Starlight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starlight Observatory clear sky chart from 2005-11-06 to 2024-12-06.

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	14% 7% 3 7% 5% 5% 5% 4 6% 7% 37%	101136
	Night	15% 7% 2 6% 4 4 4 4 5% 7% 41%	98512
Transparency	Day	42% 7% 12% 21% 14% 4	104785
	Night	40% 7% 13% 22% 15% 3	103002
Seeing	Day	22% 15% 25% 21% 12% 5%	15074
	Night	19% 13% 24% 29% 14% 2	25056

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	15% 7% 3 8% 5% 5% 4 3 5% 6% 38%	2836
	11th to 20th	Day	12% 12% 5% 11% 8% 4 4 3 5% 5% 32%	2840
	21st to end	Day	19% 10% 3 8% 4 3 4 3 4 6% 37%	3124
February	1st to 10th	Day	13% 8% 4 8% 5% 4 6% 4 5% 7% 38%	1710
	11th to 20th	Day	15% 9% 3 7% 4 5% 5% 4 5% 6% 37%	1663
	21st to end	Day	18% 9% 2 6% 4 5% 4 4 5% 6% 38%	1645
March	1st to 10th	Day	20% 9% 4 6% 4 3 4 4 5% 7% 34%	3435
	11th to 20th	Day	10% 7% 2 6% 4 3 4 3 5% 7% 48%	3480
	21st to end	Day	15% 10% 3 8% 5% 4 4 3 5% 5% 37%	3815
April	1st to 10th	Day	12% 8% 3 6% 5% 4 4 4 5% 7% 42%	2190
	11th to 20th	Day	15% 7% 2 5% 4 4 4 4 6% 7% 43%	2254
	21st to end	Day	17% 9% 3 6% 5% 4 4 4 4 6% 38%	2402
May	1st to 10th	Day	18% 8% 3 8% 6% 6% 6% 4 6% 6% 28%	4082
	11th to 20th	Day	24% 7% 3 7% 5% 4 4 4 5% 8% 30%	4081
	21st to end	Day	10% 6% 2 7% 5% 6% 6% 5% 8% 10% 36%	4476
June	1st to 10th	Day	13% 5% 2 6% 5% 5% 5% 5% 7% 9% 38%	2636
	11th to 20th	Day	11% 6% 3 7% 5% 5% 6% 5% 8% 10% 35%	2660
	21st to end	Day	9% 5% 3 7% 7% 6% 5% 5% 7% 9% 38%	2559
July	1st to 10th	Day	8% 5% 2 6% 6% 6% 6% 6% 7% 10% 39%	4120
	11th to 20th	Day	9% 5% 2 7% 6% 6% 6% 5% 9% 10% 35%	4120
	21st to end	Day	15% 9% 3 9% 6% 6% 6% 4 7% 8% 26%	4466
August	1st to 10th	Day	14% 6% 2 8% 5% 5% 6% 5% 8% 11% 30%	4104
	11th to 20th	Day	10% 5% 3 8% 6% 7% 7% 5% 8% 9% 34%	4044
	21st to end	Day	8% 4 2 6% 5% 5% 6% 4 7% 9% 43%	4212
September	1st to 10th	Day	14% 6% 2 6% 4 5% 5% 3 4 7% 43%	2280
	11th to 20th	Day	14% 7% 2 6% 5% 4 4 3 6% 7% 41%	2107
	21st to end	Day	24% 6% 2 6% 3 5% 3 4 5% 6% 34%	2076
October	1st to 10th	Day	16% 6% 3 6% 4 4 4 2 4 6% 45%	1900
	11th to 20th	Day	16% 6% 2 5% 4 4 3 3 5% 6% 46%	1900
	21st to end	Day	16% 6% 2 5% 4 3 3 3 4 5% 48%	2090
November	1st to 10th	Day	11% 7% 2 5% 3 3 3 3 5% 6% 51%	1957
	11th to 20th	Day	15% 5% 2 5% 4 3 3 4 6% 6% 47%	1803
	21st to end	Day	14% 7% 2 6% 4 4 4 3 5% 6% 46%	1600
December	1st to 10th	Day	16% 10% 2 7% 5% 4 6% 3 5% 6% 34%	1560
	11th to 20th	Day	16% 5% 3 6% 4 3 3 2 5% 7% 47%	1910
	21st to end	Day	19% 7% 3 9% 5% 4 5% 5% 6% 7% 30%	2999
January	1st to 10th	Night	12% 9% 3 7% 5% 5% 5% 4 6% 7% 36%	4109
	11th to 20th	Night	15% 10% 3 7% 5% 3 3 2 4 6% 41%	4120
	21st to end	Night	21% 9% 3 5% 4 4 4 3 4 5% 39%	4532
February	1st to 10th	Night	12% 7% 3 6% 3 5% 4 5% 5% 8% 41%	2470
	11th to 20th	Night	15% 9% 2 7% 5% 5% 4 3 5% 6% 38%	2403
	21st to end	Night	17% 8% 2 6% 4 4 4 4 6% 7% 38%	1806
