Thorn Garden Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thorn Garden Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	7% 8% 3 9% 6% 6% 6% 5% 7% 8% 36%	131201
	Night	7% 8% 3 9% 6% 6% 6% 5% 7% 8% 36%	128217
Transparency	Day	27% 41% 12% 13% 6% 1	130910
	Night	30% 39% 10% 13% 6% 1	128085
Seeing	Day	12% 11% 24% 29% 20% 5%	16758
	Night	14% 11% 23% 29% 19% 4	24974

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	9% 17% 6% 15% 9% 9% 7% 5% 7% 6% 10%	3179
	11th to 20th	Day	6% 9% 4 10% 8% 7% 9% 6% 10% 11% 21%	3224
	21st to end	Day	6% 10% 5% 13% 9% 8% 8% 6% 6% 9% 21%	4211
February	1st to 10th	Day	10% 12% 4 11% 8% 7% 7% 6% 10% 9% 16%	1800
	11th to 20th	Day	10% 13% 4 11% 8% 9% 7% 6% 6% 6% 19%	1755
	21st to end	Day	7% 15% 5% 11% 10% 9% 9% 6% 7% 6% 16%	1495
March	1st to 10th	Day	5% 10% 4 11% 7% 8% 8% 7% 8% 10% 21%	4752
	11th to 20th	Day	5% 11% 4 12% 8% 7% 7% 5% 7% 8% 26%	5000
	21st to end	Day	10% 13% 4 13% 10% 9% 8% 5% 6% 6% 16%	5570
April	1st to 10th	Day	8% 13% 4 12% 8% 8% 7% 6% 8% 9% 19%	2400
	11th to 20th	Day	8% 12% 4 11% 7% 6% 6% 5% 6% 8% 27%	2471
	21st to end	Day	9% 10% 4 10% 7% 7% 6% 5% 8% 7% 26%	2605
May	1st to 10th	Day	7% 6% 2 7% 5% 5% 6% 6% 7% 7% 42%	6320
	11th to 20th	Day	7% 6% 3 8% 6% 6% 6% 5% 7% 10% 37%	6277
	21st to end	Day	11% 8% 3 8% 6% 6% 5% 5% 7% 8% 31%	6920
June	1st to 10th	Day	9% 9% 3 8% 6% 6% 6% 5% 7% 7% 34%	3000
	11th to 20th	Day	7% 7% 3 10% 7% 8% 6% 6% 7% 9% 30%	3000
	21st to end	Day	7% 6% 3 8% 6% 5% 6% 4 8% 7% 41%	3000
July	1st to 10th	Day	4 3 1 3 3 2 3 3 4 7% 67%	6780
	11th to 20th	Day	2 3 1 3 3 2 3 2 3 5% 73%	6780
	21st to end	Day	4 3 1 4 3 3 3 3 4 6% 66%	6902
August	1st to 10th	Day	2 2 1 3 3 3 3 3 5% 7% 68%	4387
	11th to 20th	Day	5% 6% 3 10% 5% 6% 5% 4 6% 8% 42%	4156
	21st to end	Day	8% 6% 3 6% 4 4 5% 4 6% 7% 49%	4554
September	1st to 10th	Day	5% 4 2 6% 3 4 4 4 5% 9% 54%	2400
	11th to 20th	Day	6% 6% 3 9% 5% 5% 4 4 6% 7% 44%	2369
	21st to end	Day	7% 6% 2 7% 5% 5% 6% 4 7% 9% 42%	2124
October	1st to 10th	Day	8% 6% 2 8% 6% 6% 6% 6% 10% 9% 32%	3280
	11th to 20th	Day	10% 9% 4 9% 7% 7% 6% 4 7% 9% 29%	3261
	21st to end	Day	7% 8% 3 10% 7% 6% 7% 5% 9% 10% 27%	3375
November	1st to 10th	Day	10% 12% 4 13% 8% 9% 8% 6% 7% 7% 16%	1648
	11th to 20th	Day	9% 11% 4 12% 7% 7% 8% 5% 8% 8% 21%	1680
	21st to end	Day	10% 12% 5% 10% 6% 6% 9% 6% 10% 7% 20%	1680
December	1st to 10th	Day	6% 8% 3 10% 8% 8% 8% 6% 9% 12% 20%	2636
	11th to 20th	Day	8% 13% 5% 13% 9% 8% 8% 6% 8% 7% 16%	2712
	21st to end	Day	9% 15% 6% 12% 8% 8% 7% 7% 7% 7% 15%	3498
January	1st to 10th	Night	9% 13% 5% 15% 9% 9% 8% 6% 8% 6% 13%	6766
	11th to 20th	Night	6% 8% 5% 10% 7% 6% 7% 6% 8% 10% 29%	6671
	21st to end	Night	8% 10% 5% 13% 8% 7% 7% 5% 7% 6% 25%	6751
February	1st to 10th	Night	10% 12% 5% 11% 8% 7% 6% 5% 8% 7% 21%	2600
	11th to 20th	Night	9% 11% 4 11% 7% 7% 6% 5% 8% 7% 25%	2532
