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 2025-01-07.

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	13% 8% 3 7% 5% 4 4 4 6% 7% 39%	104972
	Night	14% 9% 3 7% 4 4 4 3 5% 7% 40%	104074
Transparency	Day	42% 6% 10% 24% 16% 2	108658
	Night	42% 5% 10% 23% 17% 3	108516
Seeing	Day	17% 10% 26% 24% 16% 7%	15585
	Night	17% 8% 26% 30% 17% 2	25423

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	14% 15% 6% 9% 4 4 4 3 5% 6% 30%	2575
	11th to 20th	Day	18% 9% 3 12% 6% 5% 4 3 4 5% 30%	2530
	21st to end	Day	26% 13% 8% 8% 4 3 3 3 4 6% 23%	3146
February	1st to 10th	Day	17% 10% 4 10% 5% 3 3 2 5% 6% 33%	1900
	11th to 20th	Day	16% 10% 4 7% 5% 4 5% 3 5% 6% 36%	1848
	21st to end	Day	17% 12% 5% 8% 5% 4 5% 4 5% 6% 30%	1570
March	1st to 10th	Day	24% 14% 4 7% 4 3 3 2 3 5% 32%	3120
	11th to 20th	Day	12% 7% 3 6% 4 4 4 3 5% 7% 45%	3398
	21st to end	Day	14% 8% 3 6% 5% 5% 5% 4 6% 7% 37%	3931
April	1st to 10th	Day	13% 8% 2 7% 5% 5% 4 3 6% 7% 39%	2280
	11th to 20th	Day	16% 9% 3 7% 5% 4 4 3 5% 5% 39%	2254
	21st to end	Day	17% 6% 2 7% 6% 4 4 4 5% 7% 38%	2427
May	1st to 10th	Day	18% 9% 3 7% 5% 5% 5% 5% 7% 8% 29%	4082
	11th to 20th	Day	19% 8% 2 6% 4 4 4 3 4 6% 39%	4081
	21st to end	Day	10% 7% 3 7% 6% 5% 5% 5% 7% 8% 37%	4476
June	1st to 10th	Day	10% 5% 2 6% 5% 4 4 4 6% 7% 44%	2576
	11th to 20th	Day	7% 5% 2 5% 5% 5% 5% 5% 6% 8% 45%	2660
	21st to end	Day	8% 4 2 5% 5% 5% 6% 5% 7% 11% 42%	2660
July	1st to 10th	Day	13% 7% 2 6% 4 5% 5% 4 7% 10% 38%	4440
	11th to 20th	Day	6% 5% 1 5% 5% 4 5% 4 7% 11% 48%	4440
	21st to end	Day	9% 8% 4 10% 8% 7% 7% 5% 7% 9% 27%	4481
August	1st to 10th	Day	6% 5% 2 6% 4 4 5% 4 7% 10% 48%	4104
	11th to 20th	Day	7% 5% 2 6% 4 5% 6% 5% 8% 9% 41%	3822
	21st to end	Day	5% 4 2 4 3 3 5% 4 6% 10% 55%	4180
September	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 5% 7% 47%	2226
	11th to 20th	Day	9% 5% 2 6% 4 5% 4 4 6% 8% 48%	2090
	21st to end	Day	13% 6% 2 5% 4 4 4 3 4 6% 48%	2090
October	1st to 10th	Day	10% 4 2 5% 4 3 4 3 4 6% 55%	2090
	11th to 20th	Day	12% 5% 3 6% 4 4 4 4 5% 6% 48%	1994
	21st to end	Day	17% 7% 2 6% 4 4 4 4 4 6% 41%	1881
November	1st to 10th	Day	10% 6% 2 5% 4 3 4 3 5% 6% 51%	1762
	11th to 20th	Day	15% 10% 3 6% 4 4 3 3 5% 6% 41%	1800
	21st to end	Day	18% 9% 3 6% 5% 4 3 3 6% 6% 39%	1800
December	1st to 10th	Day	17% 11% 2 6% 3 3 3 2 4 5% 44%	2930
	11th to 20th	Day	15% 11% 5% 11% 6% 5% 4 3 4 6% 29%	3346
	21st to end	Day	20% 18% 5% 9% 4 5% 4 3 4 5% 22%	3982
January	1st to 10th	Night	16% 14% 5% 9% 6% 4 4 3 6% 6% 27%	4513
	11th to 20th	Night	16% 13% 5% 12% 7% 5% 4 3 4 4 27%	4440
	21st to end	Night	24% 15% 4 7% 4 4 4 3 4 5% 27%	4505
February	1st to 10th	Night	22% 10% 5% 8% 5% 5% 4 3 5% 5% 30%	2280
	11th to 20th	Night	16% 10% 4 8% 5% 4 4 3 4 5% 36%	2218
	21st to end	Night	21% 10% 3 8% 5% 3 4 3 5% 5% 31%	1884
March	1st to 10th	Night	23% 14% 3 6% 4 3 3 3 4 5% 33%	3733
	11th to 20th	Night	13% 7% 2 6% 3 3 4 3 5% 5% 49%	3561
