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 2024-11-10.

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% 7% 36%	105564
	Night	7% 8% 3 9% 6% 6% 6% 5% 7% 7% 35%	102190
Transparency	Day	23% 49% 9% 12% 6% 1	105315
	Night	26% 48% 8% 12% 6% 1	102085
Seeing	Day	12% 11% 25% 29% 19% 4	15413
	Night	14% 11% 23% 29% 19% 4	23619

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% 16% 5% 13% 8% 8% 7% 5% 8% 7% 12%	2219
	11th to 20th	Day	9% 13% 4 10% 9% 8% 9% 6% 7% 8% 16%	2236
	21st to end	Day	9% 12% 6% 15% 11% 8% 9% 6% 6% 6% 11%	2931
February	1st to 10th	Day	10% 12% 4 11% 9% 7% 7% 5% 10% 9% 17%	1710
	11th to 20th	Day	10% 13% 4 12% 8% 9% 8% 6% 6% 6% 18%	1665
	21st to end	Day	8% 15% 5% 11% 10% 9% 9% 6% 7% 7% 17%	1422
March	1st to 10th	Day	3 10% 4 10% 8% 8% 9% 8% 8% 10% 21%	3307
	11th to 20th	Day	5% 9% 3 10% 7% 6% 6% 5% 7% 8% 34%	3490
	21st to end	Day	11% 12% 4 12% 9% 8% 7% 4 6% 6% 20%	3883
April	1st to 10th	Day	8% 13% 4 11% 8% 9% 7% 6% 8% 8% 19%	2280
	11th to 20th	Day	8% 13% 4 12% 8% 6% 6% 5% 6% 7% 25%	2346
	21st to end	Day	9% 10% 4 11% 8% 8% 6% 5% 8% 7% 24%	2475
May	1st to 10th	Day	9% 8% 3 7% 6% 6% 5% 4 6% 6% 39%	4420
	11th to 20th	Day	5% 5% 2 7% 5% 5% 5% 5% 7% 10% 45%	4377
	21st to end	Day	15% 10% 3 9% 6% 6% 5% 5% 7% 7% 26%	4830
June	1st to 10th	Day	10% 9% 4 8% 7% 6% 6% 5% 7% 7% 31%	2850
	11th to 20th	Day	7% 7% 3 10% 7% 8% 6% 6% 7% 9% 30%	2850
	21st to end	Day	6% 5% 3 8% 5% 5% 6% 4 8% 7% 42%	2850
July	1st to 10th	Day	3 3 1 4 3 3 3 3 3 5% 68%	4740
	11th to 20th	Day	3 3 1 4 3 2 3 3 3 5% 69%	4740
	21st to end	Day	6% 4 1 5% 3 3 3 3 4 6% 62%	4804
August	1st to 10th	Day	2 2 1 3 2 3 3 3 5% 7% 69%	4252
	11th to 20th	Day	5% 6% 3 10% 5% 6% 5% 4 6% 8% 42%	4026
	21st to end	Day	8% 6% 3 6% 4 4 4 3 6% 7% 49%	4414
September	1st to 10th	Day	5% 4 2 6% 3 4 4 3 5% 8% 54%	2280
	11th to 20th	Day	6% 7% 3 10% 6% 5% 5% 4 6% 7% 43%	2251
	21st to end	Day	7% 6% 2 8% 5% 5% 6% 4 7% 9% 41%	2018
October	1st to 10th	Day	8% 6% 2 8% 5% 6% 6% 6% 10% 9% 33%	3180
	11th to 20th	Day	10% 9% 4 9% 7% 7% 6% 4 7% 9% 29%	3161
	21st to end	Day	7% 8% 3 10% 7% 6% 7% 5% 9% 10% 28%	3271
November	1st to 10th	Day	10% 13% 4 13% 8% 9% 8% 6% 7% 6% 16%	1560
	11th to 20th	Day	9% 11% 3 13% 8% 7% 8% 6% 7% 8% 22%	1520
	21st to end	Day	10% 12% 5% 10% 6% 6% 9% 6% 10% 7% 20%	1520
December	1st to 10th	Day	7% 13% 4 10% 8% 7% 8% 6% 6% 8% 24%	1520
	11th to 20th	Day	6% 14% 5% 13% 9% 7% 8% 6% 8% 8% 16%	1724
	21st to end	Day	10% 14% 5% 13% 9% 8% 7% 7% 7% 7% 14%	2442
January	1st to 10th	Night	9% 13% 5% 13% 9% 8% 8% 6% 8% 6% 16%	4726
	11th to 20th	Night	8% 11% 6% 11% 8% 7% 8% 6% 7% 8% 20%	4659
	21st to end	Night	11% 12% 5% 14% 9% 8% 8% 5% 7% 7% 13%	4726
February	1st to 10th	Night	11% 12% 5% 10% 8% 7% 6% 5% 8% 7% 21%	2470
	11th to 20th	Night	9% 10% 5% 11% 7% 7% 6% 5% 8% 7% 25%	2402
