Boyce Thompson Arboretum
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boyce Thompson Arboretum clear sky chart from 2005-11-06 to 2025-07-21.

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	4 3 1 3 2 3 3 3 4 6% 68%	124565
	Night	4 3 1 3 3 3 3 3 4 6% 67%	123531
Transparency	Day	17% 4 5% 13% 27% 35%	124333
	Night	18% 3 5% 12% 26% 35%	123351
Seeing	Day	6% 7% 23% 34% 23% 6%	15834
	Night	6% 2 16% 41% 31% 4	25961

By Month

Percentage Cloud Cover SW (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	6% 5% 2 5% 2 3 3 2 3 4 65%	4098
	11th to 20th	Day	5% 3 1 3 3 2 3 2 2 3 73%	4100
	21st to end	Day	8% 5% 1 3 3 3 3 3 5% 7% 58%	4510
February	1st to 10th	Day	6% 6% 2 3 3 3 3 3 5% 5% 62%	1830
	11th to 20th	Day	9% 5% 2 5% 3 3 3 2 4 5% 58%	1950
	21st to end	Day	9% 5% 2 4 3 2 4 3 3 6% 59%	1719
March	1st to 10th	Day	7% 5% 1 3 2 3 4 3 4 4 63%	4954
	11th to 20th	Day	8% 6% 2 5% 3 3 3 2 4 5% 59%	5000
	21st to end	Day	6% 7% 2 4 3 2 3 2 3 4 63%	5486
April	1st to 10th	Day	3 3 1 3 3 2 3 2 4 7% 69%	2200
	11th to 20th	Day	3 2 1 3 2 2 2 2 3 4 76%	2327
	21st to end	Day	2 2 1 2 1 2 2 2 4 5% 77%	2484
May	1st to 10th	Day	2 1 1 2 2 2 3 2 3 4 79%	5880
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 4 81%	5841
	21st to end	Day	2 1 1 2 1 1 2 2 2 4 82%	6423
June	1st to 10th	Day	2 2 1 2 2 2 3 2 3 5% 77%	2600
	11th to 20th	Day	1 1 2 1 1 1 1 2 3 87%	2600
	21st to end	Day	2 2 1 3 2 3 3 3 4 5% 73%	2600
July	1st to 10th	Day	2 3 1 2 2 3 3 3 4 6% 71%	4240
	11th to 20th	Day	2 2 1 3 3 4 5% 5% 9% 11% 54%	4240
	21st to end	Day	3 3 1 4 4 5% 6% 5% 9% 13% 46%	4455
August	1st to 10th	Day	3 3 1 4 3 4 6% 6% 9% 11% 49%	4092
	11th to 20th	Day	2 2 1 3 3 3 4 5% 7% 10% 60%	4028
	21st to end	Day	3 2 1 3 3 4 4 4 6% 8% 63%	4146
September	1st to 10th	Day	4 3 1 3 3 4 3 3 6% 6% 63%	2090
	11th to 20th	Day	3 2 2 2 2 2 2 4 5% 76%	2090
	21st to end	Day	3 2 1 3 2 3 2 2 4 5% 74%	2090
October	1st to 10th	Day	2 1 1 1 2 2 1 3 4 81%	3362
	11th to 20th	Day	3 1 1 2 1 1 2 2 2 3 83%	3150
	21st to end	Day	1 1 2 1 2 2 2 4 5% 81%	3438
November	1st to 10th	Day	3 2 3 2 2 3 2 4 7% 72%	1959
	11th to 20th	Day	2 3 1 3 2 3 3 3 3 4 72%	1914
	21st to end	Day	6% 3 1 5% 3 3 3 3 4 6% 63%	1800
December	1st to 10th	Day	4 2 1 3 2 2 2 1 3 4 76%	2950
	11th to 20th	Day	5% 5% 2 5% 3 2 3 4 7% 8% 57%	3409
	21st to end	Day	8% 5% 1 4 3 3 2 2 4 5% 62%	4510
January	1st to 10th	Night	4 5% 2 5% 3 3 4 3 4 5% 62%	5867
	11th to 20th	Night	3 2 1 3 2 2 2 2 3 4 77%	5880
	21st to end	Night	11% 6% 2 4 3 3 4 3 4 5% 55%	6468
February	1st to 10th	Night	7% 4 2 4 3 3 3 2 5% 4 64%	2567
	11th to 20th	Night	8% 6% 2 4 3 3 3 3 4 6% 58%	2338
	21st to end	Night	8% 5% 2 5% 3 3 3 3 4 5% 59%	1910
March	1st to 10th	Night	6% 4 1 3 2 2 2 2 5% 6% 66%	4940
