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 2024-10-24.

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	10% 9% 3 8% 5% 5% 5% 4 6% 8% 35%	101904
	Night	8% 10% 4 10% 6% 5% 5% 4 6% 8% 33%	96945
Transparency	Day	45% 7% 15% 23% 9% 1	101647
	Night	47% 7% 14% 21% 9% 1	96848
Seeing	Day	13% 8% 26% 29% 19% 6%	15401
	Night	13% 6% 29% 35% 15% 2	24352

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	4 12% 5% 12% 8% 7% 5% 4 5% 8% 30%	2219
	11th to 20th	Day	5% 9% 4 10% 6% 4 8% 6% 9% 8% 31%	2199
	21st to end	Day	5% 11% 5% 10% 6% 5% 5% 3 6% 8% 35%	2698
February	1st to 10th	Day	8% 14% 5% 11% 5% 6% 5% 5% 7% 8% 27%	1520
	11th to 20th	Day	5% 16% 5% 13% 7% 5% 7% 4 5% 7% 26%	1629
	21st to end	Day	3 14% 5% 11% 8% 7% 8% 5% 5% 7% 27%	1484
March	1st to 10th	Day	4 11% 6% 17% 9% 6% 5% 4 5% 8% 25%	3138
	11th to 20th	Day	5% 11% 3 6% 4 3 4 5% 7% 11% 41%	3456
	21st to end	Day	8% 18% 6% 14% 6% 5% 5% 4 6% 6% 23%	3876
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	11% 11% 4 8% 4 3 5% 3 4 6% 40%	1986
	11th to 20th	Day	11% 8% 3 8% 5% 5% 4 4 5% 6% 39%	1748
	21st to end	Day	9% 11% 5% 9% 6% 5% 6% 5% 7% 7% 31%	1809
November	1st to 10th	Day	7% 17% 5% 10% 6% 5% 5% 5% 6% 7% 26%	1495
	11th to 20th	Day	5% 13% 6% 11% 7% 5% 5% 3 6% 7% 32%	1401
	21st to end	Day	6% 9% 3 12% 7% 6% 6% 5% 7% 8% 32%	1330
December	1st to 10th	Day	7% 14% 5% 11% 7% 8% 5% 4 5% 6% 27%	1330
	11th to 20th	Day	4 12% 5% 13% 6% 7% 6% 5% 7% 8% 27%	1682
	21st to end	Day	4 9% 2 9% 6% 4 4 4 5% 10% 42%	2442
January	1st to 10th	Night	4 13% 5% 13% 7% 6% 6% 5% 7% 7% 27%	4726
	11th to 20th	Night	4 9% 5% 12% 9% 6% 5% 6% 7% 6% 31%	4699
	21st to end	Night	6% 14% 4 9% 7% 5% 5% 5% 7% 8% 31%	4987
February	1st to 10th	Night	6% 13% 5% 12% 8% 7% 6% 4 6% 7% 28%	2660
	11th to 20th	Night	6% 14% 5% 11% 8% 5% 6% 5% 7% 7% 26%	2437
	21st to end	Night	5% 12% 4 10% 7% 7% 7% 5% 6% 7% 31%	1967
March	1st to 10th	Night	3 10% 5% 16% 9% 7% 5% 5% 8% 8% 25%	3736
	11th to 20th	Night	6% 9% 3 7% 6% 6% 6% 5% 7% 9% 36%	3522
	21st to end	Night	6% 17% 8% 18% 6% 5% 5% 5% 5% 7% 19%	3775
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	10% 9% 3 8% 5% 5% 5% 3 5% 9% 39%	2194
	11th to 20th	Night	10% 8% 3 8% 6% 4 5% 4 6% 8% 38%	2429
	21st to end	Night	7% 11% 4 8% 6% 6% 6% 4 6% 8% 32%	2613
November	1st to 10th	Night	9% 16% 5% 10% 5% 5% 5% 4 6% 7% 28%	2564
	11th to 20th	Night	7% 13% 4 10% 6% 6% 6% 4 7% 8% 29%	2778
	21st to end	Night	5% 12% 4 11% 7% 6% 5% 5% 6% 8% 31%	2850
December	1st to 10th	Night	7% 13% 5% 11% 6% 6% 4 4 5% 8% 30%	2850
	11th to 20th	Night	4 11% 5% 14% 8% 5% 5% 4 7% 9% 28%	3510
	21st to end	Night	3 9% 3 8% 5% 5% 5% 5% 7% 8% 42%	5214