	21st to end	Night	9% 15% 5% 11% 7% 7% 8% 6% 6% 6% 19%	2031
March	1st to 10th	Night	4 9% 3 8% 6% 8% 8% 7% 11% 12% 23%	3564
	11th to 20th	Night	4 6% 3 7% 6% 7% 6% 6% 7% 8% 40%	3489
	21st to end	Night	8% 11% 4 12% 8% 9% 7% 6% 7% 6% 21%	3769
April	1st to 10th	Night	8% 9% 4 11% 7% 6% 7% 6% 8% 8% 26%	1900
	11th to 20th	Night	7% 10% 3 7% 7% 6% 6% 5% 7% 8% 33%	1781
	21st to end	Night	8% 9% 3 9% 9% 6% 6% 5% 6% 8% 32%	1705
May	1st to 10th	Night	7% 6% 3 7% 4 5% 5% 3 5% 6% 50%	2540
	11th to 20th	Night	4 4 2 5% 4 4 4 3 4 8% 57%	2516
	21st to end	Night	7% 6% 3 7% 6% 6% 6% 5% 8% 8% 37%	2742
June	1st to 10th	Night	9% 6% 3 8% 7% 5% 5% 4 5% 7% 42%	1330
	11th to 20th	Night	5% 7% 2 6% 5% 5% 4 6% 6% 10% 45%	1330
	21st to end	Night	5% 4 2 4 5% 3 3 4 6% 10% 53%	1330
July	1st to 10th	Night	2 3 1 3 2 2 2 2 3 4 76%	2220
	11th to 20th	Night	1 2 1 5% 3 2 2 2 3 5% 74%	2220
	21st to end	Night	6% 3 1 3 2 2 3 2 4 5% 69%	2744
August	1st to 10th	Night	1 2 3 2 2 2 2 4 6% 76%	2680
	11th to 20th	Night	6% 5% 2 6% 4 5% 4 4 5% 8% 52%	2812
	21st to end	Night	8% 3 2 4 3 4 3 3 6% 7% 57%	3256
September	1st to 10th	Night	4 3 1 4 3 3 4 3 5% 7% 62%	1900
	11th to 20th	Night	8% 5% 2 6% 5% 5% 4 4 5% 7% 50%	1928
	21st to end	Night	7% 4 2 8% 5% 5% 5% 4 6% 8% 47%	2161
October	1st to 10th	Night	6% 5% 2 7% 4 4 6% 5% 8% 8% 44%	3800
	11th to 20th	Night	8% 8% 3 10% 7% 6% 6% 5% 6% 8% 33%	3777
	21st to end	Night	9% 7% 3 9% 7% 6% 6% 5% 8% 8% 34%	4289
November	1st to 10th	Night	10% 10% 4 11% 7% 8% 7% 6% 7% 7% 22%	2722
	11th to 20th	Night	8% 10% 4 10% 8% 7% 7% 6% 8% 8% 24%	2660
	21st to end	Night	8% 10% 4 12% 7% 8% 7% 6% 8% 7% 23%	2660
December	1st to 10th	Night	6% 12% 5% 10% 8% 7% 8% 4 7% 7% 24%	2660
	11th to 20th	Night	7% 12% 5% 12% 8% 9% 8% 7% 8% 7% 17%	3477
	21st to end	Night	8% 13% 4 13% 9% 7% 8% 6% 7% 7% 19%	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	44% 37% 8% 7% 3	2219
	11th to 20th	Day	39% 40% 6% 12% 4	2257
	21st to end	Day	47% 40% 9% 3 1	2931
February	1st to 10th	Day	24% 67% 4 3 2 1	1710
	11th to 20th	Day	26% 65% 3 4 1	1674
	21st to end	Day	19% 67% 6% 5% 2	1422
March	1st to 10th	Day	31% 45% 6% 11% 6% 1	3307
	11th to 20th	Day	23% 41% 9% 18% 7%	3490
	21st to end	Day	36% 42% 10% 9% 2	3883
April	1st to 10th	Day	19% 68% 6% 6% 1	2280
	11th to 20th	Day	17% 69% 5% 6% 2 1	2346
	21st to end	Day	16% 69% 6% 6% 2	2475
May	1st to 10th	Day	24% 44% 8% 14% 9% 1	4420
	11th to 20th	Day	16% 45% 20% 14% 5%	4380
	21st to end	Day	36% 42% 11% 9% 2	4831
June	1st to 10th	Day	14% 68% 9% 6% 3	2850
	11th to 20th	Day	11% 67% 9% 9% 3	2850
	21st to end	Day	12% 67% 10% 9% 2	2850
July	1st to 10th	Day	7% 37% 4 18% 22% 12%	4666
	11th to 20th	Day	7% 38% 7% 21% 26% 1	4659
	21st to end	Day	15% 39% 18% 18% 10%	4652
August	1st to 10th	Day	7% 38% 8% 31% 16% 1	4235
	11th to 20th	Day	22% 43% 20% 10% 4 1	4065
