Stone Garden Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stone Garden Observatory clear sky chart from 2005-11-06 to 2025-04-26.

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% 11% 4 9% 6% 6% 6% 5% 6% 7% 32%	116455
	Night	10% 13% 4 9% 6% 5% 5% 4 6% 7% 30%	120838
Transparency	Day	49% 7% 16% 20% 8% 1	116191
	Night	53% 6% 15% 18% 7% 1	120748
Seeing	Day	14% 10% 24% 27% 19% 6%	16921
	Night	18% 7% 21% 33% 19% 3	24990

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	7% 16% 7% 17% 8% 6% 6% 4 4 5% 19%	2977
	11th to 20th	Day	8% 13% 4 9% 5% 5% 5% 5% 6% 6% 34%	3159
	21st to end	Day	9% 21% 5% 10% 6% 5% 5% 5% 5% 7% 22%	3498
February	1st to 10th	Day	12% 16% 4 11% 6% 5% 4 3 5% 8% 27%	1612
	11th to 20th	Day	9% 13% 5% 10% 6% 6% 6% 4 7% 7% 27%	1755
	21st to end	Day	6% 15% 4 10% 7% 6% 6% 5% 5% 7% 29%	1485
March	1st to 10th	Day	5% 12% 5% 13% 9% 6% 6% 5% 7% 7% 25%	4472
	11th to 20th	Day	5% 11% 4 10% 6% 6% 7% 7% 9% 10% 26%	5000
	21st to end	Day	8% 13% 4 7% 5% 5% 5% 4 6% 6% 36%	5693
April	1st to 10th	Day	7% 10% 5% 14% 7% 6% 6% 5% 6% 7% 26%	2400
	11th to 20th	Day	9% 13% 5% 10% 7% 7% 7% 5% 6% 6% 27%	2564
	21st to end	Day	11% 12% 4 9% 5% 6% 6% 4 6% 6% 30%	2715
May	1st to 10th	Day	7% 9% 4 10% 6% 5% 5% 5% 7% 9% 33%	4420
	11th to 20th	Day	9% 11% 4 10% 8% 7% 7% 5% 7% 7% 25%	4592
	21st to end	Day	12% 11% 4 11% 7% 7% 6% 5% 6% 7% 25%	5398
June	1st to 10th	Day	11% 8% 3 9% 7% 7% 6% 5% 6% 8% 29%	3040
	11th to 20th	Day	14% 7% 3 8% 7% 7% 6% 5% 7% 8% 26%	3040
	21st to end	Day	11% 8% 3 8% 7% 6% 6% 5% 7% 9% 31%	3040
July	1st to 10th	Day	8% 5% 2 5% 5% 4 5% 4 6% 7% 48%	5060
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 6% 9% 52%	4754
	21st to end	Day	8% 6% 3 8% 6% 6% 7% 5% 8% 9% 34%	5138
August	1st to 10th	Day	4 3 1 4 4 3 4 3 5% 8% 61%	4514
	11th to 20th	Day	9% 5% 2 7% 4 5% 5% 4 7% 9% 42%	4247
	21st to end	Day	12% 7% 3 8% 5% 5% 6% 5% 6% 8% 36%	4521
September	1st to 10th	Day	10% 7% 2 7% 5% 6% 5% 4 7% 8% 40%	2290
	11th to 20th	Day	10% 6% 2 7% 6% 6% 5% 4 6% 9% 38%	2280
	21st to end	Day	15% 8% 3 9% 7% 6% 6% 5% 6% 7% 29%	2001
October	1st to 10th	Day	14% 14% 4 8% 6% 6% 6% 5% 7% 7% 25%	3180
	11th to 20th	Day	16% 12% 3 8% 6% 5% 5% 4 6% 8% 27%	3144
	21st to end	Day	11% 10% 4 9% 5% 5% 4 4 5% 8% 34%	2831
November	1st to 10th	Day	16% 15% 4 11% 6% 5% 5% 4 5% 4 26%	1568
	11th to 20th	Day	10% 16% 5% 12% 7% 6% 6% 4 7% 7% 19%	1596
	21st to end	Day	12% 17% 6% 10% 7% 5% 8% 5% 5% 6% 18%	1257
December	1st to 10th	Day	18% 18% 5% 11% 6% 5% 7% 4 5% 5% 17%	1960
	11th to 20th	Day	11% 22% 8% 12% 8% 6% 5% 4 5% 4 15%	2262
	21st to end	Day	5% 18% 6% 13% 7% 7% 5% 4 6% 7% 23%	2992
January	1st to 10th	Night	8% 17% 5% 13% 8% 6% 6% 5% 6% 6% 19%	6998
	11th to 20th	Night	8% 13% 4 9% 6% 5% 6% 4 6% 7% 33%	6739
	21st to end	Night	13% 20% 6% 10% 5% 5% 5% 4 6% 6% 21%	7458
February	1st to 10th	Night	11% 18% 4 10% 7% 5% 5% 4 5% 5% 25%	2784
