Watson Starview Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Watson Starview Observatory clear sky chart from 2005-11-06 to 2024-11-30.

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	10% 5% 2 5% 4 4 4 4 5% 7% 50%	101296
	Night	10% 5% 2 5% 3 3 3 3 5% 7% 54%	98419
Transparency	Day	32% 6% 10% 21% 21% 10%	105103
	Night	30% 5% 11% 21% 21% 12%	102739
Seeing	Day	12% 7% 23% 30% 22% 6%	15003
	Night	12% 7% 29% 37% 15% 1	24927

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	10% 9% 3 4 2 3 3 2 4 6% 54%	2847
	11th to 20th	Day	15% 5% 1 3 2 2 3 2 5% 5% 56%	2850
	21st to end	Day	16% 7% 3 6% 4 3 3 2 3 5% 48%	3295
February	1st to 10th	Day	16% 8% 2 5% 3 3 3 2 3 5% 50%	1900
	11th to 20th	Day	14% 8% 2 5% 4 3 3 3 5% 6% 48%	1849
	21st to end	Day	17% 5% 1 5% 3 3 3 2 4 5% 51%	1570
March	1st to 10th	Day	14% 8% 2 7% 4 4 3 3 4 5% 45%	3395
	11th to 20th	Day	18% 7% 2 6% 5% 5% 5% 4 6% 6% 36%	3480
	21st to end	Day	9% 7% 3 6% 4 4 5% 4 6% 7% 46%	3814
April	1st to 10th	Day	11% 8% 2 7% 4 4 3 4 6% 6% 45%	2090
	11th to 20th	Day	14% 6% 2 6% 5% 4 4 3 4 6% 45%	2077
	21st to end	Day	12% 5% 1 4 5% 4 5% 3 5% 6% 50%	2451
May	1st to 10th	Day	17% 7% 3 7% 4 4 5% 4 6% 8% 35%	4082
	11th to 20th	Day	13% 6% 3 7% 4 3 4 3 5% 6% 46%	4082
	21st to end	Day	10% 7% 2 7% 5% 6% 6% 5% 8% 9% 36%	4477
June	1st to 10th	Day	5% 4 1 4 4 4 5% 4 6% 9% 54%	2470
	11th to 20th	Day	6% 3 2 5% 4 3 4 4 7% 9% 54%	2470
	21st to end	Day	6% 4 1 4 5% 5% 5% 5% 7% 10% 47%	2470
July	1st to 10th	Day	6% 4 1 5% 4 5% 5% 5% 6% 8% 51%	4120
	11th to 20th	Day	4 3 1 4 3 4 4 4 6% 9% 59%	4120
	21st to end	Day	8% 5% 2 5% 4 4 5% 5% 7% 11% 44%	4465
August	1st to 10th	Day	1 1 3 2 3 3 3 5% 7% 71%	4104
	11th to 20th	Day	4 2 1 3 2 3 3 4 7% 8% 63%	3795
	21st to end	Day	4 2 1 4 4 4 5% 5% 7% 10% 53%	4020
September	1st to 10th	Day	10% 4 2 5% 4 5% 5% 4 6% 6% 49%	2090
	11th to 20th	Day	10% 4 2 7% 5% 4 4 3 6% 8% 46%	2090
	21st to end	Day	7% 3 2 5% 3 4 4 3 6% 9% 55%	2090
October	1st to 10th	Day	5% 5% 1 4 3 4 4 3 4 7% 60%	2090
	11th to 20th	Day	10% 5% 1 5% 3 3 3 3 3 5% 59%	1948
	21st to end	Day	11% 4 1 4 2 3 3 3 3 5% 61%	2090
November	1st to 10th	Day	13% 6% 1 5% 4 4 3 3 4 6% 51%	1958
	11th to 20th	Day	7% 5% 3 5% 4 3 3 3 5% 6% 56%	1845
	21st to end	Day	15% 6% 2 5% 4 3 3 2 4 7% 50%	1796
December	1st to 10th	Day	12% 6% 2 6% 3 3 3 3 4 5% 52%	1710
	11th to 20th	Day	12% 7% 2 6% 4 3 4 3 5% 6% 47%	2161
	21st to end	Day	13% 7% 2 4 3 2 3 2 3 5% 56%	3135
January	1st to 10th	Night	15% 7% 2 3 2 2 2 2 3 4 59%	4108
	11th to 20th	Night	12% 4 1 4 2 2 2 1 3 4 66%	4120
	21st to end	Night	16% 7% 3 6% 4 4 4 2 4 6% 44%	4400
February	1st to 10th	Night	12% 7% 2 6% 4 3 3 3 4 5% 52%	2280
	11th to 20th	Night	15% 7% 2 5% 3 2 3 3 4 6% 51%	2218
	21st to end	Night	16% 5% 2 5% 3 3 3 3 5% 5% 51%	1884
March	1st to 10th	Night	12% 7% 3 7% 4 3 4 3 5% 7% 46%	3459