March	1st to 10th	Night	22% 12% 3 7% 4 4 4 3 4 6% 31%	3418
	11th to 20th	Night	10% 5% 3 6% 4 4 4 4 5% 7% 47%	3480
	21st to end	Night	15% 7% 2 6% 4 3 3 3 5% 6% 45%	3817
April	1st to 10th	Night	14% 8% 2 6% 5% 5% 4 4 4 7% 41%	1988
	11th to 20th	Night	12% 5% 2 5% 4 4 5% 4 6% 8% 44%	1875
	21st to end	Night	16% 5% 2 5% 4 4 4 3 7% 8% 43%	1794
May	1st to 10th	Night	20% 8% 3 7% 4 5% 5% 4 6% 7% 32%	2813
	11th to 20th	Night	24% 9% 3 7% 4 4 4 3 5% 6% 32%	2812
	21st to end	Night	12% 5% 2 6% 4 4 5% 5% 7% 10% 40%	3079
June	1st to 10th	Night	10% 4 1 5% 4 4 5% 4 7% 11% 44%	1534
	11th to 20th	Night	11% 5% 2 6% 4 5% 4 4 6% 9% 45%	1520
	21st to end	Night	9% 4 1 5% 3 4 5% 6% 8% 10% 44%	1611
July	1st to 10th	Night	6% 5% 2 6% 5% 4 5% 5% 9% 10% 43%	2840
	11th to 20th	Night	10% 5% 2 6% 4 5% 4 5% 7% 10% 41%	2840
	21st to end	Night	12% 10% 4 9% 6% 5% 6% 4 6% 8% 29%	3087
August	1st to 10th	Night	16% 6% 3 7% 4 5% 5% 4 6% 8% 35%	2825
	11th to 20th	Night	13% 6% 2 8% 5% 5% 5% 5% 6% 7% 37%	2785
	21st to end	Night	8% 6% 2 6% 4 4 4 4 5% 9% 46%	3412
September	1st to 10th	Night	12% 4 2 6% 4 4 5% 3 5% 7% 48%	1900
	11th to 20th	Night	16% 7% 2 5% 3 3 4 3 4 8% 45%	2069
	21st to end	Night	22% 8% 2 5% 3 4 4 3 5% 5% 39%	2102
October	1st to 10th	Night	17% 6% 2 5% 4 4 4 3 4 6% 45%	2280
	11th to 20th	Night	15% 6% 2 5% 4 3 4 3 4 8% 46%	2280
	21st to end	Night	15% 4 2 5% 3 4 4 3 5% 7% 48%	2508
November	1st to 10th	Night	12% 8% 2 6% 3 3 3 4 4 6% 49%	2341
	11th to 20th	Night	14% 6% 2 5% 3 3 4 4 5% 7% 48%	2589
	21st to end	Night	14% 6% 2 5% 4 4 4 4 5% 7% 45%	2800
December	1st to 10th	Night	16% 9% 3 7% 5% 4 4 3 5% 7% 38%	2737
	11th to 20th	Night	14% 8% 2 6% 4 5% 6% 4 4 6% 40%	3285
	21st to end	Night	16% 6% 2 7% 6% 5% 5% 4 6% 7% 35%	4651

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	45% 4 9% 21% 16% 5%	2837
	11th to 20th	Day	53% 5% 7% 16% 14% 5%	2840
	21st to end	Day	49% 4 8% 20% 17% 3	3124
February	1st to 10th	Day	45% 6% 10% 20% 15% 3	1710
	11th to 20th	Day	45% 6% 10% 19% 15% 4	1672
	21st to end	Day	46% 6% 11% 16% 16% 5%	1645
March	1st to 10th	Day	48% 5% 9% 22% 12% 4	3435
	11th to 20th	Day	34% 6% 9% 25% 20% 6%	3480
	21st to end	Day	48% 7% 8% 19% 14% 4	3815
April	1st to 10th	Day	40% 7% 11% 19% 16% 6%	2190
	11th to 20th	Day	39% 6% 11% 22% 18% 4	2254
	21st to end	Day	46% 6% 11% 21% 12% 4	2402
May	1st to 10th	Day	52% 8% 10% 20% 8% 1	4120
	11th to 20th	Day	52% 7% 11% 20% 8% 1	4085
	21st to end	Day	39% 10% 16% 18% 13% 3	4489
June	1st to 10th	Day	40% 9% 14% 23% 13% 1	2636
	11th to 20th	Day	41% 10% 14% 19% 14% 1	2660
	21st to end	Day	39% 11% 14% 23% 12% 2	2559
July	1st to 10th	Day	39% 10% 17% 20% 8% 5%	4055
	11th to 20th	Day	39% 10% 17% 25% 8% 1	4051
	21st to end	Day	53% 8% 11% 19% 8% 1	4324
August	1st to 10th	Day	46% 10% 17% 21% 7%	4091
	11th to 20th	Day	43% 9% 12% 24% 12% 1	4083
	21st to end	Day	36% 9% 16% 26% 12% 1	4212