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	51% 5% 12% 20% 10% 1	2219
	11th to 20th	Day	45% 8% 10% 15% 19% 4	2220
	21st to end	Day	45% 6% 14% 27% 6% 1	2698
February	1st to 10th	Day	53% 5% 13% 16% 11% 1	1520
	11th to 20th	Day	55% 7% 13% 16% 8% 1	1638
	21st to end	Day	55% 6% 12% 18% 9%	1484
March	1st to 10th	Day	58% 5% 13% 15% 9%	3138
	11th to 20th	Day	36% 8% 18% 22% 16% 1	3457
	21st to end	Day	59% 7% 10% 17% 6%	3876
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	45% 5% 16% 24% 9% 1	1986
	11th to 20th	Day	42% 7% 16% 23% 10% 1	1748
	21st to end	Day	49% 8% 11% 23% 9% 1	1809
November	1st to 10th	Day	56% 6% 13% 18% 6% 1	1495
	11th to 20th	Day	51% 6% 14% 20% 8%	1401
	21st to end	Day	48% 8% 16% 19% 6% 2	1330
December	1st to 10th	Day	57% 6% 11% 16% 8% 2	1330
	11th to 20th	Day	50% 11% 15% 17% 6% 1	1684
	21st to end	Day	38% 5% 10% 24% 18% 4	2442
January	1st to 10th	Night	52% 7% 13% 18% 8% 2	4726
	11th to 20th	Night	51% 5% 11% 14% 16% 3	4740
	21st to end	Night	48% 6% 14% 22% 8% 1	4987
February	1st to 10th	Night	54% 6% 10% 19% 10% 1	2660
	11th to 20th	Night	54% 6% 12% 16% 11% 1	2438
	21st to end	Night	50% 5% 12% 20% 11% 1	1967
March	1st to 10th	Night	53% 8% 9% 21% 8%	3736
	11th to 20th	Night	41% 8% 19% 18% 12% 1	3522
	21st to end	Night	62% 8% 9% 13% 7%	3775
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	44% 7% 15% 22% 11% 1	2194
	11th to 20th	Night	43% 6% 14% 26% 10% 1	2429
	21st to end	Night	47% 8% 14% 22% 8% 1	2613
November	1st to 10th	Night	54% 6% 12% 20% 7% 1	2564
	11th to 20th	Night	51% 7% 12% 19% 9% 1	2778
	21st to end	Night	49% 7% 17% 19% 7% 2	2850
December	1st to 10th	Night	52% 6% 10% 20% 10% 2	2850
	11th to 20th	Night	51% 6% 16% 20% 6%	3507
	21st to end	Night	37% 6% 13% 24% 15% 5%	5214

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	4 8% 29% 33% 19% 6%	180
	11th to 20th	Day	8% 8% 32% 31% 16% 6%	180
	21st to end	Day	9% 11% 29% 25% 21% 6%	198
February	1st to 10th	Day	14% 8% 31% 31% 11% 5%	180
	11th to 20th	Day	14% 7% 32% 30% 13% 4	254
	21st to end	Day	5% 7% 32% 38% 12% 7%	296
March	1st to 10th	Day	12% 11% 33% 24% 17% 3	460
	11th to 20th	Day	9% 8% 34% 30% 13% 6%	460
	21st to end	Day	14% 8% 31% 27% 15% 3	506
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	10% 10% 33% 29% 14% 3	300
	21st to end	Day	14% 13% 33% 29% 11% 1	306
November	1st to 10th	Day	19% 6% 32% 30% 10% 4	270
	11th to 20th	Day	12% 8% 34% 34% 10% 2	213
	21st to end	Day	7% 9% 35% 29% 14% 6%	180
December	1st to 10th	Day	9% 7% 34% 27% 17% 6%	180
	11th to 20th	Day	10% 9% 37% 28% 14% 2	180
	21st to end	Day	11% 5% 25% 35% 18% 6%	198
January	1st to 10th	Night	12% 8% 33% 31% 14% 1	950
	11th to 20th	Night	12% 8% 34% 33% 11% 2	950
	21st to end	Night	14% 10% 32% 31% 12% 2	909
February	1st to 10th	Night	10% 10% 28% 32% 18% 2	760
	11th to 20th	Night	12% 6% 34% 32% 15% 1	744
	21st to end	Night	10% 5% 28% 36% 17% 4	628
March	1st to 10th	Night	10% 5% 31% 38% 14% 3	760
	11th to 20th	Night	10% 4 34% 35% 13% 3	760
	21st to end	Night	14% 5% 28% 36% 15% 2	836
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% 33% 34% 12%	804
November	1st to 10th	Night	16% 7% 35% 31% 10% 1	899
	11th to 20th	Night	14% 7% 36% 30% 11% 2	950
	21st to end	Night	11% 5% 39% 33% 11% 2	950
December	1st to 10th	Night	14% 7% 32% 32% 12% 3	950
	11th to 20th	Night	11% 7% 37% 32% 12% 2	950
	21st to end	Night	11% 4 36% 31% 16% 2	1045

Page generated at 2024-10-21 00:20:04. Version ac07

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