	11th to 20th	Night	8% 13% 5% 11% 7% 5% 5% 4 6% 7% 29%	2532
	21st to end	Night	7% 14% 6% 10% 6% 5% 5% 4 7% 7% 31%	2145
March	1st to 10th	Night	5% 15% 6% 12% 6% 5% 6% 4 6% 7% 28%	5417
	11th to 20th	Night	5% 12% 5% 10% 6% 6% 6% 5% 7% 8% 30%	4999
	21st to end	Night	10% 10% 3 7% 4 4 5% 4 6% 7% 40%	5282
April	1st to 10th	Night	8% 13% 5% 9% 6% 6% 5% 4 6% 8% 30%	2000
	11th to 20th	Night	8% 12% 4 9% 6% 6% 6% 5% 7% 7% 30%	1781
	21st to end	Night	9% 12% 4 7% 6% 5% 6% 4 6% 6% 36%	1581
May	1st to 10th	Night	10% 10% 3 9% 6% 5% 6% 5% 7% 7% 34%	2540
	11th to 20th	Night	11% 10% 4 7% 7% 7% 6% 4 6% 9% 28%	2300
	21st to end	Night	13% 13% 5% 11% 6% 5% 4 4 6% 6% 27%	2174
June	1st to 10th	Night	14% 9% 3 7% 5% 6% 6% 5% 7% 8% 29%	1140
	11th to 20th	Night	14% 8% 2 10% 6% 6% 5% 4 8% 8% 29%	1140
	21st to end	Night	10% 5% 3 9% 8% 7% 7% 5% 6% 9% 32%	1140
July	1st to 10th	Night	7% 6% 2 5% 5% 4 3 3 6% 7% 51%	1900
	11th to 20th	Night	6% 4 2 4 4 3 4 3 5% 7% 58%	2204
	21st to end	Night	12% 8% 2 5% 4 4 4 4 7% 9% 41%	2414
August	1st to 10th	Night	4 3 1 3 2 2 3 3 4 5% 70%	2402
	11th to 20th	Night	7% 4 1 4 4 4 5% 5% 6% 9% 49%	2580
	21st to end	Night	14% 8% 3 8% 5% 5% 5% 4 6% 8% 35%	3146
September	1st to 10th	Night	10% 7% 2 7% 5% 5% 5% 3 5% 8% 41%	1890
	11th to 20th	Night	10% 7% 2 6% 6% 4 5% 4 5% 7% 45%	1900
	21st to end	Night	15% 10% 3 7% 5% 5% 5% 4 5% 7% 33%	2178
October	1st to 10th	Night	15% 13% 4 10% 5% 5% 5% 4 6% 7% 25%	3800
	11th to 20th	Night	11% 10% 3 9% 5% 5% 6% 5% 7% 8% 29%	3792
	21st to end	Night	11% 10% 4 9% 6% 5% 6% 5% 6% 6% 32%	4716
November	1st to 10th	Night	14% 15% 3 9% 6% 5% 7% 5% 6% 6% 24%	2730
	11th to 20th	Night	10% 15% 5% 10% 7% 6% 6% 5% 6% 7% 23%	2800
	21st to end	Night	9% 18% 5% 12% 6% 6% 6% 4 6% 7% 21%	3122
December	1st to 10th	Night	13% 17% 4 10% 7% 7% 6% 4 6% 5% 20%	5220
	11th to 20th	Night	14% 19% 5% 12% 7% 5% 5% 4 6% 5% 17%	5930
	21st to end	Night	5% 17% 6% 12% 8% 6% 6% 4 6% 7% 22%	7964

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	67% 6% 11% 12% 5%	2977
	11th to 20th	Day	51% 6% 16% 14% 9% 3	3180
	21st to end	Day	62% 8% 14% 12% 4	3498
February	1st to 10th	Day	58% 5% 13% 12% 12% 1	1612
	11th to 20th	Day	58% 5% 16% 16% 5%	1764
	21st to end	Day	55% 5% 15% 15% 8% 2	1485
March	1st to 10th	Day	56% 5% 11% 18% 9%	4472
	11th to 20th	Day	48% 8% 16% 19% 9% 1	5000
	21st to end	Day	47% 6% 12% 23% 12% 1	5693
April	1st to 10th	Day	54% 7% 13% 19% 7% 1	2400
	11th to 20th	Day	55% 5% 10% 21% 8%	2564
	21st to end	Day	52% 5% 14% 20% 9%	2715
May	1st to 10th	Day	43% 8% 15% 24% 8% 2	4420
	11th to 20th	Day	51% 9% 12% 18% 9% 1	4595
	21st to end	Day	53% 7% 13% 21% 5% 1	5399
June	1st to 10th	Day	49% 7% 13% 20% 9% 1	3040
	11th to 20th	Day	52% 8% 13% 21% 6%	3040
	21st to end	Day	47% 8% 17% 21% 7%	3040
July	1st to 10th	Day	32% 7% 10% 26% 23% 2	4981
	11th to 20th	Day	29% 6% 20% 33% 13%	4672
	21st to end	Day	41% 7% 18% 21% 12% 1	4976
