O'Brien Dark Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the O'Brien Dark Site clear sky chart from 2005-11-06 to 2024-11-25.

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%	101259
	Night	13% 6% 2 5% 4 3 4 3 5% 7% 47%	98295
Transparency	Day	38% 7% 12% 19% 17% 7%	104973
	Night	35% 6% 12% 20% 18% 9%	102707
Seeing	Day	13% 9% 23% 28% 21% 6%	14955
	Night	15% 9% 27% 32% 16% 2	24814

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% 9% 2 5% 4 3 3 4 5% 6% 45%	2847
	11th to 20th	Day	18% 6% 2 4 3 2 3 2 3 6% 50%	2850
	21st to end	Day	23% 8% 3 6% 4 3 2 2 5% 8% 36%	3135
February	1st to 10th	Day	21% 8% 3 6% 4 3 3 3 5% 5% 37%	1819
	11th to 20th	Day	20% 8% 3 7% 4 3 3 2 4 5% 40%	1849
	21st to end	Day	18% 7% 3 6% 4 4 3 3 5% 5% 42%	1706
March	1st to 10th	Day	17% 8% 3 9% 6% 5% 5% 4 5% 7% 31%	3436
	11th to 20th	Day	20% 8% 2 7% 5% 5% 5% 4 7% 7% 30%	3480
	21st to end	Day	16% 8% 2 5% 4 4 4 3 5% 6% 41%	3814
April	1st to 10th	Day	14% 9% 3 9% 5% 4 5% 4 5% 6% 37%	2090
	11th to 20th	Day	16% 8% 3 6% 5% 5% 4 4 5% 6% 38%	2192
	21st to end	Day	11% 6% 2 6% 5% 5% 4 4 5% 7% 44%	2280
May	1st to 10th	Day	20% 8% 3 7% 5% 5% 5% 4 6% 7% 30%	3955
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 6% 7% 43%	4082
	21st to end	Day	9% 6% 3 7% 5% 5% 7% 6% 8% 11% 34%	4477
June	1st to 10th	Day	6% 5% 2 5% 5% 6% 6% 5% 8% 10% 42%	2470
	11th to 20th	Day	5% 4 1 5% 5% 5% 7% 6% 8% 10% 44%	2470
	21st to end	Day	5% 5% 3 7% 9% 7% 9% 6% 9% 10% 31%	2470
July	1st to 10th	Day	9% 4 2 5% 5% 6% 6% 5% 9% 12% 38%	4120
	11th to 20th	Day	5% 6% 2 7% 6% 6% 7% 6% 9% 11% 35%	4120
	21st to end	Day	12% 8% 3 8% 6% 6% 6% 5% 8% 10% 27%	4465
August	1st to 10th	Day	2 1 1 3 3 3 4 4 6% 10% 62%	4104
	11th to 20th	Day	3 3 1 3 4 4 6% 5% 9% 14% 48%	3950
	21st to end	Day	4 4 2 7% 6% 5% 7% 6% 8% 11% 39%	4032
September	1st to 10th	Day	6% 5% 2 7% 6% 6% 6% 5% 7% 8% 42%	2090
	11th to 20th	Day	11% 7% 3 8% 5% 5% 5% 4 6% 7% 38%	2090
	21st to end	Day	7% 3 2 7% 4 5% 5% 5% 6% 7% 48%	2090
October	1st to 10th	Day	5% 3 2 4 4 4 4 3 5% 7% 59%	1996
	11th to 20th	Day	11% 5% 2 5% 3 4 4 3 4 5% 54%	1900
	21st to end	Day	13% 5% 1 5% 3 2 2 2 5% 5% 55%	2090
November	1st to 10th	Day	11% 6% 3 6% 4 4 4 4 6% 6% 46%	1958
	11th to 20th	Day	11% 6% 2 5% 4 3 3 3 5% 6% 51%	2000
	21st to end	Day	16% 7% 2 6% 5% 4 4 4 4 4 44%	1826
December	1st to 10th	Day	15% 9% 2 8% 4 3 4 4 4 6% 42%	1710
	11th to 20th	Day	16% 8% 2 6% 5% 5% 4 3 4 5% 43%	2161
	21st to end	Day	16% 7% 3 6% 3 3 3 3 4 5% 46%	3135
January	1st to 10th	Night	18% 8% 2 5% 3 3 2 3 4 5% 48%	4108
	11th to 20th	Night	18% 6% 2 4 2 2 2 2 3 4 55%	4120
	21st to end	Night	22% 9% 2 5% 4 3 3 2 4 6% 40%	4532
February	1st to 10th	Night	19% 9% 3 7% 4 3 3 2 5% 5% 40%	2375
	11th to 20th	Night	20% 9% 2 4 3 3 3 3 4 5% 45%	2218
	21st to end	Night	20% 8% 2 6% 4 3 3 3 5% 5% 42%	1746