	11th to 20th	Night	15% 7% 3 6% 4 5% 5% 4 6% 7% 38%	3480
	21st to end	Night	8% 7% 3 8% 5% 4 4 3 5% 6% 48%	3817
April	1st to 10th	Night	8% 6% 3 4 4 5% 4 3 6% 7% 51%	2090
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 6% 6% 46%	2046
	21st to end	Night	11% 6% 2 5% 4 3 4 3 4 7% 51%	1729
May	1st to 10th	Night	19% 8% 3 6% 3 2 3 3 5% 7% 41%	2823
	11th to 20th	Night	9% 7% 2 5% 3 3 4 4 5% 8% 50%	2823
	21st to end	Night	10% 5% 2 5% 4 4 5% 4 8% 8% 45%	3090
June	1st to 10th	Night	2 3 1 4 3 3 4 4 7% 10% 60%	1710
	11th to 20th	Night	4 3 1 3 3 3 3 4 5% 10% 60%	1710
	21st to end	Night	4 4 1 3 4 4 5% 3 6% 9% 57%	1710
July	1st to 10th	Night	5% 3 1 4 3 3 3 3 6% 9% 60%	2850
	11th to 20th	Night	3 2 1 3 2 2 3 3 5% 9% 65%	2850
	21st to end	Night	4 5% 2 5% 4 4 5% 4 8% 10% 49%	3098
August	1st to 10th	Night	1 1 1 1 2 2 2 4 7% 78%	2836
	11th to 20th	Night	3 2 1 3 2 3 3 3 6% 10% 63%	3039
	21st to end	Night	4 3 1 4 3 3 4 3 5% 11% 59%	3640
September	1st to 10th	Night	9% 4 2 5% 4 4 4 4 5% 7% 53%	2090
	11th to 20th	Night	11% 5% 2 5% 4 4 5% 4 5% 8% 49%	2090
	21st to end	Night	7% 2 1 5% 3 3 4 4 5% 8% 59%	2090
October	1st to 10th	Night	5% 3 1 2 3 3 3 2 5% 6% 66%	2090
	11th to 20th	Night	10% 3 1 4 3 3 3 3 4 6% 60%	2213
	21st to end	Night	11% 4 1 4 2 3 3 3 4 4 61%	2508
November	1st to 10th	Night	12% 4 2 5% 3 3 2 3 4 5% 57%	2340
	11th to 20th	Night	6% 5% 1 5% 3 3 4 2 4 6% 60%	2550
	21st to end	Night	13% 6% 2 5% 3 3 4 3 5% 6% 50%	2600
December	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 5% 7% 54%	2470
	11th to 20th	Night	11% 8% 3 6% 3 4 4 3 4 6% 49%	3036
	21st to end	Night	14% 6% 2 4 2 3 3 2 3 4 57%	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	33% 3 6% 18% 26% 14%	2848
	11th to 20th	Day	31% 3 7% 15% 31% 14%	2850
	21st to end	Day	41% 3 4 12% 27% 12%	3295
February	1st to 10th	Day	39% 4 7% 13% 21% 16%	1900
	11th to 20th	Day	38% 4 7% 18% 20% 13%	1859
	21st to end	Day	37% 4 6% 15% 21% 17%	1570
March	1st to 10th	Day	41% 5% 6% 14% 21% 13%	3395
	11th to 20th	Day	47% 8% 11% 10% 16% 9%	3480
	21st to end	Day	37% 6% 11% 14% 23% 9%	3814
April	1st to 10th	Day	39% 6% 8% 17% 15% 14%	2090
	11th to 20th	Day	40% 5% 9% 16% 19% 11%	2077
	21st to end	Day	36% 6% 7% 20% 23% 9%	2451
May	1st to 10th	Day	47% 8% 11% 15% 14% 5%	4120
	11th to 20th	Day	38% 7% 8% 17% 25% 5%	4086
	21st to end	Day	39% 11% 13% 23% 10% 4	4489
June	1st to 10th	Day	26% 8% 14% 27% 21% 4	2470
	11th to 20th	Day	26% 7% 13% 28% 19% 6%	2470
	21st to end	Day	31% 10% 12% 23% 21% 4	2470
July	1st to 10th	Day	28% 8% 18% 36% 9% 1	4056
	11th to 20th	Day	21% 7% 14% 37% 18% 4	4051
	21st to end	Day	32% 8% 17% 31% 12% 1	4325
August	1st to 10th	Day	12% 4 12% 34% 35% 1	4090
	11th to 20th	Day	18% 6% 19% 39% 16% 2	3833
	21st to end	Day	24% 9% 18% 27% 19% 2	4019