	21st to end	Night	15% 8% 3 7% 5% 4 4 3 5% 6% 41%	3727
April	1st to 10th	Night	11% 7% 2 7% 4 4 5% 5% 7% 7% 41%	1900
	11th to 20th	Night	15% 8% 2 6% 4 4 4 3 5% 7% 42%	1875
	21st to end	Night	16% 7% 3 7% 4 4 4 4 5% 7% 39%	1753
May	1st to 10th	Night	17% 10% 4 8% 5% 5% 4 4 5% 7% 30%	2813
	11th to 20th	Night	17% 6% 1 6% 4 4 3 4 5% 7% 42%	2812
	21st to end	Night	8% 6% 2 7% 4 5% 5% 5% 7% 9% 42%	3079
June	1st to 10th	Night	10% 4 2 6% 4 4 4 4 5% 10% 48%	1598
	11th to 20th	Night	6% 5% 2 5% 4 4 5% 4 7% 8% 52%	1520
	21st to end	Night	5% 3 1 4 5% 4 4 5% 7% 10% 52%	1520
July	1st to 10th	Night	8% 6% 2 7% 4 4 4 4 5% 9% 47%	2530
	11th to 20th	Night	5% 4 2 5% 3 3 4 4 6% 11% 54%	2530
	21st to end	Night	9% 7% 3 9% 6% 6% 7% 6% 6% 9% 32%	3056
August	1st to 10th	Night	5% 4 2 6% 4 4 4 3 7% 9% 53%	2825
	11th to 20th	Night	5% 5% 2 6% 4 4 5% 4 7% 10% 47%	3003
	21st to end	Night	3 2 1 3 3 3 3 3 6% 10% 63%	3476
September	1st to 10th	Night	11% 5% 2 6% 3 4 4 3 5% 7% 51%	1934
	11th to 20th	Night	9% 5% 2 4 4 3 3 4 5% 9% 52%	2090
	21st to end	Night	13% 5% 2 5% 3 3 3 3 4 7% 52%	2090
October	1st to 10th	Night	8% 4 2 5% 4 3 3 3 4 6% 59%	2090
	11th to 20th	Night	13% 5% 2 6% 4 4 4 4 4 6% 49%	2180
	21st to end	Night	17% 7% 2 5% 4 4 3 2 4 6% 45%	2717
November	1st to 10th	Night	13% 7% 2 6% 4 4 3 2 5% 5% 50%	2536
	11th to 20th	Night	15% 6% 3 6% 5% 3 3 3 5% 7% 43%	2600
	21st to end	Night	17% 9% 3 7% 5% 4 4 3 4 6% 38%	2600
December	1st to 10th	Night	16% 10% 3 7% 4 4 3 2 4 5% 41%	4250
	11th to 20th	Night	19% 10% 3 7% 4 4 4 3 5% 6% 34%	4840
	21st to end	Night	23% 16% 5% 10% 6% 5% 5% 3 4 4 20%	6996

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	55% 4 11% 20% 9% 1	2576
	11th to 20th	Day	58% 3 9% 19% 9% 1	2530
	21st to end	Day	64% 4 8% 19% 4 1	3146
February	1st to 10th	Day	52% 3 9% 22% 11% 2	1900
	11th to 20th	Day	49% 4 9% 22% 13% 2	1858
	21st to end	Day	54% 5% 9% 19% 10% 3	1570
March	1st to 10th	Day	58% 2 6% 19% 11% 4	3120
	11th to 20th	Day	39% 5% 10% 27% 18% 2	3398
	21st to end	Day	44% 7% 13% 22% 13% 1	3931
April	1st to 10th	Day	43% 6% 10% 24% 16% 1	2280
	11th to 20th	Day	46% 5% 8% 22% 16% 3	2254
	21st to end	Day	45% 5% 10% 25% 12% 3	2427
May	1st to 10th	Day	49% 9% 14% 18% 9% 1	4120
	11th to 20th	Day	46% 7% 9% 23% 14% 1	4085
	21st to end	Day	42% 8% 12% 17% 20% 1	4489
June	1st to 10th	Day	37% 6% 10% 26% 18% 3	2576
	11th to 20th	Day	33% 8% 13% 24% 19% 2	2660
	21st to end	Day	33% 9% 14% 21% 19% 4	2660
July	1st to 10th	Day	40% 6% 12% 23% 17% 2	4371
	11th to 20th	Day	29% 7% 12% 34% 16% 2	4365
	21st to end	Day	50% 9% 12% 17% 11% 1	4339
August	1st to 10th	Day	29% 7% 14% 34% 14% 1	4091
	11th to 20th	Day	35% 7% 12% 29% 17% 1	3861
	21st to end	Day	24% 6% 13% 37% 17% 3	4179
September	1st to 10th	Day	37% 5% 10% 23% 20% 5%	2226
	11th to 20th	Day	34% 6% 10% 25% 21% 3	2090
	21st to end	Day	38% 4 8% 23% 23% 3	2090
October	1st to 10th	Day	20% 4 6% 29% 39% 3	3480
