5% 3 3 3 3 5% 8% 53%	3002
	21st to end	Day	7% 8% 3 6% 4 3 3 4 6% 7% 48%	3091
November	1st to 10th	Day	12% 12% 4 6% 4 4 5% 4 6% 6% 38%	1754
	11th to 20th	Day	6% 10% 4 9% 5% 4 6% 4 7% 7% 38%	1536
	21st to end	Day	8% 12% 4 9% 7% 5% 5% 4 6% 7% 34%	1485
December	1st to 10th	Day	7% 15% 5% 9% 6% 5% 5% 4 6% 8% 30%	1330
	11th to 20th	Day	5% 12% 5% 11% 5% 5% 5% 3 7% 9% 32%	1683
	21st to end	Day	5% 9% 3 9% 6% 6% 5% 3 6% 6% 42%	2706
January	1st to 10th	Night	3 9% 4 11% 7% 6% 7% 5% 6% 7% 35%	4417
	11th to 20th	Night	3 7% 4 11% 10% 10% 9% 6% 7% 6% 27%	4430
	21st to end	Night	4 11% 4 9% 7% 6% 6% 5% 7% 7% 34%	4873
February	1st to 10th	Night	7% 13% 5% 11% 7% 6% 4 5% 7% 7% 29%	2613
	11th to 20th	Night	6% 12% 5% 10% 6% 6% 5% 4 6% 6% 34%	2350
	21st to end	Night	6% 9% 4 8% 6% 5% 6% 4 6% 7% 39%	1884
March	1st to 10th	Night	7% 11% 4 11% 6% 3 4 3 5% 8% 38%	3734
	11th to 20th	Night	5% 7% 3 7% 5% 5% 5% 4 7% 9% 44%	3703
	21st to end	Night	11% 15% 6% 11% 5% 4 4 4 6% 7% 27%	3477
April	1st to 10th	Night	7% 8% 3 7% 6% 5% 4 5% 5% 7% 43%	1900
	11th to 20th	Night	12% 12% 4 8% 5% 4 5% 5% 6% 7% 34%	1854
	21st to end	Night	12% 7% 2 5% 4 5% 5% 4 6% 6% 43%	1678
May	1st to 10th	Night	13% 8% 2 8% 5% 4 3 4 5% 7% 40%	2540
	11th to 20th	Night	12% 5% 3 6% 5% 5% 5% 4 7% 9% 40%	2517
	21st to end	Night	16% 6% 2 5% 4 4 4 5% 5% 9% 41%	2762
June	1st to 10th	Night	11% 4 2 6% 3 4 4 4 6% 8% 49%	1439
	11th to 20th	Night	10% 6% 1 5% 4 4 4 5% 6% 8% 46%	1330
	21st to end	Night	6% 3 2 4 4 3 4 4 7% 10% 52%	1424
July	1st to 10th	Night	5% 3 1 5% 5% 5% 5% 4 7% 10% 50%	2540
	11th to 20th	Night	4 2 1 4 4 4 4 4 7% 9% 57%	2540
	21st to end	Night	2 2 1 3 3 4 4 4 6% 9% 63%	2761
August	1st to 10th	Night	7% 5% 2 4 3 3 3 2 5% 6% 58%	2528
	11th to 20th	Night	5% 3 2 5% 3 3 5% 4 6% 11% 53%	2593
	21st to end	Night	7% 4 1 4 3 3 3 3 4 8% 60%	3487
September	1st to 10th	Night	7% 4 2 4 3 2 3 3 4 6% 61%	1900
	11th to 20th	Night	9% 4 1 4 3 3 3 3 5% 7% 57%	1957
	21st to end	Night	12% 6% 2 4 3 4 3 3 4 7% 52%	2090
October	1st to 10th	Night	8% 4 2 4 3 3 3 3 4 7% 59%	3727
	11th to 20th	Night	6% 5% 2 6% 4 4 4 4 6% 8% 51%	3895
	21st to end	Night	8% 6% 2 6% 4 4 4 4 5% 8% 47%	4467
November	1st to 10th	Night	10% 11% 3 7% 4 5% 4 4 5% 7% 40%	2529
	11th to 20th	Night	7% 8% 4 9% 6% 5% 5% 4 5% 8% 38%	2641
	21st to end	Night	6% 8% 3 9% 7% 5% 5% 4 5% 8% 39%	2691
December	1st to 10th	Night	6% 10% 4 11% 7% 6% 5% 5% 6% 7% 34%	2850
	11th to 20th	Night	5% 11% 4 10% 6% 6% 6% 5% 6% 7% 33%	3514
	21st to end	Night	6% 8% 3 8% 5% 6% 5% 4 7% 7% 42%	4955

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	52% 4 20% 19% 4	2539
	11th to 20th	Day	58% 3 15% 16% 8%	2540
	21st to end	Day	45% 3 19% 29% 4	2794
February	1st to 10th	Day	56% 4 11% 19% 10%	1585
	11th to 20th	Day	54% 5% 14% 17% 9% 1	1725
	21st to end	Day	49% 4 17% 19% 10% 1	1570
March	1st to 10th	Day	51% 3 10% 29% 6%	3130
	11th to 20th	Day	44% 3 23% 24% 5%	3267
	21st to end	Day	61% 3 15% 14% 7%	4165
April	1st to 10th	Day	42% 5% 16% 24% 12% 1	2280
	11th to 20th	Day	53% 3 12% 21% 10%	2276
	21st to end	Day	46% 3 14% 25% 11%	2498
