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 2024-11-12.

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% 42%	100930
	Night	12% 7% 2 6% 4 4 4 4 6% 7% 44%	98218
Transparency	Day	38% 5% 11% 21% 19% 5%	104759
	Night	37% 4 10% 21% 21% 7%	102527
Seeing	Day	14% 6% 18% 24% 23% 15%	14651
	Night	15% 4 19% 33% 25% 5%	24631

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	17% 11% 3 7% 5% 4 4 4 5% 5% 36%	2836
	11th to 20th	Day	13% 7% 2 6% 3 4 4 3 4 4 50%	2876
	21st to end	Day	23% 10% 2 5% 3 3 3 2 4 6% 40%	3476
February	1st to 10th	Day	19% 8% 3 7% 4 3 4 3 5% 5% 38%	1900
	11th to 20th	Day	23% 8% 2 6% 3 3 3 3 4 4 40%	1848
	21st to end	Day	19% 7% 3 8% 6% 4 4 3 4 4 38%	1570
March	1st to 10th	Day	22% 8% 2 7% 4 3 4 3 5% 6% 34%	3120
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 6% 7% 43%	3403
	21st to end	Day	16% 7% 2 6% 5% 4 4 4 5% 7% 39%	3898
April	1st to 10th	Day	12% 6% 2 7% 4 4 4 4 5% 6% 46%	2280
	11th to 20th	Day	13% 5% 2 6% 5% 3 5% 3 5% 6% 46%	2254
	21st to end	Day	9% 5% 2 5% 5% 4 4 4 5% 8% 50%	2293
May	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 5% 6% 9% 43%	4082
	11th to 20th	Day	16% 7% 2 7% 4 4 4 4 6% 8% 39%	4081
	21st to end	Day	8% 5% 2 7% 5% 4 5% 4 6% 9% 45%	4476
June	1st to 10th	Day	10% 7% 2 8% 5% 5% 6% 6% 8% 10% 32%	2470
	11th to 20th	Day	5% 4 2 5% 5% 5% 6% 6% 8% 11% 43%	2470
	21st to end	Day	6% 4 2 6% 5% 7% 6% 6% 9% 10% 37%	2470
July	1st to 10th	Day	7% 5% 3 8% 6% 7% 8% 6% 10% 12% 29%	4120
	11th to 20th	Day	7% 5% 2 7% 6% 6% 6% 6% 9% 12% 35%	4120
	21st to end	Day	9% 9% 3 11% 7% 7% 8% 7% 8% 10% 21%	4466
August	1st to 10th	Day	4 2 1 5% 4 4 6% 5% 8% 13% 48%	3799
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 5% 7% 11% 46%	3729
	21st to end	Day	6% 4 2 5% 4 4 5% 5% 7% 9% 50%	4180
September	1st to 10th	Day	8% 5% 2 8% 6% 5% 6% 5% 6% 8% 40%	2197
	11th to 20th	Day	12% 6% 4 6% 4 5% 4 4 6% 7% 42%	2090
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 6% 6% 48%	2090
October	1st to 10th	Day	9% 4 2 4 4 4 4 3 6% 6% 53%	2090
	11th to 20th	Day	10% 4 2 5% 3 3 3 3 5% 6% 57%	2090
	21st to end	Day	12% 5% 2 6% 4 3 3 2 4 5% 55%	1978
November	1st to 10th	Day	13% 6% 2 5% 4 3 3 2 4 5% 53%	1762
	11th to 20th	Day	13% 6% 2 4 3 3 3 3 4 5% 53%	1719
	21st to end	Day	14% 7% 2 6% 3 3 4 3 6% 6% 46%	1710
December	1st to 10th	Day	20% 8% 2 6% 4 4 4 3 4 6% 40%	1710
	11th to 20th	Day	17% 7% 3 6% 5% 5% 3 2 3 5% 43%	2153
	21st to end	Day	19% 9% 4 7% 5% 4 4 2 4 6% 35%	3124
January	1st to 10th	Night	18% 11% 2 5% 4 4 4 3 5% 5% 39%	4109
	11th to 20th	Night	15% 10% 3 6% 3 4 4 2 4 5% 44%	4079
	21st to end	Night	20% 14% 2 5% 4 3 3 3 4 3 39%	4180
February	1st to 10th	Night	16% 8% 2 6% 5% 4 4 3 4 6% 42%	2280
	11th to 20th	Night	21% 8% 2 6% 3 3 3 3 3 4 43%	2218
	21st to end	Night	18% 7% 2 6% 4 4 4 3 5% 6% 40%	1884