	21st to end	Night	9% 14% 4 11% 7% 7% 8% 6% 6% 7% 19%	2134
March	1st to 10th	Night	4 9% 3 8% 6% 8% 8% 6% 10% 13% 26%	5139
	11th to 20th	Night	3 7% 3 8% 6% 7% 7% 6% 8% 9% 36%	4999
	21st to end	Night	8% 11% 5% 13% 8% 8% 8% 6% 8% 7% 18%	5404
April	1st to 10th	Night	7% 9% 4 11% 8% 6% 7% 6% 8% 8% 26%	2000
	11th to 20th	Night	7% 9% 3 7% 6% 6% 6% 5% 7% 9% 34%	1876
	21st to end	Night	8% 8% 3 9% 8% 6% 5% 5% 6% 8% 34%	1795
May	1st to 10th	Night	6% 5% 2 6% 4 5% 6% 4 5% 5% 52%	3640
	11th to 20th	Night	6% 6% 2 6% 5% 5% 5% 4 6% 8% 48%	3616
	21st to end	Night	5% 5% 3 6% 6% 6% 6% 6% 9% 10% 38%	3938
June	1st to 10th	Night	9% 6% 3 8% 7% 6% 4 4 5% 7% 43%	1400
	11th to 20th	Night	5% 7% 2 6% 5% 5% 4 5% 6% 10% 46%	1400
	21st to end	Night	5% 4 2 5% 5% 3 3 4 7% 10% 52%	1400
July	1st to 10th	Night	2 3 1 3 2 2 2 2 3 4 75%	3180
	11th to 20th	Night	1 2 1 3 2 2 2 1 3 5% 79%	3180
	21st to end	Night	4 2 1 2 2 2 3 2 4 6% 73%	3937
August	1st to 10th	Night	1 1 3 2 2 2 2 4 6% 76%	2765
	11th to 20th	Night	6% 5% 2 6% 4 5% 4 4 5% 8% 51%	2902
	21st to end	Night	8% 3 2 4 3 4 3 3 6% 7% 57%	3358
September	1st to 10th	Night	4 3 1 4 3 3 4 3 5% 8% 61%	2000
	11th to 20th	Night	8% 5% 2 6% 5% 5% 4 4 5% 7% 50%	2030
	21st to end	Night	7% 4 2 8% 5% 5% 5% 4 6% 7% 47%	2275
October	1st to 10th	Night	6% 5% 2 7% 4 4 6% 5% 8% 8% 45%	3920
	11th to 20th	Night	8% 8% 3 9% 7% 6% 6% 4 6% 8% 34%	3897
	21st to end	Night	9% 7% 3 9% 7% 6% 6% 5% 8% 8% 33%	4427
November	1st to 10th	Night	10% 10% 4 11% 7% 8% 8% 6% 8% 7% 22%	2870
	11th to 20th	Night	8% 10% 4 11% 8% 7% 7% 6% 9% 8% 22%	2940
	21st to end	Night	8% 9% 4 11% 8% 8% 8% 6% 8% 7% 22%	2940
December	1st to 10th	Night	5% 8% 4 9% 7% 7% 9% 5% 11% 10% 25%	4588
	11th to 20th	Night	8% 13% 5% 12% 8% 9% 8% 6% 8% 7% 16%	5489
	21st to end	Night	7% 13% 4 12% 8% 8% 8% 6% 7% 7% 19%	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	55% 27% 8% 7% 2	3179
	11th to 20th	Day	42% 31% 10% 14% 3	3245
	21st to end	Day	47% 30% 11% 9% 2	4211
February	1st to 10th	Day	26% 63% 4 3 2 1	1800
	11th to 20th	Day	27% 63% 3 5% 2	1764
	21st to end	Day	22% 64% 6% 5% 2	1495
March	1st to 10th	Day	39% 34% 8% 13% 5% 1	4752
	11th to 20th	Day	37% 31% 8% 18% 6%	5000
	21st to end	Day	49% 32% 9% 8% 2	5570
April	1st to 10th	Day	21% 65% 7% 7% 1	2400
	11th to 20th	Day	17% 66% 7% 8% 3	2471
	21st to end	Day	16% 66% 8% 8% 2	2605
May	1st to 10th	Day	25% 34% 12% 19% 10%	6320
	11th to 20th	Day	27% 34% 21% 15% 3	6280
	21st to end	Day	33% 33% 17% 13% 3	6921
June	1st to 10th	Day	13% 65% 10% 8% 3	3000
	11th to 20th	Day	12% 65% 10% 10% 3	3000
	21st to end	Day	14% 64% 11% 9% 2	3000
July	1st to 10th	Day	10% 28% 19% 18% 18% 8%	6706
	11th to 20th	Day	8% 27% 15% 25% 24% 2	6699
	21st to end	Day	13% 29% 29% 21% 8%	6708
August	1st to 10th	Day	8% 37% 9% 31% 15% 1	4370
	11th to 20th	Day	23% 42% 20% 10% 4 1	4195