September	1st to 10th	Day	34% 6% 11% 22% 17% 10%	2090
	11th to 20th	Day	36% 8% 12% 18% 20% 6%	2090
	21st to end	Day	26% 8% 10% 21% 21% 14%	2090
October	1st to 10th	Day	15% 3 6% 12% 25% 38%	3480
	11th to 20th	Day	18% 4 7% 18% 30% 23%	3214
	21st to end	Day	23% 2 4 18% 39% 13%	3428
November	1st to 10th	Day	36% 4 8% 18% 22% 11%	1958
	11th to 20th	Day	30% 5% 9% 17% 22% 17%	1845
	21st to end	Day	37% 5% 6% 17% 20% 15%	1796
December	1st to 10th	Day	36% 4 8% 19% 23% 9%	1710
	11th to 20th	Day	38% 7% 7% 12% 23% 14%	2159
	21st to end	Day	33% 3 6% 14% 28% 16%	3135
January	1st to 10th	Night	32% 2 5% 17% 27% 17%	4108
	11th to 20th	Night	26% 2 5% 16% 27% 23%	4120
	21st to end	Night	43% 4 5% 12% 23% 13%	4400
February	1st to 10th	Night	37% 5% 6% 12% 22% 18%	2280
	11th to 20th	Night	36% 4 6% 17% 21% 16%	2218
	21st to end	Night	38% 4 6% 14% 19% 19%	1884
March	1st to 10th	Night	38% 6% 8% 14% 22% 11%	3459
	11th to 20th	Night	45% 8% 11% 13% 14% 9%	3480
	21st to end	Night	38% 6% 8% 17% 22% 8%	3817
April	1st to 10th	Night	33% 6% 9% 19% 18% 15%	2090
	11th to 20th	Night	37% 7% 10% 13% 18% 15%	2046
	21st to end	Night	35% 6% 9% 20% 21% 9%	1729
May	1st to 10th	Night	44% 7% 9% 18% 14% 7%	2850
	11th to 20th	Night	32% 8% 11% 23% 18% 8%	2824
	21st to end	Night	35% 9% 14% 28% 9% 5%	3099
June	1st to 10th	Night	20% 8% 16% 30% 22% 4	1710
	11th to 20th	Night	20% 6% 16% 30% 21% 7%	1710
	21st to end	Night	24% 9% 13% 29% 20% 5%	1710
July	1st to 10th	Night	21% 8% 22% 40% 8% 1	2805
	11th to 20th	Night	16% 6% 19% 39% 19% 1	2796
	21st to end	Night	28% 7% 22% 33% 9%	2999
August	1st to 10th	Night	8% 4 12% 38% 36% 2	2826
	11th to 20th	Night	17% 5% 26% 34% 15% 2	3067
	21st to end	Night	20% 9% 21% 28% 18% 3	3640
September	1st to 10th	Night	30% 7% 13% 23% 18% 10%	2090
	11th to 20th	Night	34% 6% 15% 22% 17% 7%	2090
	21st to end	Night	23% 6% 13% 25% 18% 15%	2090
October	1st to 10th	Night	12% 2 7% 15% 21% 43%	3480
	11th to 20th	Night	17% 4 9% 16% 31% 23%	3673
	21st to end	Night	23% 3 5% 20% 34% 16%	4120
November	1st to 10th	Night	32% 4 8% 19% 23% 14%	2340
	11th to 20th	Night	27% 5% 8% 19% 23% 18%	2550
	21st to end	Night	37% 4 7% 16% 21% 15%	2600
December	1st to 10th	Night	32% 5% 8% 20% 22% 12%	2470
	11th to 20th	Night	38% 4 6% 11% 23% 18%	3037
	21st to end	Night	34% 3 4 14% 29% 16%	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	20% 13% 39% 23% 6%	270
	11th to 20th	Day	24% 12% 34% 22% 7% 1	270
	21st to end	Day	15% 17% 39% 22% 6%	297
February	1st to 10th	Day	24% 15% 37% 19% 5%	270
	11th to 20th	Day	20% 19% 39% 19% 3	270
	21st to end	Day	23% 18% 40% 18%	225
March	1st to 10th	Day	21% 14% 39% 22% 4	270
	11th to 20th	Day	18% 18% 39% 21% 5%	270
	21st to end	Day	17% 18% 40% 21% 5%	297
April	1st to 10th	Day	18% 21% 40% 17% 3	270
	11th to 20th	Day	18% 16% 40% 19% 6%	275
