Watson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Watson Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	12% 6% 2 6% 5% 4 5% 4 6% 8% 41%	127025
	Night	12% 7% 2 6% 4 4 4 3 6% 7% 44%	124368
Transparency	Day	38% 5% 12% 21% 18% 5%	130812
	Night	37% 5% 10% 21% 20% 7%	128651
Seeing	Day	14% 6% 19% 24% 22% 15%	16125
	Night	14% 5% 19% 33% 24% 5%	26134

By Month

Percentage Cloud Cover SE (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	16% 9% 3 6% 3 3 3 3 4 4 46%	4065
	11th to 20th	Day	11% 7% 5% 5% 3 3 4 3 4 5% 51%	4124
	21st to end	Day	21% 9% 3 6% 4 3 3 3 4 6% 37%	4972
February	1st to 10th	Day	18% 8% 4 7% 4 3 4 4 5% 5% 38%	2000
	11th to 20th	Day	23% 9% 2 6% 3 3 3 3 5% 4 39%	1948
	21st to end	Day	19% 7% 3 8% 5% 4 4 3 4 5% 39%	1650
March	1st to 10th	Day	19% 8% 2 8% 5% 4 5% 3 5% 6% 35%	4480
	11th to 20th	Day	12% 6% 2 6% 4 3 4 3 6% 7% 47%	4875
	21st to end	Day	15% 7% 3 7% 4 4 4 3 5% 6% 42%	5588
April	1st to 10th	Day	12% 6% 2 7% 4 4 4 4 5% 7% 46%	2400
	11th to 20th	Day	12% 5% 2 6% 5% 3 5% 3 5% 6% 48%	2374
	21st to end	Day	8% 5% 2 5% 5% 5% 5% 4 6% 8% 49%	2414
May	1st to 10th	Day	13% 7% 3 7% 5% 6% 5% 5% 6% 10% 34%	5862
	11th to 20th	Day	15% 7% 3 8% 5% 5% 5% 5% 7% 9% 31%	5861
	21st to end	Day	10% 6% 3 7% 5% 4 5% 4 6% 8% 41%	6433
June	1st to 10th	Day	11% 6% 3 8% 5% 5% 6% 6% 8% 10% 32%	2600
	11th to 20th	Day	5% 5% 2 5% 5% 6% 6% 6% 8% 12% 41%	2600
	21st to end	Day	6% 4 2 6% 5% 7% 6% 6% 9% 11% 37%	2600
July	1st to 10th	Day	6% 5% 2 7% 6% 6% 7% 6% 9% 12% 35%	5900
	11th to 20th	Day	6% 4 2 6% 5% 6% 6% 6% 10% 14% 35%	5900
	21st to end	Day	7% 7% 3 9% 7% 7% 9% 7% 10% 12% 23%	6424
August	1st to 10th	Day	4 2 1 5% 4 4 6% 5% 8% 13% 47%	3920
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 5% 7% 11% 45%	3849
	21st to end	Day	6% 4 2 5% 4 4 5% 5% 7% 9% 50%	4312
September	1st to 10th	Day	8% 5% 2 8% 6% 5% 5% 5% 6% 9% 41%	2312
	11th to 20th	Day	12% 6% 3 6% 4 5% 4 4 6% 7% 44%	2200
	21st to end	Day	10% 5% 2 6% 4 4 6% 4 6% 7% 48%	2200
October	1st to 10th	Day	9% 4 2 5% 4 4 5% 3 6% 6% 52%	2200
	11th to 20th	Day	10% 4 2 5% 3 3 3 3 4 6% 58%	2200
	21st to end	Day	12% 5% 2 6% 4 3 3 2 4 5% 54%	2082
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 3 4 5% 53%	1852
	11th to 20th	Day	13% 6% 2 4 3 3 3 3 4 6% 53%	1890
	21st to end	Day	14% 6% 2 6% 3 3 4 4 5% 7% 46%	1890
December	1st to 10th	Day	19% 7% 2 5% 3 3 3 3 4 5% 44%	3020
	11th to 20th	Day	16% 8% 3 7% 5% 5% 3 3 3 5% 42%	3463
	21st to end	Day	20% 8% 4 6% 4 4 4 2 4 6% 39%	4565
January	1st to 10th	Night	16% 9% 3 6% 3 3 4 3 5% 5% 44%	5903
	11th to 20th	Night	15% 9% 3 6% 3 4 3 2 4 6% 45%	5831
	21st to end	Night	17% 11% 2 5% 4 4 3 3 4 4 43%	5984
February	1st to 10th	Night	16% 8% 2 6% 5% 4 4 3 4 6% 42%	2400
	11th to 20th	Night	21% 8% 2 6% 3 3 3 3 4 4 42%	2338
	21st to end	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	1980
