Third Stone Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Third Stone Observatory clear sky chart from 2005-11-06 to 2025-03-27.

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	9% 9% 3 9% 6% 5% 5% 4 6% 8% 35%	112482
	Night	8% 10% 4 10% 6% 5% 5% 4 6% 8% 33%	115498
Transparency	Day	45% 7% 15% 23% 9% 1	112225
	Night	48% 6% 14% 21% 9% 1	115401
Seeing	Day	13% 8% 26% 29% 19% 5%	15804
	Night	13% 6% 29% 35% 15% 2	25053

By Month

Percentage Cloud Cover NW (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	5% 11% 4 14% 9% 7% 5% 4 6% 7% 29%	3179
	11th to 20th	Day	4 10% 4 10% 7% 5% 7% 5% 8% 9% 29%	3159
	21st to end	Day	4 11% 6% 9% 6% 5% 5% 3 6% 9% 36%	3866
February	1st to 10th	Day	8% 14% 5% 12% 6% 6% 5% 4 7% 8% 26%	1600
	11th to 20th	Day	5% 15% 5% 13% 7% 5% 7% 4 6% 7% 26%	1718
	21st to end	Day	3 13% 5% 12% 8% 7% 8% 5% 5% 7% 28%	1560
March	1st to 10th	Day	4 11% 6% 16% 9% 6% 5% 4 5% 8% 26%	3238
	11th to 20th	Day	6% 11% 3 7% 4 3 4 5% 6% 11% 40%	3565
	21st to end	Day	8% 18% 6% 15% 6% 5% 5% 4 6% 6% 23%	3943
April	1st to 10th	Day	9% 14% 4 9% 7% 6% 5% 4 7% 7% 28%	2361
	11th to 20th	Day	12% 14% 4 9% 6% 5% 5% 5% 6% 8% 27%	2440
	21st to end	Day	12% 11% 4 8% 6% 6% 4 4 7% 8% 30%	2470
May	1st to 10th	Day	16% 12% 4 8% 5% 5% 5% 5% 6% 7% 28%	4516
	11th to 20th	Day	11% 12% 3 10% 6% 6% 6% 5% 7% 8% 27%	4694
	21st to end	Day	14% 9% 3 10% 7% 6% 6% 5% 7% 8% 26%	5161
June	1st to 10th	Day	15% 7% 2 8% 5% 5% 5% 5% 7% 9% 31%	2850
	11th to 20th	Day	18% 9% 3 8% 6% 6% 6% 4 7% 8% 25%	2850
	21st to end	Day	10% 6% 2 6% 5% 4 6% 5% 9% 10% 37%	2850
July	1st to 10th	Day	9% 5% 2 5% 3 4 4 4 6% 8% 51%	4740
	11th to 20th	Day	9% 4 2 5% 4 4 4 4 5% 9% 50%	4740
	21st to end	Day	9% 4 2 5% 4 4 5% 4 6% 9% 48%	5138
August	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 4 6% 8% 43%	4439
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 11% 41%	4118
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 7% 9% 44%	4521
September	1st to 10th	Day	12% 5% 2 7% 5% 4 5% 4 6% 7% 43%	2394
	11th to 20th	Day	16% 7% 3 6% 4 3 3 3 5% 8% 42%	2090
	21st to end	Day	9% 8% 3 6% 4 5% 5% 4 6% 8% 43%	2090
October	1st to 10th	Day	8% 9% 4 8% 5% 4 5% 4 5% 7% 42%	3308
	11th to 20th	Day	9% 7% 3 8% 5% 5% 5% 5% 6% 9% 38%	2904
	21st to end	Day	8% 9% 3 8% 6% 4 5% 4 6% 7% 39%	3101
November	1st to 10th	Day	7% 17% 5% 10% 6% 5% 5% 5% 6% 8% 27%	1575