	21st to end	Day	25% 39% 10% 18% 8%	4553
September	1st to 10th	Day	16% 61% 10% 9% 5%	2400
	11th to 20th	Day	18% 59% 7% 8% 6% 1	2369
	21st to end	Day	17% 59% 10% 9% 5%	2124
October	1st to 10th	Day	31% 41% 12% 14% 3	3280
	11th to 20th	Day	38% 39% 6% 12% 5%	3261
	21st to end	Day	32% 40% 11% 10% 6% 1	3375
November	1st to 10th	Day	31% 56% 7% 4 2	1648
	11th to 20th	Day	26% 56% 6% 8% 3 1	1680
	21st to end	Day	26% 57% 5% 6% 6%	1680
December	1st to 10th	Day	45% 38% 5% 9% 2	2636
	11th to 20th	Day	45% 35% 9% 9% 2	2715
	21st to end	Day	49% 29% 11% 9% 2	3498
January	1st to 10th	Night	58% 28% 5% 7% 2	6766
	11th to 20th	Night	35% 30% 11% 17% 7%	6712
	21st to end	Night	44% 31% 12% 10% 4	6751
February	1st to 10th	Night	26% 62% 4 3 4	2600
	11th to 20th	Night	26% 62% 5% 5% 2	2534
	21st to end	Night	23% 63% 5% 5% 3	2134
March	1st to 10th	Night	33% 33% 9% 17% 7%	5139
	11th to 20th	Night	30% 31% 10% 19% 10%	5000
	21st to end	Night	49% 31% 9% 10% 2	5404
April	1st to 10th	Night	20% 64% 7% 8% 1	2000
	11th to 20th	Night	19% 63% 5% 9% 4	1876
	21st to end	Night	17% 63% 8% 7% 5%	1795
May	1st to 10th	Night	24% 30% 6% 24% 15%	3637
	11th to 20th	Night	24% 31% 18% 20% 7% 1	3616
	21st to end	Night	28% 34% 18% 16% 3	3938
June	1st to 10th	Night	14% 62% 9% 11% 3 1	1400
	11th to 20th	Night	11% 64% 8% 14% 4	1400
	21st to end	Night	12% 62% 12% 10% 3 1	1400
July	1st to 10th	Night	11% 26% 13% 25% 16% 9%	3145
	11th to 20th	Night	6% 26% 14% 26% 26% 2	3138
	21st to end	Night	14% 27% 20% 25% 13%	3826
August	1st to 10th	Night	6% 36% 10% 28% 20% 1	2757
	11th to 20th	Night	23% 42% 20% 10% 4 2	2929
	21st to end	Night	22% 39% 8% 22% 7% 1	3358
September	1st to 10th	Night	15% 59% 9% 9% 7% 1	2000
	11th to 20th	Night	18% 59% 6% 10% 7% 1	2030
	21st to end	Night	17% 58% 6% 12% 6%	2275
October	1st to 10th	Night	22% 39% 17% 19% 3	3920
	11th to 20th	Night	35% 37% 11% 15% 3	3897
	21st to end	Night	32% 39% 9% 12% 7% 1	4427
November	1st to 10th	Night	29% 55% 7% 7% 2	2870
	11th to 20th	Night	28% 56% 6% 6% 3 2	2940
	21st to end	Night	27% 57% 5% 5% 6%	2940
December	1st to 10th	Night	41% 39% 6% 11% 2	4588
	11th to 20th	Night	48% 33% 8% 8% 3	5485
	21st to end	Night	46% 29% 12% 11% 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	24% 22% 21% 22% 10% 2	200
	11th to 20th	Day	19% 15% 30% 21% 16% 1	200
	21st to end	Day	19% 15% 27% 21% 15% 3	310
February	1st to 10th	Day	19% 18% 31% 23% 8% 1	400
	11th to 20th	Day	21% 17% 26% 25% 11% 1	395
	21st to end	Day	15% 25% 25% 24% 10% 1	337
March	1st to 10th	Day	14% 16% 28% 27% 14% 1	500
	11th to 20th	Day	12% 18% 25% 27% 16% 2	500
	21st to end	Day	15% 16% 27% 31% 11% 1	550
April	1st to 10th	Day	14% 17% 27% 26% 14% 2	500
	11th to 20th	Day	12% 16% 26% 27% 16% 3	492
	21st to end	Day	15% 14% 26% 27% 13% 4	500
May	1st to 10th	Day	10% 11% 27% 31% 19% 3	503