	11th to 20th	Night	10% 5% 1 4 2 3 3 2 4 5% 60%	5000
	21st to end	Night	5% 3 1 3 3 2 3 3 6% 7% 63%	5489
April	1st to 10th	Night	4 3 1 3 3 3 3 3 4 5% 69%	2200
	11th to 20th	Night	3 3 1 3 3 3 2 2 3 3 75%	2018
	21st to end	Night	2 2 2 1 2 2 1 3 5% 79%	1911
May	1st to 10th	Night	2 3 1 2 2 2 2 2 4 5% 75%	4100
	11th to 20th	Night	1 1 1 2 2 2 2 2 3 4 80%	4074
	21st to end	Night	1 1 1 2 1 2 2 3 3 84%	4474
June	1st to 10th	Night	3 3 1 2 2 2 1 2 4 6% 75%	1800
	11th to 20th	Night	1 1 1 1 1 2 1 2 3 87%	1800
	21st to end	Night	2 3 1 4 2 3 3 3 6% 5% 71%	1800
July	1st to 10th	Night	2 3 1 5% 3 3 3 3 4 6% 68%	2950
	11th to 20th	Night	1 2 1 4 4 5% 6% 6% 10% 12% 47%	2950
	21st to end	Night	3 3 1 6% 5% 6% 8% 7% 12% 17% 33%	3109
August	1st to 10th	Night	2 3 2 5% 5% 5% 7% 7% 10% 13% 42%	2847
	11th to 20th	Night	2 2 1 3 4 4 5% 5% 8% 10% 55%	2811
	21st to end	Night	2 3 1 4 4 4 5% 4 6% 9% 58%	3504
September	1st to 10th	Night	5% 4 1 3 3 4 5% 4 6% 7% 59%	2090
	11th to 20th	Night	3 2 1 3 3 2 3 2 4 4 74%	2090
	21st to end	Night	2 2 1 3 3 2 3 2 3 5% 72%	2090
October	1st to 10th	Night	2 1 1 2 2 2 2 2 4 5% 78%	3613
	11th to 20th	Night	2 1 1 1 1 1 1 2 4 85%	3778
	21st to end	Night	3 2 1 2 1 2 2 2 3 4 79%	4120
November	1st to 10th	Night	4 3 1 3 2 2 3 3 4 6% 71%	2340
	11th to 20th	Night	3 2 1 4 2 3 2 2 3 3 75%	2484
	21st to end	Night	7% 4 1 4 3 3 3 2 4 5% 64%	2600
December	1st to 10th	Night	5% 2 1 2 2 2 2 2 4 6% 72%	4240
	11th to 20th	Night	7% 4 2 5% 3 3 3 2 4 5% 62%	4811
	21st to end	Night	7% 5% 1 4 3 3 3 3 3 3 64%	6468

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	23% 3 3 9% 41% 21%	4099
	11th to 20th	Day	17% 3 3 15% 26% 37%	4100
	21st to end	Day	24% 5% 4 11% 28% 28%	4510
February	1st to 10th	Day	24% 3 6% 12% 22% 33%	1830
	11th to 20th	Day	28% 4 5% 13% 26% 25%	1960
	21st to end	Day	25% 4 3 13% 25% 29%	1719
March	1st to 10th	Day	22% 3 4 10% 37% 24%	4954
	11th to 20th	Day	26% 4 4 16% 26% 23%	5000
	21st to end	Day	26% 3 2 12% 35% 22%	5486
April	1st to 10th	Day	14% 3 3 11% 36% 33%	2200
	11th to 20th	Day	13% 2 3 8% 28% 46%	2330
	21st to end	Day	10% 2 3 10% 31% 44%	2484
May	1st to 10th	Day	11% 2 3 9% 28% 47%	5880
	11th to 20th	Day	8% 2 3 6% 28% 53%	5844
	21st to end	Day	9% 1 1 3 15% 69%	6424
June	1st to 10th	Day	11% 2 2 5% 26% 53%	2600
	11th to 20th	Day	7% 1 1 2 15% 74%	2600
	21st to end	Day	14% 4 5% 8% 18% 52%	2600
July	1st to 10th	Day	14% 3 5% 8% 16% 52%	4176
	11th to 20th	Day	18% 8% 13% 28% 24% 10%	4171
	21st to end	Day	23% 9% 16% 32% 15% 5%	4315
August	1st to 10th	Day	22% 8% 18% 25% 14% 13%	4078
	11th to 20th	Day	18% 7% 10% 23% 27% 15%	4067
	21st to end	Day	17% 6% 11% 18% 25% 23%	4145
September	1st to 10th	Day	22% 5% 8% 16% 23% 27%	2090
