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 2024-02-25.

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% 10% 4 8% 7% 4 6% 6% 6% 8% 30%	77504
	Night	11% 12% 5% 8% 7% 4 6% 6% 6% 7% 28%	81727
Transparency	Day	51% 7% 16% 19% 6% 1	77198
	Night	54% 7% 15% 17% 7% 1	81613
Seeing	Day	14% 10% 24% 27% 20% 6%	15336
	Night	18% 7% 21% 32% 19% 3	23451

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	8% 15% 6% 13% 11% 6% 6% 7% 5% 6% 18%	2073
	11th to 20th	Day	10% 16% 6% 8% 6% 3 4 6% 5% 6% 31%	2199
	21st to end	Day	11% 20% 6% 10% 8% 5% 7% 6% 5% 7% 16%	2442
February	1st to 10th	Day	13% 16% 5% 9% 7% 4 4 3 5% 8% 27%	1531
	11th to 20th	Day	10% 12% 4 9% 7% 4 7% 5% 7% 7% 27%	1665
	21st to end	Day	6% 12% 6% 9% 8% 4 6% 6% 5% 7% 31%	1369
March	1st to 10th	Day	8% 13% 6% 7% 6% 4 6% 6% 6% 8% 30%	1781
	11th to 20th	Day	6% 13% 5% 8% 7% 5% 6% 7% 7% 8% 29%	1980
	21st to end	Day	7% 13% 5% 8% 8% 5% 6% 6% 5% 7% 28%	2256
April	1st to 10th	Day	7% 9% 5% 12% 9% 4 6% 7% 6% 7% 25%	2160
	11th to 20th	Day	10% 12% 5% 9% 8% 5% 7% 6% 5% 6% 26%	2304
	21st to end	Day	13% 11% 5% 8% 6% 5% 6% 5% 5% 6% 30%	2503
May	1st to 10th	Day	11% 11% 5% 9% 8% 4 5% 5% 6% 7% 29%	2520
	11th to 20th	Day	10% 9% 4 7% 8% 4 6% 6% 7% 8% 33%	2594
	21st to end	Day	11% 7% 3 8% 8% 5% 7% 7% 6% 8% 30%	3042
June	1st to 10th	Day	12% 7% 3 8% 8% 5% 7% 7% 6% 8% 29%	2880
	11th to 20th	Day	15% 7% 4 7% 8% 5% 7% 6% 6% 8% 26%	2880
	21st to end	Day	10% 7% 4 6% 7% 4 6% 7% 7% 9% 32%	2880
July	1st to 10th	Day	11% 6% 3 7% 7% 4 6% 6% 7% 8% 34%	2880
	11th to 20th	Day	11% 6% 3 6% 6% 4 6% 6% 7% 10% 35%	2714
	21st to end	Day	9% 5% 3 5% 5% 4 6% 6% 7% 9% 42%	2938
August	1st to 10th	Day	7% 5% 2 5% 6% 3 6% 5% 6% 9% 46%	2579
	11th to 20th	Day	7% 5% 2 6% 5% 4 4 6% 6% 9% 45%	2431
	21st to end	Day	11% 7% 3 8% 7% 5% 6% 7% 7% 8% 31%	2574
September	1st to 10th	Day	10% 7% 2 6% 6% 4 6% 6% 7% 8% 39%	2170
	11th to 20th	Day	10% 6% 2 6% 7% 4 6% 5% 6% 9% 39%	2160
	21st to end	Day	14% 7% 3 8% 8% 4 6% 6% 6% 7% 30%	1897
October	1st to 10th	Day	14% 10% 4 6% 7% 3 5% 6% 7% 8% 31%	1800
	11th to 20th	Day	13% 10% 3 8% 7% 5% 5% 7% 6% 8% 28%	1796
	21st to end	Day	15% 12% 4 8% 5% 3 5% 4 4 8% 31%	1634
November	1st to 10th	Day	17% 14% 5% 9% 7% 4 5% 5% 5% 4 26%	1488
	11th to 20th	Day	10% 15% 5% 12% 8% 5% 7% 5% 7% 7% 19%	1517
	21st to end	Day	12% 16% 6% 10% 8% 4 8% 7% 5% 6% 18%	1195
December	1st to 10th	Day	12% 12% 6% 10% 10% 4 8% 5% 6% 6% 22%	1140
	11th to 20th	Day	13% 21% 6% 8% 7% 5% 5% 6% 5% 5% 18%	1442
	21st to end	Day	7% 15% 4 9% 10% 5% 6% 5% 6% 6% 25%	2090
January	1st to 10th	Night	10% 15% 5% 9% 10% 5% 7% 7% 6% 7% 19%	4888
	11th to 20th	Night	10% 15% 5% 9% 7% 4 6% 5% 4 5% 30%	4699
	21st to end	Night	16% 21% 6% 10% 8% 4 5% 5% 5% 5% 15%	5214
February	1st to 10th	Night	12% 17% 5% 9% 8% 4 5% 5% 5% 6% 24%	2645
	11th to 20th	Night	9% 13% 5% 10% 8% 3 5% 5% 5% 7% 28%	2402