March	1st to 10th	Night	21% 10% 3 7% 4 3 4 3 5% 6% 33%	3419
	11th to 20th	Night	23% 8% 3 7% 5% 4 4 3 5% 6% 32%	3480
	21st to end	Night	14% 7% 3 8% 5% 4 4 3 4 5% 43%	3817
April	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 6% 8% 43%	2090
	11th to 20th	Night	14% 6% 2 6% 4 4 3 3 5% 9% 45%	1937
	21st to end	Night	14% 7% 2 6% 3 3 4 3 5% 7% 47%	1900
May	1st to 10th	Night	22% 8% 3 7% 5% 4 4 3 5% 7% 33%	2930
	11th to 20th	Night	13% 7% 3 7% 3 4 4 4 5% 7% 44%	2823
	21st to end	Night	11% 6% 2 6% 5% 5% 5% 4 8% 10% 39%	3090
June	1st to 10th	Night	6% 4 1 4 3 4 4 4 7% 10% 54%	1710
	11th to 20th	Night	3 3 1 4 3 3 4 5% 6% 12% 57%	1710
	21st to end	Night	4 4 1 5% 5% 4 6% 5% 8% 11% 46%	1710
July	1st to 10th	Night	6% 3 1 4 4 4 5% 5% 8% 13% 46%	2850
	11th to 20th	Night	4 4 2 6% 5% 4 5% 5% 7% 11% 47%	2850
	21st to end	Night	10% 9% 3 8% 4 5% 5% 4 6% 10% 36%	3098
August	1st to 10th	Night	1 1 2 2 2 3 2 5% 10% 72%	2836
	11th to 20th	Night	2 2 1 3 2 3 4 4 8% 14% 58%	2884
	21st to end	Night	4 3 1 4 4 4 5% 5% 8% 13% 48%	3618
September	1st to 10th	Night	5% 6% 2 5% 4 4 4 4 7% 10% 50%	2090
	11th to 20th	Night	9% 7% 2 7% 5% 5% 4 4 6% 7% 44%	2090
	21st to end	Night	8% 5% 1 4 3 3 3 3 5% 9% 56%	2090
October	1st to 10th	Night	4 4 1 4 3 3 4 2 5% 8% 62%	2163
	11th to 20th	Night	10% 4 2 3 3 2 4 3 4 6% 60%	2280
	21st to end	Night	12% 5% 2 5% 2 3 2 2 3 4 60%	2508
November	1st to 10th	Night	10% 6% 2 4 4 3 4 3 5% 6% 53%	2340
	11th to 20th	Night	11% 5% 2 6% 5% 3 3 3 5% 6% 51%	2400
	21st to end	Night	16% 7% 2 6% 4 3 4 3 5% 6% 45%	2445
December	1st to 10th	Night	17% 9% 2 5% 4 4 3 3 4 6% 43%	2470
	11th to 20th	Night	17% 9% 3 6% 4 4 3 3 4 6% 42%	3036
	21st to end	Night	16% 8% 2 5% 3 3 4 2 4 5% 49%	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	40% 5% 6% 15% 24% 10%	2848
	11th to 20th	Day	37% 4 5% 18% 29% 7%	2850
	21st to end	Day	48% 3 4 16% 17% 11%	3135
February	1st to 10th	Day	49% 4 6% 13% 17% 11%	1819
	11th to 20th	Day	47% 4 7% 14% 17% 11%	1859
	21st to end	Day	45% 5% 9% 14% 15% 12%	1706
March	1st to 10th	Day	50% 7% 11% 12% 16% 4	3436
	11th to 20th	Day	50% 7% 11% 12% 15% 5%	3480
	21st to end	Day	43% 6% 9% 17% 21% 4	3814
April	1st to 10th	Day	47% 7% 7% 16% 15% 9%	2090
	11th to 20th	Day	45% 6% 9% 14% 17% 9%	2192
	21st to end	Day	39% 6% 10% 19% 20% 6%	2280
May	1st to 10th	Day	52% 8% 11% 15% 10% 4	3990
	11th to 20th	Day	39% 7% 10% 18% 23% 2	4086
	21st to end	Day	40% 10% 13% 27% 10% 1	4489
June	1st to 10th	Day	33% 10% 17% 23% 15% 2	2470
	11th to 20th	Day	31% 10% 17% 23% 15% 4	2470
	21st to end	Day	42% 12% 15% 21% 9% 1	2470
July	1st to 10th	Day	34% 12% 25% 24% 4	4056
	11th to 20th	Day	38% 12% 17% 22% 10% 1	4051
	21st to end	Day	48% 11% 17% 19% 5% 1	4325
August	1st to 10th	Day	16% 7% 17% 37% 23%	4090
	11th to 20th	Day	23% 9% 31% 29% 6% 1	3989
	21st to end	Day	33% 15% 16% 21% 13% 2	4031