	11th to 20th	Day	5% 13% 5% 11% 7% 5% 5% 3 5% 7% 32%	1474
	21st to end	Day	6% 10% 4 13% 7% 6% 6% 4 7% 7% 30%	1400
December	1st to 10th	Day	11% 15% 5% 10% 7% 7% 5% 4 6% 8% 24%	2290
	11th to 20th	Day	7% 13% 5% 13% 7% 6% 6% 4 6% 7% 24%	2642
	21st to end	Day	3 7% 3 11% 7% 5% 4 4 6% 9% 42%	3498
January	1st to 10th	Night	3 12% 6% 15% 9% 7% 6% 4 6% 7% 26%	6766
	11th to 20th	Night	3 10% 5% 12% 8% 6% 5% 5% 6% 6% 34%	6739
	21st to end	Night	5% 13% 4 9% 6% 5% 5% 4 7% 9% 33%	7119
February	1st to 10th	Night	5% 12% 5% 12% 8% 7% 6% 4 6% 7% 27%	2800
	11th to 20th	Night	6% 13% 5% 11% 8% 6% 6% 5% 7% 7% 26%	2568
	21st to end	Night	5% 12% 4 10% 7% 7% 7% 5% 6% 7% 31%	2067
March	1st to 10th	Night	3 10% 5% 16% 8% 7% 5% 5% 8% 8% 25%	3856
	11th to 20th	Night	6% 10% 3 8% 6% 6% 6% 5% 7% 9% 35%	3633
	21st to end	Night	6% 17% 8% 18% 6% 5% 5% 5% 5% 7% 19%	3846
April	1st to 10th	Night	7% 14% 4 9% 7% 6% 6% 4 7% 8% 29%	1819
	11th to 20th	Night	12% 10% 4 10% 7% 6% 5% 5% 7% 8% 27%	1691
	21st to end	Night	12% 10% 4 9% 5% 4 5% 4 6% 8% 34%	1710
May	1st to 10th	Night	22% 13% 4 9% 4 4 4 3 4 5% 27%	2444
	11th to 20th	Night	11% 8% 4 9% 7% 5% 7% 5% 6% 8% 30%	2199
	21st to end	Night	15% 11% 3 9% 7% 6% 7% 5% 8% 7% 22%	2414
June	1st to 10th	Night	17% 8% 4 8% 5% 6% 5% 5% 7% 8% 26%	1330
	11th to 20th	Night	17% 9% 4 8% 8% 5% 6% 5% 7% 7% 23%	1330
	21st to end	Night	8% 6% 3 7% 6% 6% 5% 5% 9% 7% 36%	1330
July	1st to 10th	Night	11% 6% 2 6% 4 4 5% 4 6% 8% 44%	2220
	11th to 20th	Night	7% 5% 2 6% 5% 5% 5% 5% 7% 10% 45%	2220
	21st to end	Night	10% 5% 2 5% 4 4 4 4 7% 9% 45%	2414
August	1st to 10th	Night	14% 6% 2 6% 4 4 5% 4 6% 8% 42%	2488
	11th to 20th	Night	9% 6% 2 6% 4 4 6% 5% 7% 11% 39%	2716
	21st to end	Night	11% 5% 2 6% 5% 5% 4 4 6% 9% 42%	3146
September	1st to 10th	Night	11% 6% 3 7% 5% 5% 5% 4 5% 6% 43%	1783
	11th to 20th	Night	14% 7% 2 7% 4 4 4 3 6% 6% 43%	2090
	21st to end	Night	9% 7% 3 7% 6% 4 4 4 6% 9% 42%	2090
October	1st to 10th	Night	8% 7% 3 7% 5% 5% 6% 4 5% 9% 43%	3666
	11th to 20th	Night	8% 8% 3 7% 5% 4 5% 4 7% 8% 40%	4021
	21st to end	Night	7% 11% 4 8% 6% 6% 6% 4 6% 7% 35%	4457
November	1st to 10th	Night	9% 16% 5% 10% 5% 5% 5% 4 6% 7% 29%	2703
	11th to 20th	Night	6% 13% 4 11% 7% 6% 6% 4 7% 8% 28%	2925
	21st to end	Night	5% 12% 5% 11% 7% 6% 5% 5% 6% 8% 30%	3000