	21st to end	Night	8% 12% 6% 9% 6% 4 5% 5% 6% 7% 32%	1981
March	1st to 10th	Night	8% 13% 5% 9% 7% 3 7% 6% 6% 7% 29%	2178
	11th to 20th	Night	7% 13% 6% 9% 7% 4 7% 5% 6% 7% 30%	1979
	21st to end	Night	7% 11% 5% 9% 7% 5% 8% 8% 7% 7% 27%	2100
April	1st to 10th	Night	9% 12% 5% 9% 6% 4 5% 5% 6% 8% 29%	1800
	11th to 20th	Night	9% 11% 4 9% 7% 4 7% 7% 7% 8% 27%	1601
	21st to end	Night	10% 11% 4 6% 7% 4 5% 6% 6% 6% 35%	1457
May	1st to 10th	Night	12% 10% 4 8% 6% 5% 6% 5% 7% 8% 30%	1440
	11th to 20th	Night	11% 7% 4 6% 8% 5% 7% 6% 6% 8% 32%	1298
	21st to end	Night	13% 8% 4 7% 6% 3 5% 6% 8% 9% 31%	1230
June	1st to 10th	Night	15% 7% 4 6% 6% 5% 6% 6% 7% 8% 29%	1080
	11th to 20th	Night	15% 7% 3 8% 7% 5% 5% 5% 7% 8% 29%	1080
	21st to end	Night	9% 4 4 8% 9% 5% 7% 6% 6% 9% 32%	1080
July	1st to 10th	Night	10% 7% 3 6% 7% 4 6% 6% 9% 9% 34%	1080
	11th to 20th	Night	11% 6% 3 6% 6% 4 5% 5% 6% 8% 41%	1244
	21st to end	Night	10% 6% 3 4 5% 4 4 5% 6% 9% 46%	1379
August	1st to 10th	Night	6% 5% 2 4 4 2 4 5% 6% 8% 52%	1360
	11th to 20th	Night	6% 4 1 4 5% 3 5% 6% 6% 9% 51%	1465
	21st to end	Night	12% 8% 4 7% 7% 4 5% 6% 6% 9% 33%	1782
September	1st to 10th	Night	11% 7% 3 6% 7% 4 5% 5% 5% 9% 40%	1790
	11th to 20th	Night	10% 6% 3 6% 6% 3 5% 5% 5% 7% 46%	1800
	21st to end	Night	15% 9% 4 5% 6% 3 5% 5% 5% 8% 34%	2062
October	1st to 10th	Night	11% 10% 4 8% 6% 4 5% 5% 7% 8% 32%	2160
	11th to 20th	Night	16% 10% 4 7% 7% 4 5% 6% 7% 7% 29%	2163
	21st to end	Night	14% 11% 4 7% 8% 3 6% 6% 6% 7% 30%	2719
November	1st to 10th	Night	15% 14% 4 8% 7% 4 7% 6% 6% 6% 25%	2590
	11th to 20th	Night	11% 14% 6% 9% 8% 4 6% 7% 5% 7% 23%	2660
	21st to end	Night	10% 16% 5% 11% 7% 4 6% 6% 6% 6% 22%	2965
December	1st to 10th	Night	10% 12% 6% 9% 8% 5% 7% 6% 5% 6% 26%	3040
	11th to 20th	Night	15% 17% 5% 9% 8% 4 6% 5% 6% 6% 19%	3750
	21st to end	Night	6% 14% 5% 10% 10% 5% 6% 6% 7% 8% 23%	5566

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	65% 8% 11% 13% 3	2073
	11th to 20th	Day	55% 7% 15% 8% 11% 5%	2220
	21st to end	Day	67% 9% 12% 8% 3 1	2442
February	1st to 10th	Day	59% 5% 14% 11% 11% 1	1531
	11th to 20th	Day	58% 6% 16% 16% 5%	1674
	21st to end	Day	53% 5% 16% 15% 9% 2	1369
March	1st to 10th	Day	51% 5% 13% 19% 10% 1	1781
	11th to 20th	Day	51% 5% 18% 19% 6%	1980
	21st to end	Day	54% 6% 16% 19% 5%	2256
April	1st to 10th	Day	55% 8% 13% 18% 6% 1	2160
	11th to 20th	Day	57% 5% 10% 21% 7%	2304
	21st to end	Day	54% 5% 14% 19% 9%	2503
May	1st to 10th	Day	54% 6% 13% 20% 6%	2520
	11th to 20th	Day	48% 6% 15% 21% 9% 1	2597
	21st to end	Day	49% 7% 15% 21% 6% 1	3043
June	1st to 10th	Day	49% 8% 14% 20% 9% 1	2880
	11th to 20th	Day	52% 9% 13% 20% 6%	2880
	21st to end	Day	45% 8% 17% 22% 7%	2880
July	1st to 10th	Day	44% 8% 15% 25% 8% 1	2801
	11th to 20th	Day	43% 8% 20% 24% 4	2632
	21st to end	Day	33% 9% 24% 27% 7%	2775
August	1st to 10th	Day	33% 9% 24% 27% 7%	2561