September	1st to 10th	Day	37% 11% 14% 18% 14% 6%	2090
	11th to 20th	Day	43% 8% 13% 16% 16% 4	2090
	21st to end	Day	32% 8% 12% 18% 16% 13%	2090
October	1st to 10th	Day	17% 4 7% 13% 22% 39%	3321
	11th to 20th	Day	21% 4 6% 16% 33% 20%	3140
	21st to end	Day	25% 4 5% 16% 35% 15%	3428
November	1st to 10th	Day	36% 7% 10% 16% 21% 10%	1958
	11th to 20th	Day	33% 5% 10% 15% 23% 14%	2000
	21st to end	Day	43% 5% 6% 16% 18% 11%	1826
December	1st to 10th	Day	43% 5% 9% 16% 19% 7%	1710
	11th to 20th	Day	43% 5% 6% 14% 22% 10%	2159
	21st to end	Day	41% 5% 5% 14% 25% 10%	3135
January	1st to 10th	Night	38% 5% 6% 19% 20% 12%	4108
	11th to 20th	Night	36% 3 6% 17% 27% 11%	4120
	21st to end	Night	47% 3 5% 16% 20% 10%	4532
February	1st to 10th	Night	47% 4 6% 13% 17% 14%	2375
	11th to 20th	Night	43% 4 7% 15% 18% 12%	2218
	21st to end	Night	46% 5% 8% 13% 15% 14%	1746
March	1st to 10th	Night	52% 6% 8% 13% 16% 5%	3419
	11th to 20th	Night	53% 6% 9% 14% 13% 5%	3480
	21st to end	Night	43% 6% 9% 14% 22% 7%	3817
April	1st to 10th	Night	41% 6% 10% 17% 15% 11%	2090
	11th to 20th	Night	38% 5% 11% 17% 16% 12%	1937
	21st to end	Night	38% 6% 10% 20% 21% 7%	1900
May	1st to 10th	Night	51% 7% 10% 18% 10% 4	2959
	11th to 20th	Night	38% 7% 11% 19% 21% 4	2824
	21st to end	Night	37% 10% 16% 27% 9% 1	3099
June	1st to 10th	Night	23% 9% 18% 30% 19% 1	1710
	11th to 20th	Night	20% 8% 20% 30% 18% 4	1710
	21st to end	Night	29% 11% 20% 27% 13% 1	1710
July	1st to 10th	Night	27% 11% 31% 26% 4	2805
	11th to 20th	Night	28% 11% 23% 28% 11%	2796
	21st to end	Night	41% 11% 20% 23% 5%	2999
August	1st to 10th	Night	9% 6% 17% 43% 24% 1	2826
	11th to 20th	Night	15% 8% 34% 33% 9% 1	2911
	21st to end	Night	27% 14% 21% 27% 10% 1	3618
September	1st to 10th	Night	29% 10% 22% 20% 13% 7%	2090
	11th to 20th	Night	37% 8% 15% 18% 16% 6%	2090
	21st to end	Night	26% 7% 15% 21% 19% 13%	2090
October	1st to 10th	Night	15% 4 7% 17% 17% 40%	3605
	11th to 20th	Night	16% 3 6% 21% 31% 23%	3779
	21st to end	Night	22% 3 5% 18% 36% 16%	4120
November	1st to 10th	Night	33% 5% 12% 20% 20% 11%	2340
	11th to 20th	Night	35% 4 9% 17% 21% 14%	2400
	21st to end	Night	41% 5% 8% 15% 19% 12%	2445
December	1st to 10th	Night	43% 5% 8% 19% 16% 10%	2470
	11th to 20th	Night	44% 5% 7% 10% 20% 14%	3037
	21st to end	Night	39% 4 6% 13% 29% 8%	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% 21% 35% 17% 3 1	270
	11th to 20th	Day	27% 16% 33% 19% 3 2	270
	21st to end	Day	24% 22% 33% 18% 3	297
February	1st to 10th	Day	28% 18% 37% 13% 3	270
	11th to 20th	Day	32% 21% 30% 16% 1	270
	21st to end	Day	27% 28% 34% 10% 1	225
March	1st to 10th	Day	25% 24% 33% 14% 3	270
	11th to 20th	Day	18% 29% 35% 15% 3	270
	21st to end	Day	21% 18% 42% 16% 3	297
April	1st to 10th	Day	19% 22% 40% 16% 3	270
	11th to 20th	Day	24% 19% 29% 19% 8% 1	390