March	1st to 10th	Night	21% 9% 3 7% 4 5% 4 3 5% 6% 33%	5373
	11th to 20th	Night	11% 6% 2 5% 3 3 4 3 5% 6% 51%	5083
	21st to end	Night	13% 6% 2 7% 6% 4 4 3 5% 5% 44%	5383
April	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 7% 47%	2000
	11th to 20th	Night	12% 5% 2 6% 3 2 3 2 5% 7% 51%	1975
	21st to end	Night	9% 6% 2 5% 4 4 3 4 5% 6% 51%	1985
May	1st to 10th	Night	12% 7% 3 8% 5% 6% 5% 4 6% 9% 37%	4033
	11th to 20th	Night	16% 8% 3 8% 5% 6% 6% 4 6% 7% 31%	4032
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 8% 10% 42%	4421
June	1st to 10th	Night	6% 5% 2 7% 6% 6% 6% 5% 8% 10% 39%	1800
	11th to 20th	Night	5% 2 1 4 4 6% 5% 4 7% 11% 50%	1800
	21st to end	Night	4 4 2 6% 4 4 5% 6% 8% 11% 45%	1800
July	1st to 10th	Night	6% 6% 3 7% 5% 6% 6% 5% 9% 12% 35%	4058
	11th to 20th	Night	5% 4 2 5% 4 5% 6% 6% 9% 16% 37%	4057
	21st to end	Night	6% 7% 3 9% 7% 7% 7% 6% 9% 12% 27%	4427
August	1st to 10th	Night	3 3 1 4 4 4 4 4 8% 12% 51%	3210
	11th to 20th	Night	5% 5% 2 6% 4 4 5% 4 6% 9% 51%	3200
	21st to end	Night	7% 4 2 4 4 4 4 3 6% 8% 53%	3586
September	1st to 10th	Night	7% 5% 2 6% 5% 5% 6% 5% 7% 7% 43%	2066
	11th to 20th	Night	11% 5% 2 7% 4 4 4 4 7% 7% 45%	2200
	21st to end	Night	7% 5% 2 6% 3 4 4 3 6% 9% 51%	2200
October	1st to 10th	Night	7% 5% 2 5% 3 4 4 2 3 5% 59%	2200
	11th to 20th	Night	9% 5% 2 5% 3 3 4 3 4 6% 58%	2200
	21st to end	Night	13% 6% 2 5% 4 3 3 2 3 3 55%	2752
November	1st to 10th	Night	11% 5% 2 4 3 3 3 3 4 6% 56%	2666
	11th to 20th	Night	12% 6% 2 5% 3 3 2 3 4 6% 55%	2730
	21st to end	Night	13% 6% 2 6% 3 4 4 3 5% 6% 48%	2730
December	1st to 10th	Night	18% 7% 2 5% 4 3 3 2 4 5% 47%	4380
	11th to 20th	Night	17% 8% 2 6% 4 3 3 2 4 5% 45%	4952
	21st to end	Night	19% 8% 2 6% 4 4 4 3 5% 6% 40%	6633

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% 2 4 25% 22% 3	4066
	11th to 20th	Day	37% 2 6% 23% 29% 3	4124
	21st to end	Day	49% 3 6% 15% 20% 7%	4972
February	1st to 10th	Day	47% 4 6% 19% 20% 4	2000
	11th to 20th	Day	48% 3 4 19% 20% 6%	1958
	21st to end	Day	49% 3 5% 14% 22% 7%	1650
March	1st to 10th	Day	49% 3 7% 15% 19% 6%	4480
	11th to 20th	Day	37% 3 6% 17% 23% 15%	4875
	21st to end	Day	44% 2 9% 19% 22% 4	5590
April	1st to 10th	Day	38% 3 10% 22% 19% 7%	2400
	11th to 20th	Day	37% 3 6% 21% 25% 8%	2374
	21st to end	Day	33% 4 11% 26% 24% 3	2414
May	1st to 10th	Day	44% 5% 13% 23% 13% 2	5897
	11th to 20th	Day	47% 7% 15% 22% 9% 1	5865
	21st to end	Day	40% 5% 12% 25% 17% 1	6447
June	1st to 10th	Day	43% 6% 16% 19% 14% 1	2600
	11th to 20th	Day	32% 7% 19% 25% 15% 2	2600
	21st to end	Day	35% 9% 18% 24% 11% 3	2600
July	1st to 10th	Day	35% 12% 23% 22% 8%	5835
	11th to 20th	Day	32% 14% 30% 18% 5%	5831
	21st to end	Day	46% 13% 22% 16% 3	6240
August	1st to 10th	Day	25% 7% 17% 36% 16% 1	3907
	11th to 20th	Day	30% 6% 19% 28% 14% 2	3885