	11th to 20th	Day	34% 8% 24% 27% 7%	2431
	21st to end	Day	48% 8% 20% 20% 4	2574
September	1st to 10th	Day	41% 8% 25% 21% 5%	2170
	11th to 20th	Day	42% 7% 20% 25% 6%	2160
	21st to end	Day	52% 8% 16% 19% 5% 1	1897
October	1st to 10th	Day	51% 6% 23% 17% 4	1800
	11th to 20th	Day	53% 7% 18% 20% 2	1796
	21st to end	Day	53% 6% 18% 19% 4	1634
November	1st to 10th	Day	61% 6% 13% 13% 6%	1488
	11th to 20th	Day	64% 6% 14% 11% 4	1517
	21st to end	Day	68% 6% 12% 12% 2	1195
December	1st to 10th	Day	63% 6% 14% 14% 4	1140
	11th to 20th	Day	69% 6% 11% 9% 4	1444
	21st to end	Day	57% 7% 16% 14% 6%	2090
January	1st to 10th	Night	62% 8% 15% 14% 2	4888
	11th to 20th	Night	57% 6% 11% 7% 12% 7%	4740
	21st to end	Night	71% 8% 9% 8% 3	5214
February	1st to 10th	Night	62% 5% 12% 11% 9% 1	2645
	11th to 20th	Night	57% 4 18% 16% 6%	2404
	21st to end	Night	51% 4 16% 18% 9% 2	1981
March	1st to 10th	Night	53% 6% 14% 16% 10% 1	2178
	11th to 20th	Night	52% 5% 13% 21% 8% 1	1980
	21st to end	Night	53% 7% 14% 19% 6% 1	2100
April	1st to 10th	Night	52% 6% 12% 21% 9% 1	1800
	11th to 20th	Night	53% 6% 11% 23% 7%	1601
	21st to end	Night	49% 6% 12% 21% 12%	1457
May	1st to 10th	Night	52% 6% 16% 19% 7%	1440
	11th to 20th	Night	48% 8% 12% 21% 11% 1	1298
	21st to end	Night	47% 8% 14% 23% 8%	1230
June	1st to 10th	Night	50% 7% 13% 19% 10% 1	1080
	11th to 20th	Night	53% 6% 14% 21% 6%	1080
	21st to end	Night	46% 6% 19% 24% 5%	1080
July	1st to 10th	Night	44% 8% 16% 22% 10% 1	1050
	11th to 20th	Night	42% 6% 20% 26% 6%	1203
	21st to end	Night	33% 8% 23% 26% 9%	1302
August	1st to 10th	Night	31% 5% 23% 30% 10% 1	1353
	11th to 20th	Night	30% 7% 21% 34% 8% 1	1465
	21st to end	Night	48% 7% 20% 20% 4 1	1782
September	1st to 10th	Night	42% 6% 20% 25% 6% 1	1790
	11th to 20th	Night	38% 7% 16% 31% 9%	1800
	21st to end	Night	49% 7% 16% 19% 9%	2062
October	1st to 10th	Night	49% 7% 19% 19% 5%	2160
	11th to 20th	Night	53% 6% 19% 18% 4	2163
	21st to end	Night	54% 7% 16% 19% 5%	2719
November	1st to 10th	Night	59% 7% 14% 14% 5%	2590
	11th to 20th	Night	60% 8% 14% 13% 6%	2660
	21st to end	Night	62% 6% 15% 13% 4	2965
December	1st to 10th	Night	59% 7% 13% 15% 5%	3040
	11th to 20th	Night	67% 8% 12% 10% 4	3747
	21st to end	Night	57% 8% 16% 13% 6%	5566

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	14% 7% 21% 35% 17% 6%	180
	11th to 20th	Day	17% 12% 20% 29% 16% 6%	180
	21st to end	Day	21% 11% 26% 21% 17% 4	198
February	1st to 10th	Day	22% 18% 25% 20% 11% 3	232
	11th to 20th	Day	17% 11% 28% 26% 15% 3	365
	21st to end	Day	12% 17% 30% 23% 13% 5%	293
March	1st to 10th	Day	12% 14% 28% 25% 16% 5%	404
	11th to 20th	Day	12% 16% 27% 25% 16% 3	420
	21st to end	Day	17% 17% 26% 18% 17% 5%	462
April	1st to 10th	Day	11% 17% 29% 26% 14% 3	420
	11th to 20th	Day	15% 15% 27% 22% 14% 7%	412
	21st to end	Day	14% 16% 27% 26% 12% 4	420