May	1st to 10th	Day	47% 3 13% 25% 11% 2	4430
	11th to 20th	Day	41% 3 11% 31% 15% 1	4392
	21st to end	Day	41% 3 14% 31% 10%	4825
June	1st to 10th	Day	37% 4 18% 28% 11% 2	2736
	11th to 20th	Day	38% 5% 16% 28% 12% 1	2850
	21st to end	Day	31% 5% 19% 31% 13% 1	2748
July	1st to 10th	Day	31% 3 19% 33% 12% 2	4361
	11th to 20th	Day	20% 3 13% 37% 24% 3	4355
	21st to end	Day	21% 3 11% 39% 22% 4	4651
August	1st to 10th	Day	31% 3 10% 32% 18% 6%	4396
	11th to 20th	Day	31% 3 18% 34% 13% 2	4287
	21st to end	Day	27% 3 12% 40% 17% 2	4179
September	1st to 10th	Day	31% 3 12% 32% 19% 3	2280
	11th to 20th	Day	34% 4 14% 30% 16% 2	2223
	21st to end	Day	34% 3 11% 32% 18% 2	2090
October	1st to 10th	Day	30% 2 10% 28% 26% 4	3237
	11th to 20th	Day	32% 2 14% 39% 13%	3002
	21st to end	Day	36% 4 13% 35% 12% 1	3091
November	1st to 10th	Day	47% 4 11% 25% 11% 2	1754
	11th to 20th	Day	44% 3 20% 23% 9% 1	1536
	21st to end	Day	49% 4 22% 18% 7%	1485
December	1st to 10th	Day	53% 4 16% 21% 6%	1330
	11th to 20th	Day	47% 5% 26% 18% 4	1680
	21st to end	Day	42% 2 11% 30% 13% 1	2706
January	1st to 10th	Night	48% 4 20% 20% 8%	4417
	11th to 20th	Night	56% 3 16% 18% 7%	4430
	21st to end	Night	47% 4 20% 23% 5%	4873
February	1st to 10th	Night	53% 5% 14% 21% 7%	2613
	11th to 20th	Night	49% 5% 14% 21% 10% 1	2351
	21st to end	Night	44% 4 14% 25% 12% 1	1884
March	1st to 10th	Night	46% 2 11% 34% 7%	3734
	11th to 20th	Night	37% 2 21% 30% 9%	3703
	21st to end	Night	57% 2 14% 19% 7%	3477
April	1st to 10th	Night	41% 4 13% 25% 15% 2	1900
	11th to 20th	Night	49% 3 10% 24% 12% 2	1854
	21st to end	Night	42% 4 12% 28% 14% 1	1678
May	1st to 10th	Night	44% 3 11% 24% 15% 3	2540
	11th to 20th	Night	42% 2 16% 26% 13%	2518
	21st to end	Night	42% 3 13% 31% 10% 2	2762
June	1st to 10th	Night	34% 4 16% 29% 15% 3	1439
	11th to 20th	Night	34% 6% 18% 26% 14% 2	1330
	21st to end	Night	27% 5% 19% 33% 14% 2	1424
July	1st to 10th	Night	30% 5% 23% 30% 10% 2	2500
	11th to 20th	Night	24% 3 12% 33% 24% 4	2492
	21st to end	Night	19% 2 14% 37% 22% 7%	2674
August	1st to 10th	Night	29% 2 13% 30% 21% 5%	2520
	11th to 20th	Night	26% 3 20% 36% 13% 2	2617
	21st to end	Night	24% 3 13% 36% 21% 4	3487
September	1st to 10th	Night	25% 3 10% 33% 25% 5%	1900
	11th to 20th	Night	28% 4 14% 29% 21% 4	1957
	21st to end	Night	34% 3 10% 31% 19% 2	2090
October	1st to 10th	Night	27% 3 9% 31% 24% 5%	3727
	11th to 20th	Night	31% 2 10% 39% 17% 1	3895
	21st to end	Night	35% 3 12% 36% 12% 2	4467
November	1st to 10th	Night	44% 4 12% 24% 13% 2	2529
	11th to 20th	Night	45% 4 18% 25% 7% 2	2641
	21st to end	Night	44% 4 20% 24% 8%	2691
December	1st to 10th	Night	50% 5% 18% 21% 6%	2850
	11th to 20th	Night	49% 4 19% 22% 6%	3516
	21st to end	Night	42% 3 11% 29% 13% 2	4955

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	9% 6% 39% 29% 12% 5%	270
	11th to 20th	Day	9% 10% 40% 27% 13% 1	270
	21st to end	Day	12% 11% 30% 33% 13%	297
February	1st to 10th	Day	10% 9% 43% 34% 5%	270
	11th to 20th	Day	13% 6% 43% 29% 9% 1	270
	21st to end	Day	8% 10% 37% 32% 12% 1	225
March	1st to 10th	Day	18% 9% 34% 24% 12% 2	270