	11th to 20th	Day	12% 2 4 11% 34% 37%	2090
	21st to end	Day	14% 3 3 9% 27% 43%	2090
October	1st to 10th	Day	9% 2 3 9% 19% 58%	3362
	11th to 20th	Day	9% 3 1 7% 32% 49%	3150
	21st to end	Day	8% 2 2 6% 27% 55%	3438
November	1st to 10th	Day	13% 2 3 12% 38% 32%	1959
	11th to 20th	Day	16% 3 3 11% 27% 39%	1914
	21st to end	Day	23% 3 4 14% 25% 31%	1800
December	1st to 10th	Day	15% 3 3 15% 40% 25%	2950
	11th to 20th	Day	22% 3 5% 11% 30% 28%	3408
	21st to end	Day	25% 3 4 20% 32% 17%	4510
January	1st to 10th	Night	23% 3 3 9% 40% 21%	5867
	11th to 20th	Night	13% 2 2 15% 33% 35%	5880
	21st to end	Night	30% 5% 3 10% 27% 24%	6468
February	1st to 10th	Night	22% 3 5% 11% 25% 34%	2567
	11th to 20th	Night	27% 3 4 15% 27% 24%	2339
	21st to end	Night	25% 4 3 10% 24% 33%	1910
March	1st to 10th	Night	20% 2 3 13% 37% 25%	4940
	11th to 20th	Night	25% 3 5% 17% 28% 23%	5000
	21st to end	Night	18% 4 4 16% 35% 23%	5489
April	1st to 10th	Night	18% 3 3 9% 35% 33%	2200
	11th to 20th	Night	14% 2 3 7% 27% 46%	2018
	21st to end	Night	9% 2 3 9% 29% 48%	1911
May	1st to 10th	Night	12% 2 3 11% 30% 42%	4097
	11th to 20th	Night	8% 2 2 6% 24% 58%	4075
	21st to end	Night	7% 1 1 3 13% 74%	4474
June	1st to 10th	Night	12% 2 3 4 24% 56%	1800
	11th to 20th	Night	6% 1 2 3 13% 75%	1800
	21st to end	Night	15% 4 4 6% 16% 56%	1800
July	1st to 10th	Night	19% 3 5% 9% 15% 50%	2905
	11th to 20th	Night	22% 8% 12% 25% 21% 11%	2896
	21st to end	Night	30% 10% 19% 24% 12% 5%	3011
August	1st to 10th	Night	26% 9% 20% 21% 9% 14%	2838
	11th to 20th	Night	20% 6% 12% 21% 23% 17%	2838
	21st to end	Night	21% 5% 9% 16% 22% 26%	3504
September	1st to 10th	Night	22% 6% 8% 20% 19% 25%	2090
	11th to 20th	Night	16% 3 4 10% 28% 39%	2090
	21st to end	Night	15% 3 4 10% 25% 43%	2090
October	1st to 10th	Night	10% 2 4 8% 17% 58%	3613
	11th to 20th	Night	7% 2 1 9% 23% 58%	3778
	21st to end	Night	11% 2 2 7% 21% 57%	4120
November	1st to 10th	Night	15% 2 3 10% 32% 38%	2340
	11th to 20th	Night	15% 3 3 12% 25% 42%	2484
	21st to end	Night	23% 3 4 11% 28% 32%	2600
December	1st to 10th	Night	15% 3 3 15% 34% 30%	4240
	11th to 20th	Night	26% 3 3 9% 26% 33%	4811
	21st to end	Night	24% 3 4 16% 31% 22%	6468

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	13% 7% 29% 34% 17%	300
	11th to 20th	Day	13% 7% 28% 34% 17% 2	300
	21st to end	Day	8% 11% 31% 33% 15% 2	330
February	1st to 10th	Day	9% 12% 25% 35% 15% 3	330
	11th to 20th	Day	13% 11% 33% 31% 12%	495
	21st to end	Day	13% 15% 33% 29% 10%	414
March	1st to 10th	Day	11% 14% 34% 30% 10% 2	500
	11th to 20th	Day	10% 14% 28% 32% 13% 2	500
	21st to end	Day	8% 15% 32% 32% 11% 1	550
April	1st to 10th	Day	6% 13% 31% 31% 17% 3	500
	11th to 20th	Day	5% 18% 31% 31% 12% 3	492
	21st to end	Day	3 14% 32% 31% 15% 5%	500