	11th to 20th	Day	12% 9% 21% 34% 20% 4	528
	21st to end	Day	12% 8% 27% 30% 19% 5%	577
June	1st to 10th	Day	12% 9% 29% 30% 17% 4	730
	11th to 20th	Day	8% 11% 34% 30% 14% 3	730
	21st to end	Day	7% 6% 26% 31% 23% 7%	730
July	1st to 10th	Day	7% 4 20% 37% 28% 4	721
	11th to 20th	Day	3 6% 23% 33% 28% 7%	718
	21st to end	Day	2 3 18% 32% 32% 13%	590
August	1st to 10th	Day	5% 4 13% 29% 36% 13%	527
	11th to 20th	Day	3 2 13% 30% 40% 12%	517
	21st to end	Day	7% 5% 20% 32% 26% 10%	583
September	1st to 10th	Day	8% 3 19% 35% 25% 10%	530
	11th to 20th	Day	10% 6% 16% 33% 29% 5%	530
	21st to end	Day	13% 7% 22% 31% 25% 3	530
October	1st to 10th	Day	15% 8% 20% 27% 24% 6%	530
	11th to 20th	Day	17% 15% 20% 25% 19% 4	530
	21st to end	Day	17% 17% 26% 25% 12% 3	486
November	1st to 10th	Day	24% 15% 25% 25% 11%	220
	11th to 20th	Day	23% 12% 21% 22% 20% 2	220
	21st to end	Day	19% 14% 22% 28% 13% 4	220
December	1st to 10th	Day	24% 11% 23% 22% 19% 2	204
	11th to 20th	Day	20% 21% 25% 23% 12% 1	200
	21st to end	Day	24% 15% 30% 25% 4 1	220
January	1st to 10th	Night	22% 20% 26% 20% 10% 2	900
	11th to 20th	Night	18% 17% 21% 26% 17% 1	885
	21st to end	Night	18% 13% 24% 25% 17% 4	880
February	1st to 10th	Night	19% 16% 27% 26% 10% 2	800
	11th to 20th	Night	18% 14% 23% 29% 16% 1	784
	21st to end	Night	19% 18% 24% 25% 11% 3	660
March	1st to 10th	Night	13% 15% 26% 29% 16% 3	800
	11th to 20th	Night	13% 15% 28% 26% 15% 3	800
	21st to end	Night	14% 13% 26% 31% 14% 3	880
April	1st to 10th	Night	15% 12% 26% 29% 16% 2	800
	11th to 20th	Night	10% 11% 29% 31% 16% 2	688
	21st to end	Night	10% 10% 31% 32% 12% 5%	595
May	1st to 10th	Night	7% 8% 26% 34% 22% 3	400
	11th to 20th	Night	8% 7% 20% 37% 24% 4	394
	21st to end	Night	9% 6% 22% 40% 22% 2	433
June	1st to 10th	Night	11% 6% 23% 37% 23% 1	400
	11th to 20th	Night	8% 5% 29% 39% 17% 3	400
	21st to end	Night	7% 3 26% 32% 28% 5%	400
July	1st to 10th	Night	6% 2 17% 39% 33% 4	390
	11th to 20th	Night	3 3 18% 43% 29% 4	388
	21st to end	Night	3 1 20% 37% 33% 7%	438
August	1st to 10th	Night	3 4 12% 30% 41% 9%	489
	11th to 20th	Night	4 1 8% 35% 39% 12%	594
	21st to end	Night	6% 3 19% 31% 33% 7%	721
September	1st to 10th	Night	6% 3 14% 35% 31% 11%	800
	11th to 20th	Night	10% 3 14% 31% 28% 13%	800
	21st to end	Night	12% 5% 18% 31% 27% 7%	800
October	1st to 10th	Night	12% 7% 15% 30% 28% 8%	800
	11th to 20th	Night	14% 11% 19% 27% 23% 7%	800
	21st to end	Night	19% 12% 21% 29% 16% 4	880
November	1st to 10th	Night	21% 14% 24% 28% 12% 1	820
	11th to 20th	Night	18% 19% 25% 23% 13% 2	840
	21st to end	Night	18% 16% 25% 24% 12% 4	840
December	1st to 10th	Night	19% 13% 25% 24% 17% 3	808
	11th to 20th	Night	19% 18% 28% 26% 9% 1	877
	21st to end	Night	21% 18% 30% 24% 6% 1	990

Page generated at 2025-11-29 22:00:57. Version ac07

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