August	1st to 10th	Day	22% 6% 18% 40% 14% 1	4496
	11th to 20th	Day	36% 7% 20% 29% 7%	4289
	21st to end	Day	47% 5% 26% 19% 3	4520
September	1st to 10th	Day	40% 8% 25% 22% 5%	2290
	11th to 20th	Day	43% 7% 19% 25% 6%	2280
	21st to end	Day	53% 8% 16% 18% 4 1	2001
October	1st to 10th	Day	58% 6% 18% 13% 6%	3180
	11th to 20th	Day	55% 5% 21% 18% 2	3144
	21st to end	Day	49% 6% 25% 14% 6%	2831
November	1st to 10th	Day	61% 6% 13% 13% 6%	1568
	11th to 20th	Day	64% 6% 14% 12% 4	1596
	21st to end	Day	68% 6% 12% 13% 2	1257
December	1st to 10th	Day	72% 5% 12% 9% 2	1960
	11th to 20th	Day	74% 4 11% 6% 4	2264
	21st to end	Day	61% 6% 15% 14% 4	2992
January	1st to 10th	Night	65% 6% 13% 11% 5%	6998
	11th to 20th	Night	53% 5% 14% 13% 10% 5%	6780
	21st to end	Night	66% 6% 15% 11% 3	7458
February	1st to 10th	Night	61% 5% 12% 11% 10% 1	2784
	11th to 20th	Night	56% 4 19% 16% 5%	2534
	21st to end	Night	52% 4 15% 18% 9% 1	2145
March	1st to 10th	Night	54% 5% 11% 18% 11%	5417
	11th to 20th	Night	49% 7% 10% 23% 11%	5000
	21st to end	Night	42% 5% 11% 25% 16%	5282
April	1st to 10th	Night	51% 6% 11% 23% 9% 1	2000
	11th to 20th	Night	51% 6% 11% 23% 9% 1	1781
	21st to end	Night	48% 6% 12% 22% 12% 1	1581
May	1st to 10th	Night	45% 7% 14% 25% 8% 1	2540
	11th to 20th	Night	49% 8% 15% 21% 7% 1	2300
	21st to end	Night	55% 7% 12% 21% 5%	2174
June	1st to 10th	Night	50% 7% 12% 19% 11% 1	1140
	11th to 20th	Night	52% 6% 13% 22% 6%	1140
	21st to end	Night	47% 6% 18% 24% 5%	1140
July	1st to 10th	Night	33% 5% 10% 27% 24% 1	1870
	11th to 20th	Night	27% 4 18% 39% 12%	2163
	21st to end	Night	39% 5% 23% 18% 14% 1	2337
August	1st to 10th	Night	18% 3 15% 43% 20% 1	2395
	11th to 20th	Night	31% 6% 18% 36% 9%	2604
	21st to end	Night	48% 4 25% 19% 3	3146
September	1st to 10th	Night	41% 6% 20% 27% 7% 1	1890
	11th to 20th	Night	38% 6% 15% 31% 9%	1900
	21st to end	Night	50% 7% 15% 19% 9%	2178
October	1st to 10th	Night	58% 5% 16% 16% 5%	3800
	11th to 20th	Night	52% 4 20% 20% 4	3792
	21st to end	Night	53% 5% 21% 18% 4	4716
November	1st to 10th	Night	60% 7% 14% 14% 5%	2730
	11th to 20th	Night	60% 7% 14% 13% 6%	2800
	21st to end	Night	62% 6% 15% 13% 4	3122
December	1st to 10th	Night	67% 5% 12% 12% 3	5220
	11th to 20th	Night	70% 6% 13% 8% 3	5927
	21st to end	Night	61% 6% 16% 12% 4	7964

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	15% 6% 20% 36% 17% 7%	200
	11th to 20th	Day	17% 12% 19% 33% 16% 5%	200
	21st to end	Day	20% 10% 25% 22% 19% 3	220
February	1st to 10th	Day	20% 16% 25% 20% 14% 4	255
	11th to 20th	Day	16% 11% 26% 27% 18% 3	395
	21st to end	Day	14% 18% 30% 22% 11% 5%	331
March	1st to 10th	Day	11% 14% 27% 25% 17% 5%	480
	11th to 20th	Day	12% 15% 28% 26% 16% 3	500
	21st to end	Day	15% 16% 26% 21% 17% 5%	550
April	1st to 10th	Day	12% 17% 28% 27% 14% 3	500
	11th to 20th	Day	15% 15% 28% 22% 14% 6%	492
	21st to end	Day	13% 16% 27% 27% 14% 4	481