	11th to 20th	Day	14% 14% 33% 24% 13% 3	311
	21st to end	Day	13% 10% 35% 26% 14% 1	506
April	1st to 10th	Day	10% 17% 38% 27% 7%	460
	11th to 20th	Day	22% 14% 35% 21% 8% 1	474
	21st to end	Day	17% 12% 28% 28% 14% 2	650
May	1st to 10th	Day	15% 8% 30% 28% 16% 3	653
	11th to 20th	Day	18% 8% 25% 25% 19% 5%	675
	21st to end	Day	17% 7% 28% 23% 19% 6%	734
June	1st to 10th	Day	13% 10% 27% 28% 18% 4	680
	11th to 20th	Day	11% 12% 36% 27% 12% 2	680
	21st to end	Day	9% 7% 26% 35% 19% 5%	680
July	1st to 10th	Day	6% 5% 25% 37% 23% 3	671
	11th to 20th	Day	6% 6% 26% 40% 18% 4	668
	21st to end	Day	4 5% 28% 35% 22% 6%	739
August	1st to 10th	Day	7% 5% 24% 34% 24% 6%	676
	11th to 20th	Day	8% 5% 24% 36% 22% 5%	659
	21st to end	Day	8% 7% 27% 32% 19% 6%	539
September	1st to 10th	Day	10% 8% 30% 34% 14% 3	490
	11th to 20th	Day	15% 9% 25% 28% 22% 1	433
	21st to end	Day	20% 8% 29% 28% 12% 3	300
October	1st to 10th	Day	22% 6% 28% 25% 15% 4	300
	11th to 20th	Day	13% 10% 35% 24% 14% 4	300
	21st to end	Day	16% 11% 41% 23% 8% 2	330
November	1st to 10th	Day	23% 8% 29% 25% 13% 1	297
	11th to 20th	Day	9% 13% 33% 35% 10% 1	270
	21st to end	Day	14% 8% 40% 28% 10%	270
December	1st to 10th	Day	11% 9% 40% 27% 11% 2	270
	11th to 20th	Day	13% 11% 43% 26% 5% 2	270
	21st to end	Day	9% 6% 40% 29% 13% 3	297
January	1st to 10th	Night	8% 8% 38% 34% 10% 1	950
	11th to 20th	Night	10% 10% 35% 36% 9% 1	950
	21st to end	Night	10% 11% 36% 31% 11% 1	1045
February	1st to 10th	Night	12% 8% 35% 34% 10% 1	903
	11th to 20th	Night	12% 6% 40% 30% 12% 1	744
	21st to end	Night	9% 6% 27% 40% 15% 3	628
March	1st to 10th	Night	9% 11% 35% 34% 10% 1	760
	11th to 20th	Night	10% 6% 39% 32% 11% 1	755
	21st to end	Night	14% 9% 29% 36% 12%	627
April	1st to 10th	Night	13% 9% 36% 34% 7%	570
	11th to 20th	Night	18% 10% 36% 30% 6%	563
	21st to end	Night	15% 6% 33% 33% 12%	570
May	1st to 10th	Night	14% 4 29% 38% 14% 1	570
	11th to 20th	Night	15% 5% 25% 36% 17% 1	561
	21st to end	Night	19% 5% 24% 36% 15% 1	617
June	1st to 10th	Night	11% 8% 32% 35% 14% 1	570
	11th to 20th	Night	12% 5% 38% 35% 10%	570
	21st to end	Night	7% 5% 25% 46% 16% 1	570
July	1st to 10th	Night	7% 3 25% 46% 19% 1	555
	11th to 20th	Night	5% 5% 30% 47% 13%	552
	21st to end	Night	4 3 27% 49% 16%	624
August	1st to 10th	Night	6% 4 27% 48% 14% 1	567
	11th to 20th	Night	8% 3 24% 45% 20% 1	564
	21st to end	Night	6% 4 27% 44% 18% 1	627
September	1st to 10th	Night	9% 4 28% 42% 15% 2	570
	11th to 20th	Night	10% 5% 28% 43% 14% 1	570
	21st to end	Night	13% 6% 30% 39% 11% 1	570
October	1st to 10th	Night	15% 8% 28% 35% 14%	713
	11th to 20th	Night	14% 8% 27% 37% 13%	826
	21st to end	Night	14% 8% 35% 33% 10% 1	1045
November	1st to 10th	Night	15% 8% 32% 36% 8% 1	973
	11th to 20th	Night	12% 8% 39% 30% 9% 1	950
	21st to end	Night	9% 9% 37% 36% 7% 1	950
December	1st to 10th	Night	9% 7% 36% 36% 10% 2	950
	11th to 20th	Night	11% 11% 33% 35% 9% 1	950
	21st to end	Night	15% 8% 33% 32% 12%	1045

Page generated at 2024-11-07 05:55:44. Version ac07

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