December	1st to 10th	Night	10% 15% 5% 11% 7% 5% 4 4 5% 7% 26%	4890
	11th to 20th	Night	7% 13% 6% 14% 7% 6% 5% 4 6% 9% 23%	5550
	21st to end	Night	3 8% 3 9% 6% 5% 6% 5% 7% 8% 41%	7458

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	53% 4 13% 19% 10% 1	3179
	11th to 20th	Day	49% 6% 9% 13% 18% 4	3180
	21st to end	Day	45% 6% 16% 27% 6%	3866
February	1st to 10th	Day	55% 5% 13% 15% 12% 1	1600
	11th to 20th	Day	55% 6% 13% 17% 9% 1	1727
	21st to end	Day	54% 5% 12% 19% 10%	1560
March	1st to 10th	Day	56% 5% 13% 16% 9%	3238
	11th to 20th	Day	37% 8% 18% 21% 16% 1	3566
	21st to end	Day	59% 7% 10% 17% 6%	3943
April	1st to 10th	Day	53% 8% 13% 16% 9% 1	2361
	11th to 20th	Day	53% 5% 11% 21% 9%	2440
	21st to end	Day	50% 7% 13% 22% 6% 2	2470
May	1st to 10th	Day	54% 7% 10% 23% 6%	4516
	11th to 20th	Day	50% 8% 11% 23% 8%	4698
	21st to end	Day	51% 10% 14% 17% 8% 1	5162
June	1st to 10th	Day	47% 8% 14% 23% 7% 1	2850
	11th to 20th	Day	55% 9% 14% 17% 5% 1	2850
	21st to end	Day	38% 11% 22% 23% 6% 1	2850
July	1st to 10th	Day	30% 7% 16% 30% 15% 2	4666
	11th to 20th	Day	31% 6% 18% 34% 10%	4659
	21st to end	Day	32% 6% 18% 27% 14% 2	4976
August	1st to 10th	Day	38% 7% 19% 31% 6%	4421
	11th to 20th	Day	35% 7% 24% 27% 7%	4159
	21st to end	Day	36% 5% 25% 28% 6% 1	4520
September	1st to 10th	Day	38% 7% 19% 25% 9% 3	2394
	11th to 20th	Day	40% 6% 16% 30% 8% 1	2090
	21st to end	Day	38% 7% 17% 26% 10% 3	2090
October	1st to 10th	Day	42% 4 19% 29% 6% 1	3308
	11th to 20th	Day	41% 5% 16% 26% 11% 1	2904
	21st to end	Day	43% 5% 11% 33% 7%	3101
November	1st to 10th	Day	55% 6% 13% 19% 6% 1	1575
	11th to 20th	Day	52% 6% 14% 20% 8%	1474
	21st to end	Day	50% 8% 16% 18% 6% 2	1400
December	1st to 10th	Day	59% 5% 17% 13% 5% 1	2290
	11th to 20th	Day	57% 8% 16% 14% 4 1	2644
	21st to end	Day	39% 4 10% 27% 17% 3	3498
January	1st to 10th	Night	56% 5% 13% 18% 8% 2	6766
	11th to 20th	Night	50% 4 12% 15% 17% 3	6780
	21st to end	Night	46% 5% 17% 23% 7% 1	7119
February	1st to 10th	Night	54% 6% 10% 19% 10% 1	2800
	11th to 20th	Night	54% 6% 11% 16% 12% 1	2569
	21st to end	Night	50% 5% 12% 20% 11% 1	2067
March	1st to 10th	Night	52% 8% 10% 21% 9%	3856
	11th to 20th	Night	42% 8% 19% 18% 12% 1	3633
	21st to end	Night	62% 8% 9% 13% 7%	3846
April	1st to 10th	Night	51% 7% 13% 18% 10% 1	1819
	11th to 20th	Night	52% 6% 11% 20% 9% 2	1691