March	1st to 10th	Night	24% 9% 3 7% 4 3 4 3 4 5% 34%	3733
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 5% 7% 48%	3555
	21st to end	Night	15% 7% 3 7% 6% 4 5% 4 5% 6% 40%	3761
April	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 7% 47%	1900
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 5% 7% 49%	1875
	21st to end	Night	9% 6% 2 4 4 4 3 4 6% 6% 52%	1886
May	1st to 10th	Night	9% 5% 3 7% 5% 5% 5% 4 6% 9% 42%	2813
	11th to 20th	Night	16% 7% 2 6% 4 5% 5% 3 6% 8% 38%	2812
	21st to end	Night	6% 3 1 5% 4 4 4 5% 9% 11% 48%	3079
June	1st to 10th	Night	6% 5% 1 7% 6% 6% 6% 5% 9% 10% 38%	1710
	11th to 20th	Night	4 2 1 4 4 6% 5% 4 7% 12% 51%	1710
	21st to end	Night	4 4 2 6% 4 4 5% 6% 7% 11% 46%	1710
July	1st to 10th	Night	7% 6% 3 8% 6% 6% 6% 5% 9% 12% 33%	2840
	11th to 20th	Night	6% 5% 2 5% 5% 5% 6% 6% 9% 14% 37%	2840
	21st to end	Night	8% 9% 4 10% 7% 7% 7% 6% 8% 9% 25%	3087
August	1st to 10th	Night	3 3 1 4 4 4 4 4 8% 12% 52%	3111
	11th to 20th	Night	5% 5% 2 6% 4 4 5% 3 6% 9% 52%	3100
	21st to end	Night	6% 4 2 4 4 4 4 3 6% 8% 54%	3476
September	1st to 10th	Night	7% 5% 2 6% 5% 5% 6% 5% 7% 7% 44%	1962
	11th to 20th	Night	12% 5% 2 7% 4 4 4 4 6% 7% 44%	2090
	21st to end	Night	7% 5% 2 6% 3 4 4 3 6% 9% 51%	2090
October	1st to 10th	Night	7% 4 2 5% 3 3 3 2 3 5% 60%	2090
	11th to 20th	Night	9% 5% 2 5% 3 3 4 3 4 6% 57%	2090
	21st to end	Night	13% 5% 2 5% 4 3 3 2 3 4 56%	2614
November	1st to 10th	Night	11% 4 2 4 3 3 4 3 4 6% 55%	2536
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 4 6% 54%	2484
	21st to end	Night	13% 6% 2 6% 3 4 4 3 5% 6% 47%	2470
December	1st to 10th	Night	18% 7% 2 6% 5% 4 4 3 5% 6% 41%	2470
	11th to 20th	Night	17% 7% 2 5% 4 3 3 2 3 5% 48%	3042
	21st to end	Night	18% 9% 3 7% 4 4 4 4 6% 7% 34%	4532

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	51% 2 5% 15% 23% 4	2837
	11th to 20th	Day	39% 2 6% 26% 23% 3	2876
	21st to end	Day	48% 3 6% 17% 21% 6%	3476
February	1st to 10th	Day	48% 3 6% 19% 20% 4	1900
	11th to 20th	Day	48% 3 4 19% 20% 6%	1858
	21st to end	Day	50% 2 5% 13% 22% 7%	1570
March	1st to 10th	Day	52% 3 8% 14% 16% 8%	3120
	11th to 20th	Day	42% 3 7% 19% 23% 7%	3403
	21st to end	Day	45% 2 11% 16% 20% 5%	3898
April	1st to 10th	Day	39% 3 10% 21% 19% 8%	2280
	11th to 20th	Day	38% 3 6% 20% 24% 8%	2254
	21st to end	Day	32% 3 10% 27% 25% 3	2293
May	1st to 10th	Day	37% 4 13% 26% 18% 3	4120
	11th to 20th	Day	43% 4 11% 28% 12% 1	4085
	21st to end	Day	36% 3 14% 28% 17% 2	4489
June	1st to 10th	Day	43% 6% 16% 20% 14% 1	2470
	11th to 20th	Day	31% 6% 19% 26% 16% 2	2470
	21st to end	Day	36% 9% 17% 24% 11% 3	2470
July	1st to 10th	Day	39% 12% 25% 18% 6% 1	4055
	11th to 20th	Day	37% 10% 23% 22% 7% 1	4051
	21st to end	Day	54% 10% 16% 15% 5%	4324
August	1st to 10th	Day	24% 6% 16% 37% 16% 1	3786
	11th to 20th	Day	30% 6% 19% 28% 15% 2	3765