	21st to end	Day	28% 8% 17% 20% 24% 3	4311
September	1st to 10th	Day	39% 6% 14% 19% 16% 6%	2312
	11th to 20th	Day	39% 5% 12% 22% 20% 3	2200
	21st to end	Day	34% 6% 11% 21% 22% 6%	2200
October	1st to 10th	Day	24% 4 6% 14% 34% 19%	3590
	11th to 20th	Day	19% 2 3 17% 32% 27%	3568
	21st to end	Day	24% 3 7% 20% 36% 11%	3344
November	1st to 10th	Day	34% 3 7% 17% 29% 10%	1852
	11th to 20th	Day	33% 2 7% 20% 28% 10%	1890
	21st to end	Day	37% 3 7% 21% 24% 7%	1890
December	1st to 10th	Day	43% 3 6% 18% 26% 5%	3020
	11th to 20th	Day	47% 3 8% 19% 18% 6%	3460
	21st to end	Day	47% 3 6% 25% 18% 2	4565
January	1st to 10th	Night	44% 2 4 25% 22% 3	5903
	11th to 20th	Night	43% 3 4 21% 23% 7%	5831
	21st to end	Night	46% 3 4 16% 26% 5%	5984
February	1st to 10th	Night	45% 3 5% 18% 22% 7%	2400
	11th to 20th	Night	47% 3 5% 18% 19% 9%	2339
	21st to end	Night	44% 3 7% 16% 20% 10%	1980
March	1st to 10th	Night	51% 3 6% 15% 17% 7%	5373
	11th to 20th	Night	33% 3 8% 16% 23% 16%	5083
	21st to end	Night	43% 3 5% 23% 19% 7%	5383
April	1st to 10th	Night	35% 4 11% 22% 21% 8%	2000
	11th to 20th	Night	34% 3 6% 22% 24% 11%	1975
	21st to end	Night	33% 3 11% 25% 21% 6%	1985
May	1st to 10th	Night	44% 6% 14% 20% 15% 2	4060
	11th to 20th	Night	50% 6% 14% 20% 9% 1	4033
	21st to end	Night	35% 5% 16% 26% 16% 2	4429
June	1st to 10th	Night	36% 7% 17% 23% 16% 2	1800
	11th to 20th	Night	26% 7% 24% 24% 17% 2	1800
	21st to end	Night	30% 8% 18% 24% 15% 4	1800
July	1st to 10th	Night	37% 11% 22% 21% 9% 1	4015
	11th to 20th	Night	29% 15% 31% 19% 4 1	4006
	21st to end	Night	45% 13% 23% 14% 4	4297
August	1st to 10th	Night	22% 7% 17% 39% 14% 1	3199
	11th to 20th	Night	29% 7% 19% 29% 14% 3	3230
	21st to end	Night	28% 6% 16% 20% 26% 4	3586
September	1st to 10th	Night	36% 6% 17% 18% 17% 6%	2066
	11th to 20th	Night	36% 6% 11% 22% 21% 4	2200
	21st to end	Night	29% 6% 9% 24% 25% 7%	2200
October	1st to 10th	Night	24% 4 6% 17% 29% 20%	3590
	11th to 20th	Night	18% 2 4 17% 31% 28%	3571
	21st to end	Night	23% 2 3 23% 33% 15%	4440
November	1st to 10th	Night	31% 3 8% 19% 25% 14%	2666
	11th to 20th	Night	32% 2 5% 21% 28% 12%	2730
	21st to end	Night	37% 2 7% 20% 24% 9%	2730
December	1st to 10th	Night	40% 3 6% 21% 26% 4	4380
	11th to 20th	Night	42% 4 9% 17% 24% 5%	4954
	21st to end	Night	46% 3 5% 27% 18% 2	6633

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	23% 16% 32% 21% 8%	303
	11th to 20th	Day	19% 18% 35% 16% 11% 1	300
	21st to end	Day	21% 15% 40% 16% 7% 2	330
February	1st to 10th	Day	21% 15% 35% 18% 10% 1	300
	11th to 20th	Day	36% 11% 31% 18% 4	300
	21st to end	Day	25% 14% 38% 19% 3	249
March	1st to 10th	Day	25% 14% 31% 21% 8% 1	300
	11th to 20th	Day	20% 11% 33% 26% 8% 2	447
	21st to end	Day	18% 9% 34% 27% 11%	550
April	1st to 10th	Day	18% 11% 27% 26% 14% 3	500
	11th to 20th	Day	16% 7% 30% 28% 16% 3	492