May	1st to 10th	Day	14% 13% 28% 27% 15% 3	463
	11th to 20th	Day	12% 9% 24% 31% 19% 5%	610
	21st to end	Day	13% 8% 22% 27% 24% 6%	868
June	1st to 10th	Day	15% 10% 23% 30% 19% 3	870
	11th to 20th	Day	15% 7% 24% 29% 19% 5%	870
	21st to end	Day	13% 6% 24% 28% 24% 6%	870
July	1st to 10th	Day	13% 6% 18% 30% 26% 7%	856
	11th to 20th	Day	13% 6% 20% 26% 28% 7%	681
	21st to end	Day	9% 6% 18% 32% 27% 7%	739
August	1st to 10th	Day	8% 4 17% 32% 28% 10%	562
	11th to 20th	Day	8% 7% 25% 27% 21% 11%	477
	21st to end	Day	14% 8% 25% 30% 16% 7%	539
September	1st to 10th	Day	13% 8% 25% 26% 22% 7%	490
	11th to 20th	Day	12% 8% 23% 26% 23% 9%	490
	21st to end	Day	18% 11% 32% 23% 13% 3	490
October	1st to 10th	Day	22% 9% 23% 25% 15% 6%	490
	11th to 20th	Day	20% 10% 29% 24% 17% 1	485
	21st to end	Day	18% 12% 26% 28% 13% 4	247
November	1st to 10th	Day	30% 12% 22% 23% 8% 5%	200
	11th to 20th	Day	21% 9% 23% 22% 17% 9%	200
	21st to end	Day	21% 10% 26% 28% 12% 4	200
December	1st to 10th	Day	18% 5% 18% 30% 26% 5%	200
	11th to 20th	Day	14% 9% 19% 32% 23% 5%	200
	21st to end	Day	22% 6% 13% 37% 14% 8%	220
January	1st to 10th	Night	22% 9% 23% 33% 11% 2	1002
	11th to 20th	Night	22% 7% 26% 29% 15% 1	900
	21st to end	Night	21% 10% 25% 29% 13% 2	990
February	1st to 10th	Night	22% 11% 20% 30% 15% 3	820
	11th to 20th	Night	17% 6% 24% 36% 15% 2	784
	21st to end	Night	16% 9% 24% 32% 16% 2	660
March	1st to 10th	Night	15% 11% 23% 33% 16% 3	800
	11th to 20th	Night	13% 10% 24% 36% 16% 2	800
	21st to end	Night	13% 9% 19% 30% 25% 4	880
April	1st to 10th	Night	16% 8% 17% 34% 22% 3	800
	11th to 20th	Night	14% 6% 21% 35% 18% 5%	593
	21st to end	Night	15% 5% 26% 33% 20% 1	409
May	1st to 10th	Night	16% 6% 22% 37% 16% 3	380
	11th to 20th	Night	13% 4 18% 40% 23% 3	374
	21st to end	Night	15% 8% 16% 33% 26% 2	411
June	1st to 10th	Night	16% 6% 22% 32% 21% 2	380
	11th to 20th	Night	17% 4 21% 30% 25% 3	380
	21st to end	Night	11% 3 17% 38% 29% 2	380
July	1st to 10th	Night	12% 4 14% 37% 32% 1	370
	11th to 20th	Night	12% 3 14% 37% 30% 3	368
	21st to end	Night	12% 3 13% 36% 30% 6%	416
August	1st to 10th	Night	8% 2 11% 34% 37% 8%	378
	11th to 20th	Night	9% 1 15% 37% 34% 5%	420
	21st to end	Night	17% 4 16% 34% 26% 4	627
September	1st to 10th	Night	14% 4 17% 34% 29% 3	750
	11th to 20th	Night	13% 4 14% 32% 30% 7%	760
	21st to end	Night	21% 7% 21% 33% 17% 2	760
October	1st to 10th	Night	20% 5% 19% 32% 19% 4	760
	11th to 20th	Night	22% 5% 23% 30% 18% 2	760
	21st to end	Night	21% 10% 25% 31% 13% 1	836
November	1st to 10th	Night	24% 10% 24% 30% 11% 2	780
	11th to 20th	Night	21% 11% 26% 30% 11% 2	800
	21st to end	Night	20% 10% 28% 29% 11% 2	1052
December	1st to 10th	Night	18% 6% 21% 36% 18% 2	1100
	11th to 20th	Night	21% 8% 24% 32% 14% 2	1100
	21st to end	Night	18% 7% 25% 33% 15% 2	1210

Page generated at 2025-04-23 06:25:04. Version ac07

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