	21st to end	Day	28% 8% 17% 20% 24% 3	4179
September	1st to 10th	Day	40% 6% 15% 18% 16% 6%	2197
	11th to 20th	Day	40% 5% 11% 20% 20% 3	2090
	21st to end	Day	35% 5% 10% 22% 21% 7%	2090
October	1st to 10th	Day	24% 4 6% 14% 34% 19%	3480
	11th to 20th	Day	19% 2 3 17% 31% 27%	3458
	21st to end	Day	23% 3 7% 20% 36% 11%	3240
November	1st to 10th	Day	35% 2 7% 17% 29% 10%	1762
	11th to 20th	Day	34% 2 7% 19% 27% 10%	1719
	21st to end	Day	38% 3 7% 21% 24% 7%	1710
December	1st to 10th	Day	47% 3 8% 18% 20% 5%	1710
	11th to 20th	Day	47% 2 5% 18% 21% 6%	2150
	21st to end	Day	52% 3 7% 18% 18% 2	3124
January	1st to 10th	Night	48% 2 4 18% 24% 4	4109
	11th to 20th	Night	44% 2 4 24% 19% 7%	4079
	21st to end	Night	51% 2 3 15% 24% 5%	4180
February	1st to 10th	Night	45% 3 5% 18% 22% 7%	2280
	11th to 20th	Night	46% 2 5% 17% 20% 9%	2219
	21st to end	Night	46% 3 7% 15% 20% 10%	1884
March	1st to 10th	Night	53% 2 7% 15% 15% 7%	3733
	11th to 20th	Night	35% 3 9% 18% 24% 10%	3555
	21st to end	Night	46% 2 6% 20% 20% 6%	3761
April	1st to 10th	Night	35% 4 11% 21% 21% 8%	1900
	11th to 20th	Night	35% 3 6% 21% 23% 11%	1875
	21st to end	Night	32% 3 10% 26% 22% 7%	1886
May	1st to 10th	Night	39% 4 12% 22% 20% 2	2840
	11th to 20th	Night	44% 4 13% 25% 12% 1	2813
	21st to end	Night	27% 5% 17% 30% 18% 3	3087
June	1st to 10th	Night	37% 7% 16% 23% 15% 2	1710
	11th to 20th	Night	24% 7% 25% 25% 17% 2	1710
	21st to end	Night	30% 8% 18% 24% 16% 5%	1710
July	1st to 10th	Night	41% 11% 23% 18% 7% 1	2795
	11th to 20th	Night	30% 13% 25% 24% 6% 1	2786
	21st to end	Night	52% 11% 17% 14% 6% 1	2988
August	1st to 10th	Night	22% 6% 16% 40% 14% 1	3100
	11th to 20th	Night	28% 6% 19% 29% 15% 3	3130
	21st to end	Night	27% 6% 16% 21% 27% 4	3476
September	1st to 10th	Night	36% 6% 17% 18% 17% 6%	1962
	11th to 20th	Night	37% 6% 11% 21% 21% 4	2090
	21st to end	Night	29% 5% 9% 25% 24% 8%	2090
October	1st to 10th	Night	23% 4 6% 17% 30% 21%	3480
	11th to 20th	Night	18% 2 4 17% 30% 28%	3461
	21st to end	Night	23% 2 4 23% 33% 15%	4302
November	1st to 10th	Night	31% 3 9% 19% 24% 14%	2536
	11th to 20th	Night	33% 2 5% 20% 27% 12%	2484
	21st to end	Night	38% 2 8% 20% 25% 8%	2470
December	1st to 10th	Night	45% 3 8% 19% 20% 4	2470
	11th to 20th	Night	42% 2 6% 16% 29% 5%	3044
	21st to end	Night	49% 3 5% 22% 18% 2	4532

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% 15% 31% 21% 9%	270
	11th to 20th	Day	19% 17% 34% 16% 12% 1	270
	21st to end	Day	21% 15% 40% 16% 6% 2	297
February	1st to 10th	Day	23% 15% 33% 18% 11%	270
	11th to 20th	Day	34% 11% 32% 19% 4	270
	21st to end	Day	28% 14% 38% 18% 2	225
March	1st to 10th	Day	24% 12% 32% 22% 8% 1	270
	11th to 20th	Day	19% 10% 33% 27% 8% 2	409
	21st to end	Day	18% 8% 34% 27% 12%	506
April	1st to 10th	Day	18% 11% 27% 27% 15% 3	460
	11th to 20th	Day	17% 7% 29% 28% 17% 3	452