May	1st to 10th	Day	15% 13% 27% 27% 14% 3	423
	11th to 20th	Day	12% 8% 22% 32% 20% 6%	563
	21st to end	Day	12% 7% 23% 27% 25% 6%	805
June	1st to 10th	Day	15% 9% 23% 30% 20% 3	810
	11th to 20th	Day	15% 7% 24% 30% 19% 5%	810
	21st to end	Day	12% 5% 24% 28% 24% 6%	810
July	1st to 10th	Day	13% 6% 18% 29% 26% 7%	796
	11th to 20th	Day	14% 6% 19% 26% 27% 7%	631
	21st to end	Day	8% 6% 18% 32% 29% 7%	684
August	1st to 10th	Day	8% 5% 16% 33% 28% 10%	519
	11th to 20th	Day	8% 7% 25% 27% 23% 10%	437
	21st to end	Day	13% 8% 24% 30% 17% 8%	495
September	1st to 10th	Day	14% 8% 26% 26% 20% 5%	450
	11th to 20th	Day	12% 8% 23% 26% 23% 8%	450
	21st to end	Day	17% 10% 32% 23% 14% 3	450
October	1st to 10th	Day	20% 9% 23% 25% 16% 6%	450
	11th to 20th	Day	19% 10% 30% 22% 18% 2	446
	21st to end	Day	19% 13% 26% 26% 13% 4	223
November	1st to 10th	Day	31% 12% 22% 23% 8% 6%	180
	11th to 20th	Day	19% 9% 23% 23% 15% 10%	180
	21st to end	Day	21% 10% 26% 27% 12% 4	180
December	1st to 10th	Day	16% 5% 18% 31% 26% 5%	180
	11th to 20th	Day	13% 9% 18% 32% 23% 4	180
	21st to end	Day	21% 5% 14% 37% 15% 8%	198
January	1st to 10th	Night	23% 9% 22% 33% 11% 2	947
	11th to 20th	Night	22% 7% 26% 28% 16% 1	850
	21st to end	Night	21% 10% 25% 29% 13% 2	935
February	1st to 10th	Night	22% 11% 20% 30% 15% 2	778
	11th to 20th	Night	18% 6% 25% 34% 15% 1	744
	21st to end	Night	16% 10% 22% 33% 17% 2	610
March	1st to 10th	Night	16% 10% 23% 32% 16% 3	720
	11th to 20th	Night	13% 10% 24% 35% 16% 2	720
	21st to end	Night	13% 9% 18% 30% 25% 5%	792
April	1st to 10th	Night	17% 8% 16% 34% 22% 3	720
	11th to 20th	Night	15% 6% 21% 34% 19% 5%	533
	21st to end	Night	16% 5% 27% 32% 20% 1	377
May	1st to 10th	Night	17% 6% 23% 35% 16% 3	360
	11th to 20th	Night	12% 4 17% 40% 24% 3	354
	21st to end	Night	16% 6% 15% 33% 27% 2	389
June	1st to 10th	Night	16% 6% 21% 33% 21% 3	360
	11th to 20th	Night	17% 4 21% 30% 25% 3	360
	21st to end	Night	11% 3 18% 37% 29% 2	360
July	1st to 10th	Night	12% 4 15% 37% 32% 1	350
	11th to 20th	Night	12% 2 15% 37% 30% 3	348
	21st to end	Night	12% 3 13% 36% 31% 6%	394
August	1st to 10th	Night	8% 2 11% 34% 38% 8%	358
	11th to 20th	Night	9% 1 15% 37% 34% 5%	397
	21st to end	Night	17% 3 16% 34% 26% 4	594
September	1st to 10th	Night	15% 4 17% 34% 28% 3	710
	11th to 20th	Night	13% 4 14% 31% 31% 7%	720
	21st to end	Night	20% 7% 20% 32% 18% 2	720
October	1st to 10th	Night	19% 5% 19% 32% 20% 4	720
	11th to 20th	Night	22% 5% 23% 29% 19% 2	720
	21st to end	Night	22% 10% 25% 30% 13% 1	792
November	1st to 10th	Night	24% 10% 24% 29% 11% 2	740
	11th to 20th	Night	21% 11% 26% 30% 11% 1	760
	21st to end	Night	20% 10% 29% 29% 10% 2	995
December	1st to 10th	Night	18% 6% 21% 36% 18% 2	1040
	11th to 20th	Night	21% 8% 24% 32% 13% 2	1040
	21st to end	Night	18% 7% 25% 33% 15% 2	1144

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