	21st to end	Day	16% 15% 38% 22% 8% 1	460
May	1st to 10th	Day	15% 15% 39% 23% 7%	575
	11th to 20th	Day	14% 9% 30% 32% 13% 1	675
	21st to end	Day	9% 9% 32% 33% 14% 2	734
June	1st to 10th	Day	5% 3 21% 36% 28% 7%	680
	11th to 20th	Day	8% 2 19% 40% 25% 7%	680
	21st to end	Day	7% 2 16% 33% 32% 10%	680
July	1st to 10th	Day	8% 2 11% 33% 34% 13%	671
	11th to 20th	Day	4 1 6% 37% 42% 11%	668
	21st to end	Day	6% 1 6% 35% 41% 11%	739
August	1st to 10th	Day	3 1 5% 30% 49% 13%	676
	11th to 20th	Day	5% 4 26% 50% 15%	613
	21st to end	Day	7% 2 7% 22% 39% 23%	456
September	1st to 10th	Day	7% 1 10% 29% 41% 12%	300
	11th to 20th	Day	8% 3 10% 31% 34% 14%	300
	21st to end	Day	8% 2 10% 37% 32% 11%	300
October	1st to 10th	Day	7% 3 19% 38% 27% 5%	300
	11th to 20th	Day	12% 8% 22% 35% 19% 5%	300
	21st to end	Day	22% 10% 27% 21% 15% 4	330
November	1st to 10th	Day	17% 8% 25% 35% 13% 1	300
	11th to 20th	Day	16% 12% 30% 34% 8% 1	300
	21st to end	Day	19% 22% 31% 20% 6% 2	282
December	1st to 10th	Day	20% 11% 36% 20% 7% 4	270
	11th to 20th	Day	26% 17% 33% 19% 3 1	270
	21st to end	Day	24% 16% 35% 19% 5% 1	297
January	1st to 10th	Night	22% 15% 32% 25% 6%	950
	11th to 20th	Night	22% 16% 31% 24% 7%	950
	21st to end	Night	22% 15% 34% 25% 5%	1045
February	1st to 10th	Night	25% 15% 35% 22% 3	855
	11th to 20th	Night	26% 14% 34% 22% 4	744
	21st to end	Night	27% 17% 40% 14% 2	490
March	1st to 10th	Night	21% 16% 38% 21% 4	570
	11th to 20th	Night	19% 12% 38% 25% 5%	570
	21st to end	Night	16% 12% 46% 20% 6%	627
April	1st to 10th	Night	16% 14% 41% 25% 3	570
	11th to 20th	Night	17% 14% 35% 27% 7%	563
	21st to end	Night	18% 12% 38% 24% 8%	570
May	1st to 10th	Night	15% 11% 40% 26% 6% 1	570
	11th to 20th	Night	14% 6% 33% 35% 12% 1	561
	21st to end	Night	8% 5% 34% 37% 15% 1	617
June	1st to 10th	Night	7% 2 16% 49% 24% 3	570
	11th to 20th	Night	3 2 24% 52% 18% 2	570
	21st to end	Night	5% 1 16% 44% 30% 4	570
July	1st to 10th	Night	4 2 12% 45% 32% 5%	555
	11th to 20th	Night	3 9% 51% 32% 4	552
	21st to end	Night	4 1 9% 54% 26% 5%	624
August	1st to 10th	Night	2 1 6% 57% 31% 3	567
	11th to 20th	Night	5% 6% 50% 35% 4	564
	21st to end	Night	6% 1 8% 36% 42% 7%	627
September	1st to 10th	Night	6% 2 8% 36% 40% 8%	570
	11th to 20th	Night	10% 2 9% 35% 39% 5%	570
	21st to end	Night	10% 2 14% 35% 34% 5%	570
October	1st to 10th	Night	5% 3 16% 43% 28% 6%	643
	11th to 20th	Night	10% 4 26% 40% 18% 2	760
	21st to end	Night	16% 7% 27% 30% 18% 2	836
November	1st to 10th	Night	12% 6% 28% 35% 18% 1	780
	11th to 20th	Night	13% 11% 30% 33% 13%	800
	21st to end	Night	20% 11% 30% 26% 12% 1	889
December	1st to 10th	Night	23% 10% 31% 26% 11%	950
	11th to 20th	Night	25% 14% 29% 26% 5%	950
	21st to end	Night	22% 13% 30% 27% 7%	1045

Page generated at 2024-11-22 07:02:27. Version ac07

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