	21st to end	Day	11% 6% 28% 31% 20% 3	500
May	1st to 10th	Day	12% 5% 24% 31% 19% 9%	503
	11th to 20th	Day	11% 4 19% 28% 24% 14%	527
	21st to end	Day	11% 2 15% 29% 22% 21%	573
June	1st to 10th	Day	13% 3 9% 25% 30% 20%	530
	11th to 20th	Day	8% 1 10% 22% 27% 32%	530
	21st to end	Day	9% 2 7% 20% 28% 34%	530
July	1st to 10th	Day	9% 1 3 16% 31% 39%	525
	11th to 20th	Day	7% 2 2 15% 30% 45%	524
	21st to end	Day	6% 3 6% 13% 38% 35%	576
August	1st to 10th	Day	8% 3 6% 19% 31% 32%	526
	11th to 20th	Day	11% 2 3 17% 37% 30%	519
	21st to end	Day	13% 1 6% 19% 34% 27%	583
September	1st to 10th	Day	8% 2 8% 24% 38% 21%	530
	11th to 20th	Day	13% 2 8% 20% 37% 20%	530
	21st to end	Day	10% 2 13% 24% 36% 15%	530
October	1st to 10th	Day	11% 2 15% 35% 26% 11%	530
	11th to 20th	Day	10% 4 19% 31% 29% 7%	530
	21st to end	Day	14% 8% 19% 31% 24% 4	445
November	1st to 10th	Day	18% 5% 23% 29% 17% 6%	330
	11th to 20th	Day	15% 9% 34% 28% 12% 2	330
	21st to end	Day	11% 14% 33% 26% 13% 3	330
December	1st to 10th	Day	27% 8% 34% 21% 10%	330
	11th to 20th	Day	22% 11% 31% 23% 10% 2	330
	21st to end	Day	25% 11% 28% 24% 9% 2	363
January	1st to 10th	Night	21% 8% 33% 32% 5%	805
	11th to 20th	Night	22% 10% 31% 30% 7%	800
	21st to end	Night	20% 8% 35% 33% 4	880
February	1st to 10th	Night	19% 6% 30% 39% 7%	800
	11th to 20th	Night	26% 8% 29% 32% 4	783
	21st to end	Night	20% 10% 34% 33% 3	660
March	1st to 10th	Night	21% 7% 29% 36% 7%	800
	11th to 20th	Night	17% 7% 31% 33% 10% 1	800
	21st to end	Night	15% 8% 29% 37% 11% 1	847
April	1st to 10th	Night	14% 6% 31% 35% 14% 1	600
	11th to 20th	Night	15% 5% 28% 35% 16% 1	593
	21st to end	Night	12% 4 23% 40% 21% 1	600
May	1st to 10th	Night	10% 4 19% 39% 24% 4	600
	11th to 20th	Night	12% 3 14% 36% 30% 5%	591
	21st to end	Night	10% 1 7% 37% 38% 7%	650
June	1st to 10th	Night	8% 2 5% 37% 40% 7%	600
	11th to 20th	Night	4 1 7% 28% 45% 15%	600
	21st to end	Night	5% 2 4 27% 51% 12%	600
July	1st to 10th	Night	9% 1 3 20% 49% 18%	585
	11th to 20th	Night	5% 1 2 16% 54% 22%	582
	21st to end	Night	7% 3 3 19% 51% 17%	657
August	1st to 10th	Night	6% 3 2 21% 52% 17%	597
	11th to 20th	Night	9% 2 2 23% 52% 12%	594
	21st to end	Night	13% 1 3 24% 45% 13%	660
September	1st to 10th	Night	8% 2 5% 27% 49% 10%	666
	11th to 20th	Night	13% 2 6% 26% 47% 6%	800
	21st to end	Night	9% 2 9% 34% 42% 4	800
October	1st to 10th	Night	9% 2 14% 41% 29% 5%	800
	11th to 20th	Night	11% 3 19% 40% 24% 3	800
	21st to end	Night	16% 3 19% 39% 21% 3	880
November	1st to 10th	Night	12% 5% 17% 41% 22% 3	820
	11th to 20th	Night	14% 4 26% 38% 17% 1	840
	21st to end	Night	16% 5% 25% 40% 13% 1	840
December	1st to 10th	Night	21% 8% 27% 35% 9% 1	840
	11th to 20th	Night	22% 5% 26% 35% 11% 1	840
	21st to end	Night	24% 7% 25% 35% 8%	924

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