	21st to end	Day	16% 14% 40% 22% 8%	631
May	1st to 10th	Day	12% 13% 40% 28% 7%	653
	11th to 20th	Day	15% 8% 29% 34% 12% 2	675
	21st to end	Day	14% 7% 29% 34% 15% 2	734
June	1st to 10th	Day	6% 3 17% 43% 26% 6%	680
	11th to 20th	Day	6% 1 18% 49% 22% 4	680
	21st to end	Day	5% 2 11% 39% 33% 9%	680
July	1st to 10th	Day	7% 2 9% 36% 34% 11%	671
	11th to 20th	Day	2 1 5% 37% 46% 10%	668
	21st to end	Day	4 6% 35% 44% 11%	739
August	1st to 10th	Day	2 4 33% 46% 14%	676
	11th to 20th	Day	5% 1 4 36% 41% 13%	522
	21st to end	Day	7% 2 3 26% 37% 26%	444
September	1st to 10th	Day	11% 1 4 24% 39% 21%	300
	11th to 20th	Day	7% 1 4 26% 43% 19%	300
	21st to end	Day	10% 2 7% 32% 35% 14%	300
October	1st to 10th	Day	5% 2 12% 37% 35% 9%	300
	11th to 20th	Day	9% 5% 23% 34% 25% 4	300
	21st to end	Day	19% 10% 25% 25% 20% 2	330
November	1st to 10th	Day	22% 6% 29% 29% 9% 4	300
	11th to 20th	Day	11% 11% 33% 28% 15% 3	300
	21st to end	Day	18% 17% 35% 17% 12% 1	299
December	1st to 10th	Day	16% 7% 32% 30% 11% 4	270
	11th to 20th	Day	16% 17% 38% 23% 6%	270
	21st to end	Day	18% 13% 40% 22% 6% 1	297
January	1st to 10th	Night	17% 10% 36% 30% 8%	950
	11th to 20th	Night	17% 11% 34% 31% 7%	950
	21st to end	Night	16% 12% 38% 28% 6%	965
February	1st to 10th	Night	17% 13% 38% 28% 4	760
	11th to 20th	Night	19% 13% 41% 24% 2	744
	21st to end	Night	21% 12% 45% 20% 2	628
March	1st to 10th	Night	17% 13% 39% 28% 2	599
	11th to 20th	Night	14% 11% 40% 30% 5%	570
	21st to end	Night	10% 14% 47% 26% 3	627
April	1st to 10th	Night	11% 12% 46% 28% 2	570
	11th to 20th	Night	15% 12% 45% 25% 4	563
	21st to end	Night	11% 12% 45% 25% 7%	570
May	1st to 10th	Night	12% 11% 44% 28% 5%	570
	11th to 20th	Night	12% 6% 35% 36% 10% 1	561
	21st to end	Night	8% 7% 36% 36% 12%	617
June	1st to 10th	Night	3 2 22% 47% 24% 1	570
	11th to 20th	Night	5% 1 27% 54% 13% 1	570
	21st to end	Night	4 2 16% 50% 24% 3	570
July	1st to 10th	Night	6% 1 13% 49% 28% 3	555
	11th to 20th	Night	3 1 9% 55% 30% 2	552
	21st to end	Night	2 1 8% 54% 32% 4	624
August	1st to 10th	Night	2 5% 55% 35% 4	567
	11th to 20th	Night	6% 1 5% 52% 32% 4	564
	21st to end	Night	5% 4 46% 40% 5%	627
September	1st to 10th	Night	9% 1 9% 42% 34% 4	570
	11th to 20th	Night	11% 2 8% 39% 37% 3	570
	21st to end	Night	8% 1 11% 41% 35% 4	570
October	1st to 10th	Night	6% 2 14% 48% 26% 4	570
	11th to 20th	Night	12% 3 26% 40% 17% 2	693
	21st to end	Night	13% 9% 28% 32% 18% 1	836
November	1st to 10th	Night	14% 4 24% 42% 14% 1	780
	11th to 20th	Night	9% 7% 31% 38% 13% 1	950
	21st to end	Night	16% 11% 31% 30% 11% 1	1000
December	1st to 10th	Night	13% 8% 31% 37% 9% 1	950
	11th to 20th	Night	19% 9% 33% 35% 5%	950
	21st to end	Night	16% 10% 34% 32% 8%	1045

Page generated at 2024-11-27 16:30:50. Version ac07

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