	21st to end	Day	25% 40% 10% 18% 7%	4413
September	1st to 10th	Day	15% 63% 8% 9% 5%	2280
	11th to 20th	Day	18% 62% 6% 7% 6% 1	2251
	21st to end	Day	16% 61% 9% 8% 5%	2018
October	1st to 10th	Day	30% 41% 11% 14% 3	3180
	11th to 20th	Day	38% 40% 6% 12% 4	3161
	21st to end	Day	32% 41% 11% 10% 6% 1	3271
November	1st to 10th	Day	30% 58% 6% 4 2	1560
	11th to 20th	Day	22% 62% 4 7% 3 1	1520
	21st to end	Day	22% 62% 5% 5% 6%	1520
December	1st to 10th	Day	27% 61% 3 6% 2	1520
	11th to 20th	Day	32% 50% 9% 7% 2	1727
	21st to end	Day	41% 39% 11% 7% 2	2442
January	1st to 10th	Night	44% 39% 6% 8% 3	4726
	11th to 20th	Night	36% 38% 7% 12% 7%	4700
	21st to end	Night	47% 40% 8% 4 1	4726
February	1st to 10th	Night	23% 66% 4 3 4	2470
	11th to 20th	Night	24% 66% 5% 4 2	2404
	21st to end	Night	21% 66% 5% 5% 3	2031
March	1st to 10th	Night	27% 43% 8% 15% 5% 1	3564
	11th to 20th	Night	20% 41% 10% 18% 10% 1	3490
	21st to end	Night	39% 41% 9% 10% 2	3769
April	1st to 10th	Night	19% 67% 7% 6% 1	1900
	11th to 20th	Night	19% 67% 4 7% 3	1781
	21st to end	Night	17% 66% 6% 6% 5%	1705
May	1st to 10th	Night	21% 41% 5% 20% 13% 1	2540
	11th to 20th	Night	13% 40% 15% 21% 10% 1	2516
	21st to end	Night	26% 42% 15% 13% 4 1	2742
June	1st to 10th	Night	14% 65% 7% 11% 2 1	1330
	11th to 20th	Night	10% 67% 7% 12% 4	1330
	21st to end	Night	10% 65% 11% 10% 3 1	1330
July	1st to 10th	Night	8% 36% 4 21% 19% 13%	2185
	11th to 20th	Night	7% 36% 9% 18% 29% 1	2178
	21st to end	Night	16% 36% 11% 20% 17%	2657
August	1st to 10th	Night	5% 37% 9% 27% 20% 1	2672
	11th to 20th	Night	22% 43% 20% 10% 4 1	2839
	21st to end	Night	22% 40% 8% 22% 7% 1	3256
September	1st to 10th	Night	13% 62% 8% 9% 7% 1	1900
	11th to 20th	Night	18% 61% 5% 8% 6% 1	1928
	21st to end	Night	16% 61% 5% 12% 6%	2161
October	1st to 10th	Night	22% 40% 17% 18% 3	3800
	11th to 20th	Night	35% 38% 10% 14% 2	3777
	21st to end	Night	32% 40% 9% 12% 7% 1	4289
November	1st to 10th	Night	26% 58% 7% 7% 2	2722
	11th to 20th	Night	23% 61% 5% 6% 3 2	2660
	21st to end	Night	23% 62% 4 5% 6%	2660
December	1st to 10th	Night	27% 61% 4 5% 2	2660
	11th to 20th	Night	32% 50% 8% 8% 2	3473
	21st to end	Night	37% 38% 10% 11% 3	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	24% 22% 22% 20% 10% 2	180
	11th to 20th	Day	21% 16% 30% 18% 14% 1	180
	21st to end	Day	20% 16% 29% 20% 14% 1	283
February	1st to 10th	Day	20% 19% 32% 21% 8% 1	370
	11th to 20th	Day	21% 18% 27% 23% 10% 1	365
	21st to end	Day	15% 25% 26% 23% 10% 1	312
March	1st to 10th	Day	13% 17% 29% 28% 12% 1	460
	11th to 20th	Day	12% 18% 25% 26% 17% 2	460
	21st to end	Day	15% 16% 28% 30% 10% 1	506
April	1st to 10th	Day	15% 18% 28% 25% 12% 2	460
	11th to 20th	Day	13% 17% 27% 26% 14% 2	452
	21st to end	Day	16% 16% 28% 28% 11% 1	460
May	1st to 10th	Day	10% 11% 27% 32% 19% 1	463