	21st to end	Night	46% 6% 15% 23% 10% 1	1710
May	1st to 10th	Night	59% 5% 7% 21% 7%	2444
	11th to 20th	Night	47% 10% 12% 23% 8%	2199
	21st to end	Night	54% 12% 13% 14% 7% 1	2414
June	1st to 10th	Night	52% 10% 12% 19% 7% 1	1330
	11th to 20th	Night	56% 10% 12% 18% 4 1	1330
	21st to end	Night	40% 11% 19% 22% 7% 1	1330
July	1st to 10th	Night	37% 7% 18% 22% 15% 1	2185
	11th to 20th	Night	34% 8% 16% 31% 9% 2	2178
	21st to end	Night	34% 6% 21% 27% 12% 1	2337
August	1st to 10th	Night	38% 7% 18% 33% 5%	2481
	11th to 20th	Night	37% 8% 23% 25% 7% 1	2741
	21st to end	Night	38% 6% 20% 29% 6% 1	3146
September	1st to 10th	Night	40% 7% 15% 24% 12% 3	1783
	11th to 20th	Night	39% 6% 14% 28% 9% 3	2090
	21st to end	Night	38% 6% 14% 26% 14% 3	2090
October	1st to 10th	Night	40% 5% 19% 25% 11% 1	3666
	11th to 20th	Night	40% 5% 14% 31% 9% 1	4021
	21st to end	Night	47% 6% 14% 26% 6% 1	4457
November	1st to 10th	Night	54% 5% 12% 21% 7% 1	2703
	11th to 20th	Night	52% 7% 12% 19% 9% 1	2925
	21st to end	Night	51% 7% 16% 18% 7% 1	3000
December	1st to 10th	Night	58% 5% 15% 16% 6% 1	4890
	11th to 20th	Night	58% 5% 16% 15% 6%	5547
	21st to end	Night	38% 5% 12% 26% 16% 3	7458

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	5% 10% 28% 34% 19% 6%	200
	11th to 20th	Day	8% 9% 35% 30% 15% 5%	200
	21st to end	Day	10% 11% 30% 25% 19% 5%	220
February	1st to 10th	Day	13% 8% 31% 34% 10% 5%	200
	11th to 20th	Day	13% 6% 33% 31% 13% 4	283
	21st to end	Day	5% 8% 32% 37% 11% 6%	324
March	1st to 10th	Day	11% 10% 33% 26% 18% 3	500
	11th to 20th	Day	9% 8% 34% 30% 13% 6%	500
	21st to end	Day	15% 8% 31% 27% 15% 3	531
April	1st to 10th	Day	12% 15% 34% 25% 11% 3	460
	11th to 20th	Day	16% 12% 25% 29% 14% 5%	452
	21st to end	Day	15% 10% 27% 31% 12% 3	460
May	1st to 10th	Day	15% 10% 24% 31% 16% 4	577
	11th to 20th	Day	11% 8% 23% 35% 21% 4	675
	21st to end	Day	16% 5% 22% 28% 21% 7%	734
June	1st to 10th	Day	18% 7% 24% 32% 16% 3	680
	11th to 20th	Day	20% 8% 23% 26% 19% 5%	680
	21st to end	Day	12% 4 22% 29% 25% 9%	680
July	1st to 10th	Day	14% 4 17% 31% 25% 9%	671
	11th to 20th	Day	15% 5% 18% 31% 25% 6%	668
	21st to end	Day	8% 4 19% 29% 30% 10%	739
August	1st to 10th	Day	12% 5% 20% 30% 23% 11%	512
	11th to 20th	Day	9% 4 23% 32% 26% 7%	477
	21st to end	Day	13% 6% 19% 29% 24% 8%	539