September	1st to 10th	Day	41% 7% 13% 17% 18% 5%	2280
	11th to 20th	Day	41% 6% 9% 22% 19% 3	2107
	21st to end	Day	50% 5% 10% 18% 16% 2	2076
October	1st to 10th	Day	33% 4 9% 20% 26% 8%	3160
	11th to 20th	Day	28% 4 10% 25% 19% 15%	3140
	21st to end	Day	27% 3 9% 29% 19% 13%	3428
November	1st to 10th	Day	34% 4 10% 23% 25% 4	1957
	11th to 20th	Day	36% 5% 9% 20% 23% 7%	1803
	21st to end	Day	39% 5% 10% 23% 21% 3	1600
December	1st to 10th	Day	49% 6% 9% 18% 15% 3	1560
	11th to 20th	Day	37% 4 8% 25% 20% 6%	1906
	21st to end	Day	50% 6% 12% 18% 12% 2	2999
January	1st to 10th	Night	41% 6% 12% 19% 19% 3	4109
	11th to 20th	Night	44% 5% 8% 22% 17% 5%	4120
	21st to end	Night	48% 4 6% 23% 16% 3	4532
February	1st to 10th	Night	39% 6% 13% 24% 15% 2	2470
	11th to 20th	Night	46% 5% 9% 18% 17% 5%	2405
	21st to end	Night	44% 6% 11% 21% 12% 5%	1806
March	1st to 10th	Night	53% 6% 9% 18% 11% 3	3418
	11th to 20th	Night	35% 6% 9% 26% 22% 3	3480
	21st to end	Night	39% 6% 12% 22% 17% 4	3817
April	1st to 10th	Night	42% 7% 13% 19% 16% 4	1988
	11th to 20th	Night	36% 7% 15% 21% 19% 4	1875
	21st to end	Night	39% 6% 14% 26% 13% 2	1794
May	1st to 10th	Night	49% 9% 13% 18% 9% 2	2840
	11th to 20th	Night	50% 8% 13% 22% 6% 1	2813
	21st to end	Night	35% 10% 21% 23% 7% 4	3087
June	1st to 10th	Night	33% 12% 14% 27% 13% 1	1534
	11th to 20th	Night	36% 9% 18% 24% 11% 2	1520
	21st to end	Night	30% 11% 20% 23% 14% 1	1611
July	1st to 10th	Night	33% 11% 21% 21% 11% 3	2795
	11th to 20th	Night	36% 10% 20% 25% 8% 1	2786
	21st to end	Night	50% 9% 15% 18% 8%	2988
August	1st to 10th	Night	44% 9% 18% 22% 5%	2814
	11th to 20th	Night	42% 8% 16% 21% 12% 1	2812
	21st to end	Night	33% 7% 17% 30% 10% 2	3412
September	1st to 10th	Night	36% 8% 14% 20% 19% 4	1900
	11th to 20th	Night	38% 6% 10% 27% 16% 2	2069
	21st to end	Night	46% 6% 11% 20% 16% 2	2102
October	1st to 10th	Night	39% 4 10% 20% 22% 5%	3800
	11th to 20th	Night	30% 4 10% 19% 24% 13%	3779
	21st to end	Night	23% 3 12% 28% 24% 9%	4120
November	1st to 10th	Night	36% 5% 9% 22% 22% 6%	2341
	11th to 20th	Night	36% 5% 9% 23% 23% 5%	2589
	21st to end	Night	38% 6% 11% 23% 19% 4	2800
December	1st to 10th	Night	46% 5% 10% 22% 14% 3	2737
	11th to 20th	Night	42% 5% 10% 21% 20% 2	3288
	21st to end	Night	43% 7% 14% 20% 15% 1	4651

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	31% 29% 25% 11% 3 1	270
	11th to 20th	Day	36% 27% 26% 9% 1	270
	21st to end	Day	35% 28% 27% 8% 2	297
February	1st to 10th	Day	29% 32% 28% 9% 2	270
	11th to 20th	Day	46% 26% 19% 8% 1	270
	21st to end	Day	39% 23% 25% 12% 1	310
March	1st to 10th	Day	29% 25% 25% 18% 4	460
	11th to 20th	Day	29% 26% 25% 15% 5% 1	460
	21st to end	Day	26% 25% 27% 16% 5% 1	506
April	1st to 10th	Day	28% 27% 22% 18% 6%	460
	11th to 20th	Day	24% 19% 27% 18% 9% 3	452
	21st to end	Day	28% 21% 24% 16% 9% 2	460