	11th to 20th	Day	11% 9% 21% 35% 20% 3	488
	21st to end	Day	13% 8% 28% 30% 17% 5%	533
June	1st to 10th	Day	13% 9% 31% 30% 15% 3	680
	11th to 20th	Day	8% 11% 36% 30% 12% 3	680
	21st to end	Day	6% 6% 28% 33% 22% 5%	680
July	1st to 10th	Day	7% 4 20% 38% 27% 4	671
	11th to 20th	Day	3 7% 24% 33% 27% 6%	668
	21st to end	Day	2 3 20% 33% 31% 10%	545
August	1st to 10th	Day	5% 4 14% 29% 36% 12%	487
	11th to 20th	Day	3 2 13% 30% 40% 12%	477
	21st to end	Day	7% 6% 21% 34% 26% 7%	539
September	1st to 10th	Day	9% 4 19% 37% 24% 7%	490
	11th to 20th	Day	11% 7% 16% 35% 27% 4	490
	21st to end	Day	13% 7% 22% 32% 23% 3	490
October	1st to 10th	Day	15% 8% 22% 27% 23% 6%	490
	11th to 20th	Day	17% 15% 20% 26% 19% 4	490
	21st to end	Day	17% 17% 27% 25% 11% 3	448
November	1st to 10th	Day	26% 14% 26% 23% 11%	198
	11th to 20th	Day	21% 13% 23% 21% 19% 3	180
	21st to end	Day	19% 17% 22% 26% 13% 4	180
December	1st to 10th	Day	23% 11% 24% 22% 17% 2	180
	11th to 20th	Day	19% 20% 26% 23% 11% 1	180
	21st to end	Day	25% 15% 30% 25% 4 1	198
January	1st to 10th	Night	22% 21% 27% 18% 10% 2	850
	11th to 20th	Night	19% 17% 22% 25% 16% 1	837
	21st to end	Night	19% 13% 24% 24% 16% 3	836
February	1st to 10th	Night	20% 16% 27% 26% 10% 2	760
	11th to 20th	Night	17% 15% 23% 28% 15% 1	744
	21st to end	Night	18% 19% 25% 24% 11% 3	628
March	1st to 10th	Night	13% 15% 26% 28% 15% 3	760
	11th to 20th	Night	13% 15% 28% 25% 15% 3	760
	21st to end	Night	14% 13% 27% 31% 13% 3	836
April	1st to 10th	Night	16% 12% 26% 28% 16% 2	760
	11th to 20th	Night	11% 12% 29% 31% 15% 2	653
	21st to end	Night	10% 10% 32% 32% 11% 4	565
May	1st to 10th	Night	7% 8% 26% 34% 22% 3	380
	11th to 20th	Night	8% 7% 21% 36% 25% 4	374
	21st to end	Night	9% 6% 23% 40% 21% 2	411
June	1st to 10th	Night	12% 6% 24% 37% 21% 1	380
	11th to 20th	Night	8% 5% 30% 39% 16% 2	380
	21st to end	Night	6% 3 26% 33% 27% 4	380
July	1st to 10th	Night	6% 2 16% 38% 34% 4	370
	11th to 20th	Night	2 4 19% 42% 29% 4	368
	21st to end	Night	3 1 20% 37% 32% 7%	416
August	1st to 10th	Night	3 4 12% 30% 41% 9%	464
	11th to 20th	Night	4 1 7% 35% 40% 12%	564
	21st to end	Night	6% 4 20% 32% 32% 6%	685
September	1st to 10th	Night	6% 3 15% 36% 31% 10%	760
	11th to 20th	Night	11% 4 14% 31% 28% 13%	760
	21st to end	Night	11% 5% 19% 31% 26% 8%	760
October	1st to 10th	Night	12% 7% 16% 29% 29% 8%	760
	11th to 20th	Night	14% 11% 18% 26% 23% 8%	760
	21st to end	Night	19% 12% 21% 28% 16% 4	836
November	1st to 10th	Night	21% 13% 24% 28% 12% 1	778
	11th to 20th	Night	17% 19% 26% 22% 14% 3	760
	21st to end	Night	18% 18% 27% 22% 11% 4	760
December	1st to 10th	Night	19% 13% 26% 24% 15% 3	760
	11th to 20th	Night	19% 18% 28% 26% 8% 1	829
	21st to end	Night	21% 19% 31% 22% 6% 1	935

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