	21st to end	Day	12% 7% 27% 32% 20% 3	460
May	1st to 10th	Day	11% 5% 24% 30% 20% 10%	463
	11th to 20th	Day	11% 3 17% 29% 25% 16%	487
	21st to end	Day	10% 2 13% 29% 23% 22%	529
June	1st to 10th	Day	13% 2 9% 25% 30% 21%	490
	11th to 20th	Day	8% 1 10% 20% 27% 34%	490
	21st to end	Day	9% 2 8% 21% 28% 32%	490
July	1st to 10th	Day	9% 1 3 15% 31% 40%	485
	11th to 20th	Day	7% 1 2 15% 29% 45%	484
	21st to end	Day	6% 3 6% 13% 37% 35%	532
August	1st to 10th	Day	7% 3 6% 18% 32% 34%	486
	11th to 20th	Day	10% 2 3 18% 37% 29%	479
	21st to end	Day	14% 1 4 17% 35% 29%	539
September	1st to 10th	Day	9% 2 7% 22% 39% 22%	490
	11th to 20th	Day	14% 3 9% 20% 36% 20%	490
	21st to end	Day	11% 2 12% 22% 37% 16%	490
October	1st to 10th	Day	12% 1 14% 35% 27% 12%	490
	11th to 20th	Day	10% 4 19% 32% 28% 7%	490
	21st to end	Day	14% 7% 18% 31% 25% 4	408
November	1st to 10th	Day	20% 4 23% 30% 17% 6%	300
	11th to 20th	Day	16% 10% 30% 30% 10% 3	273
	21st to end	Day	11% 11% 32% 29% 14% 3	270
December	1st to 10th	Day	25% 7% 34% 22% 11%	270
	11th to 20th	Day	24% 10% 30% 24% 10% 2	270
	21st to end	Day	26% 10% 30% 24% 9% 1	297
January	1st to 10th	Night	22% 8% 32% 33% 5%	760
	11th to 20th	Night	22% 10% 30% 31% 7%	760
	21st to end	Night	21% 8% 34% 33% 4	836
February	1st to 10th	Night	19% 6% 30% 39% 6%	760
	11th to 20th	Night	26% 7% 29% 34% 4	743
	21st to end	Night	21% 10% 34% 32% 3	628
March	1st to 10th	Night	21% 6% 29% 36% 7%	760
	11th to 20th	Night	17% 6% 31% 34% 11% 1	760
	21st to end	Night	15% 8% 28% 37% 11% 1	805
April	1st to 10th	Night	14% 5% 31% 35% 15% 1	570
	11th to 20th	Night	16% 5% 27% 35% 16% 1	563
	21st to end	Night	12% 3 23% 40% 20% 1	570
May	1st to 10th	Night	10% 4 20% 38% 25% 4	570
	11th to 20th	Night	12% 2 13% 36% 32% 5%	561
	21st to end	Night	9% 1 7% 37% 40% 7%	617
June	1st to 10th	Night	8% 2 5% 37% 40% 7%	570
	11th to 20th	Night	3 1 6% 28% 46% 15%	570
	21st to end	Night	5% 1 4 27% 51% 12%	570
July	1st to 10th	Night	9% 1 2 20% 50% 18%	555
	11th to 20th	Night	5% 1 2 16% 55% 22%	552
	21st to end	Night	7% 3 3 20% 50% 17%	624
August	1st to 10th	Night	5% 2 2 21% 52% 18%	567
	11th to 20th	Night	10% 2 2 24% 52% 11%	564
	21st to end	Night	14% 1 2 23% 47% 14%	627
September	1st to 10th	Night	9% 1 5% 25% 50% 10%	632
	11th to 20th	Night	14% 2 6% 26% 46% 6%	760
	21st to end	Night	9% 2 9% 33% 43% 4	760
October	1st to 10th	Night	9% 1 13% 41% 30% 5%	760
	11th to 20th	Night	12% 3 19% 40% 24% 3	760
	21st to end	Night	15% 3 18% 39% 22% 3	836
November	1st to 10th	Night	12% 4 17% 42% 22% 3	780
	11th to 20th	Night	15% 4 25% 39% 17% 1	765
	21st to end	Night	16% 4 24% 40% 14% 1	760
December	1st to 10th	Night	21% 8% 26% 35% 10% 1	760
	11th to 20th	Night	23% 4 27% 35% 11% 1	760
	21st to end	Night	25% 7% 25% 36% 7%	836

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