May	1st to 10th	Day	3 13% 32% 32% 16% 3	503
	11th to 20th	Day	3 9% 31% 34% 18% 5%	528
	21st to end	Day	2 8% 30% 38% 18% 2	573
June	1st to 10th	Day	3 5% 23% 39% 25% 5%	530
	11th to 20th	Day	2 5% 28% 35% 23% 7%	530
	21st to end	Day	3 2 19% 45% 25% 6%	530
July	1st to 10th	Day	3 11% 43% 36% 8%	526
	11th to 20th	Day	3 1 8% 34% 40% 13%	524
	21st to end	Day	5% 5% 32% 39% 18%	532
August	1st to 10th	Day	2 5% 31% 44% 17%	487
	11th to 20th	Day	4 7% 29% 42% 17%	477
	21st to end	Day	2 1 7% 24% 46% 20%	539
September	1st to 10th	Day	5% 1 12% 35% 35% 11%	490
	11th to 20th	Day	4 2 16% 37% 33% 7%	490
	21st to end	Day	4 4 20% 36% 26% 9%	490
October	1st to 10th	Day	4 5% 21% 38% 27% 5%	414
	11th to 20th	Day	5% 6% 19% 37% 28% 6%	300
	21st to end	Day	2 8% 19% 38% 24% 8%	330
November	1st to 10th	Day	6% 5% 24% 43% 20% 2	300
	11th to 20th	Day	4 7% 25% 40% 22% 3	300
	21st to end	Day	8% 9% 28% 38% 15% 2	300
December	1st to 10th	Day	10% 5% 26% 37% 21% 2	300
	11th to 20th	Day	6% 7% 29% 41% 15% 2	300
	21st to end	Day	16% 8% 28% 32% 13% 2	330
January	1st to 10th	Night	10% 3 21% 45% 19% 3	800
	11th to 20th	Night	7% 3 25% 41% 20% 4	800
	21st to end	Night	14% 5% 23% 39% 17% 3	880
February	1st to 10th	Night	9% 4 23% 40% 22% 3	800
	11th to 20th	Night	11% 4 23% 46% 15% 1	784
	21st to end	Night	9% 5% 26% 42% 16% 2	660
March	1st to 10th	Night	6% 5% 28% 45% 15% 1	800
	11th to 20th	Night	9% 3 19% 39% 24% 6%	800
	21st to end	Night	5% 4 20% 45% 24% 2	880
April	1st to 10th	Night	4 4 17% 49% 23% 2	800
	11th to 20th	Night	4 4 24% 45% 22% 1	634
	21st to end	Night	2 3 21% 47% 23% 4	600
May	1st to 10th	Night	3 3 16% 49% 27% 3	600
	11th to 20th	Night	2 2 16% 49% 28% 3	591
	21st to end	Night	3 2 12% 51% 29% 3	650
June	1st to 10th	Night	2 2 7% 48% 39% 2	600
	11th to 20th	Night	1 1 8% 53% 35% 3	600
	21st to end	Night	2 2 8% 38% 47% 4	600
July	1st to 10th	Night	2 1 7% 32% 52% 5%	585
	11th to 20th	Night	2 1 4 23% 63% 7%	582
	21st to end	Night	4 3 18% 61% 13%	624
August	1st to 10th	Night	2 3 17% 66% 11%	567
	11th to 20th	Night	3 1 3 24% 59% 10%	564
	21st to end	Night	3 4 22% 60% 10%	664
September	1st to 10th	Night	5% 1 5% 30% 48% 10%	760
	11th to 20th	Night	3 1 8% 39% 42% 7%	760
	21st to end	Night	2 2 9% 46% 33% 7%	760
October	1st to 10th	Night	4 2 13% 45% 33% 3	760
	11th to 20th	Night	4 1 14% 45% 31% 4	760
	21st to end	Night	3 2 16% 41% 30% 8%	836
November	1st to 10th	Night	5% 2 16% 46% 27% 5%	780
	11th to 20th	Night	3 3 20% 45% 28% 3	800
	21st to end	Night	8% 4 20% 42% 23% 3	800
December	1st to 10th	Night	10% 4 19% 39% 27% 2	800
	11th to 20th	Night	11% 3 19% 44% 20% 2	800
	21st to end	Night	12% 2 21% 43% 19% 3	880

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