May	1st to 10th	Day	26% 19% 27% 14% 10% 4	463
	11th to 20th	Day	26% 16% 25% 19% 9% 5%	487
	21st to end	Day	17% 12% 24% 26% 14% 7%	529
June	1st to 10th	Day	16% 12% 27% 24% 14% 6%	490
	11th to 20th	Day	14% 11% 28% 22% 17% 8%	490
	21st to end	Day	10% 10% 32% 29% 15% 4	490
July	1st to 10th	Day	14% 5% 22% 27% 20% 11%	485
	11th to 20th	Day	6% 4 26% 32% 23% 9%	484
	21st to end	Day	14% 6% 25% 25% 20% 9%	532
August	1st to 10th	Day	17% 7% 21% 22% 21% 11%	486
	11th to 20th	Day	15% 5% 23% 26% 21% 11%	479
	21st to end	Day	12% 5% 17% 32% 21% 14%	539
September	1st to 10th	Day	17% 7% 21% 22% 22% 11%	490
	11th to 20th	Day	15% 4 19% 30% 22% 9%	490
	21st to end	Day	30% 11% 19% 22% 14% 4	490
October	1st to 10th	Day	21% 8% 24% 27% 18% 3	490
	11th to 20th	Day	20% 15% 25% 23% 15% 2	490
	21st to end	Day	22% 16% 29% 22% 9% 2	539
November	1st to 10th	Day	20% 15% 32% 21% 9% 2	495
	11th to 20th	Day	26% 19% 28% 21% 6% 1	318
	21st to end	Day	31% 18% 31% 16% 3 2	200
December	1st to 10th	Day	27% 29% 26% 13% 5% 1	190
	11th to 20th	Day	31% 22% 26% 15% 5% 1	180
	21st to end	Day	32% 22% 29% 12% 5%	263
January	1st to 10th	Night	23% 26% 30% 18% 3	760
	11th to 20th	Night	25% 19% 28% 25% 3	760
	21st to end	Night	21% 22% 36% 19% 2	836
February	1st to 10th	Night	20% 22% 32% 24% 2	760
	11th to 20th	Night	26% 26% 31% 16% 1	743
	21st to end	Night	26% 20% 35% 17% 2	628
March	1st to 10th	Night	22% 19% 33% 22% 4 1	760
	11th to 20th	Night	26% 19% 28% 20% 5% 1	760
	21st to end	Night	21% 18% 32% 22% 6%	836
April	1st to 10th	Night	20% 22% 28% 25% 6%	760
	11th to 20th	Night	18% 16% 29% 26% 11%	751
	21st to end	Night	20% 14% 28% 30% 8%	654
May	1st to 10th	Night	18% 10% 26% 30% 15% 1	570
	11th to 20th	Night	21% 10% 21% 31% 16% 2	561
	21st to end	Night	17% 3 18% 36% 24% 3	617
June	1st to 10th	Night	11% 3 18% 40% 25% 3	394
	11th to 20th	Night	13% 3 15% 41% 26% 2	380
	21st to end	Night	10% 3 19% 44% 21% 3	471
July	1st to 10th	Night	13% 1 14% 38% 28% 6%	555
	11th to 20th	Night	8% 2 11% 35% 39% 5%	552
	21st to end	Night	9% 3 14% 37% 33% 4	624
August	1st to 10th	Night	16% 2 10% 36% 33% 3	567
	11th to 20th	Night	13% 3 9% 38% 33% 5%	564
	21st to end	Night	11% 4 13% 35% 33% 4	798
September	1st to 10th	Night	14% 4 12% 36% 28% 7%	760
	11th to 20th	Night	18% 4 14% 31% 28% 5%	760
	21st to end	Night	26% 7% 17% 29% 20% 1	760
October	1st to 10th	Night	19% 9% 16% 31% 22% 3	760
	11th to 20th	Night	19% 14% 23% 28% 16% 1	760
	21st to end	Night	18% 13% 24% 31% 12% 1	836
November	1st to 10th	Night	17% 11% 26% 33% 11% 2	780
	11th to 20th	Night	21% 15% 26% 28% 8% 1	800
	21st to end	Night	19% 17% 26% 29% 8% 1	800
December	1st to 10th	Night	25% 20% 27% 24% 5%	783
	11th to 20th	Night	19% 19% 28% 29% 5%	760
	21st to end	Night	22% 17% 27% 30% 3	836

Page generated at 2024-12-03 06:45:12. Version ac07

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