September	1st to 10th	Day	14% 8% 27% 24% 22% 5%	490
	11th to 20th	Day	19% 7% 27% 24% 18% 5%	490
	21st to end	Day	12% 13% 30% 26% 14% 4	490
October	1st to 10th	Day	16% 8% 26% 28% 16% 6%	386
	11th to 20th	Day	11% 10% 32% 29% 14% 3	300
	21st to end	Day	13% 13% 33% 29% 11% 1	330
November	1st to 10th	Day	18% 5% 32% 29% 11% 4	300
	11th to 20th	Day	11% 8% 36% 33% 11% 2	236
	21st to end	Day	8% 9% 33% 31% 14% 6%	200
December	1st to 10th	Day	12% 7% 33% 27% 16% 5%	200
	11th to 20th	Day	11% 10% 35% 29% 14% 3	200
	21st to end	Day	10% 6% 24% 35% 19% 6%	220
January	1st to 10th	Night	11% 8% 34% 32% 15% 1	1000
	11th to 20th	Night	12% 8% 35% 33% 11% 2	1000
	21st to end	Night	13% 10% 33% 31% 11% 2	956
February	1st to 10th	Night	10% 9% 28% 33% 18% 2	800
	11th to 20th	Night	11% 6% 34% 33% 15% 1	784
	21st to end	Night	9% 5% 30% 35% 17% 4	660
March	1st to 10th	Night	10% 5% 30% 39% 14% 2	800
	11th to 20th	Night	10% 4 34% 36% 13% 3	800
	21st to end	Night	14% 5% 29% 36% 14% 2	862
April	1st to 10th	Night	13% 8% 30% 32% 16% 1	679
	11th to 20th	Night	16% 7% 27% 38% 11% 1	563
	21st to end	Night	16% 8% 25% 37% 14% 1	570
May	1st to 10th	Night	19% 7% 25% 35% 13% 1	437
	11th to 20th	Night	13% 5% 25% 35% 20% 2	374
	21st to end	Night	23% 5% 21% 31% 17% 2	411
June	1st to 10th	Night	20% 9% 20% 35% 14% 3	380
	11th to 20th	Night	21% 7% 21% 35% 16% 1	380
	21st to end	Night	11% 3 24% 38% 22% 2	380
July	1st to 10th	Night	14% 2 15% 46% 22% 1	370
	11th to 20th	Night	13% 3 19% 40% 23% 2	368
	21st to end	Night	10% 5% 17% 38% 29% 2	416
August	1st to 10th	Night	12% 3 20% 30% 30% 4	378
	11th to 20th	Night	12% 4 18% 38% 25% 2	505
	21st to end	Night	11% 5% 21% 40% 22% 1	627
September	1st to 10th	Night	14% 3 21% 40% 19% 2	629
	11th to 20th	Night	17% 4 22% 38% 18% 1	760
	21st to end	Night	11% 5% 29% 40% 14% 2	760
October	1st to 10th	Night	15% 4 28% 37% 14% 2	760
	11th to 20th	Night	14% 5% 32% 37% 11% 2	760
	21st to end	Night	13% 8% 32% 34% 12%	836
November	1st to 10th	Night	16% 6% 36% 31% 10% 1	948
	11th to 20th	Night	13% 7% 36% 31% 11% 2	1000
	21st to end	Night	11% 5% 38% 33% 11% 2	1000
December	1st to 10th	Night	15% 7% 33% 32% 11% 3	1000
	11th to 20th	Night	11% 7% 36% 32% 13% 2	1000
	21st to end	Night	11% 4 36% 31% 15% 2	1100

Page generated at 2025-03-24 07:04:32. Version ac07

Copyright © 2025 Attilla Danko.