	11th to 20th	Day	26% 4 10% 27% 31% 3	3288
	21st to end	Day	33% 4 8% 27% 24% 5%	3081
November	1st to 10th	Day	33% 3 10% 29% 20% 5%	1762
	11th to 20th	Day	43% 3 8% 23% 18% 4	1800
	21st to end	Day	47% 3 9% 23% 17% 1	1800
December	1st to 10th	Day	45% 3 9% 26% 16% 1	2930
	11th to 20th	Day	57% 4 9% 19% 10% 2	3343
	21st to end	Day	64% 5% 11% 14% 6%	3982
January	1st to 10th	Night	56% 6% 11% 17% 11% 1	4513
	11th to 20th	Night	61% 4 8% 17% 8% 1	4440
	21st to end	Night	60% 3 8% 18% 10% 1	4505
February	1st to 10th	Night	56% 4 10% 20% 8% 1	2280
	11th to 20th	Night	51% 4 8% 21% 14% 1	2219
	21st to end	Night	55% 5% 10% 16% 11% 3	1884
March	1st to 10th	Night	55% 4 7% 21% 12% 2	3733
	11th to 20th	Night	37% 5% 11% 26% 18% 3	3561
	21st to end	Night	43% 6% 9% 26% 14% 1	3727
April	1st to 10th	Night	38% 7% 11% 23% 17% 3	1900
	11th to 20th	Night	42% 4 10% 20% 19% 6%	1875
	21st to end	Night	44% 5% 10% 24% 14% 3	1753
May	1st to 10th	Night	51% 8% 11% 20% 7% 2	2840
	11th to 20th	Night	40% 8% 13% 27% 11% 1	2813
	21st to end	Night	35% 9% 13% 23% 16% 3	3087
June	1st to 10th	Night	32% 7% 10% 27% 20% 4	1598
	11th to 20th	Night	28% 7% 15% 23% 22% 5%	1520
	21st to end	Night	24% 9% 17% 24% 19% 7%	1520
July	1st to 10th	Night	34% 8% 10% 26% 19% 3	2490
	11th to 20th	Night	24% 6% 12% 35% 19% 3	2482
	21st to end	Night	46% 9% 12% 20% 12% 1	2958
August	1st to 10th	Night	25% 7% 15% 37% 14% 1	2814
	11th to 20th	Night	29% 7% 14% 31% 18% 1	3030
	21st to end	Night	17% 6% 15% 35% 23% 4	3476
September	1st to 10th	Night	33% 6% 9% 25% 22% 6%	1934
	11th to 20th	Night	29% 6% 9% 27% 24% 5%	2090
	21st to end	Night	33% 5% 8% 24% 25% 5%	2090
October	1st to 10th	Night	20% 3 7% 19% 48% 4	3480
	11th to 20th	Night	25% 3 7% 24% 38% 4	3613
	21st to end	Night	31% 3 8% 22% 30% 6%	4467
November	1st to 10th	Night	37% 3 9% 24% 23% 4	2536
	11th to 20th	Night	41% 4 8% 21% 21% 5%	2600
	21st to end	Night	48% 4 9% 21% 17% 1	2600
December	1st to 10th	Night	47% 3 10% 24% 15%	4250
	11th to 20th	Night	50% 4 13% 20% 12% 1	4842
	21st to end	Night	68% 4 8% 14% 6% 1	6996

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	25% 16% 36% 16% 6%	288
	11th to 20th	Day	27% 23% 33% 13% 3	270
	21st to end	Day	26% 21% 38% 12% 3	297
February	1st to 10th	Day	23% 21% 37% 15% 3	270
	11th to 20th	Day	27% 23% 37% 10% 2 1	270
	21st to end	Day	32% 16% 36% 12% 4	225
March	1st to 10th	Day	26% 20% 35% 16% 3	270
	11th to 20th	Day	22% 18% 38% 13% 7% 1	404
	21st to end	Day	27% 17% 32% 18% 6%	506
April	1st to 10th	Day	20% 17% 29% 24% 8% 2	460
	11th to 20th	Day	21% 19% 27% 21% 10% 3	452
	21st to end	Day	21% 14% 36% 17% 9% 2	460
May	1st to 10th	Day	22% 11% 33% 20% 9% 5%	463
	11th to 20th	Day	21% 8% 26% 24% 14% 7%	487
	21st to end	Day	13% 6% 23% 29% 18% 11%	529
June	1st to 10th	Day	12% 6% 24% 29% 21% 8%	596
	11th to 20th	Day	7% 5% 22% 34% 23% 9%	680
	21st to end	Day	10% 4 27% 29% 21% 10%	680
July	1st to 10th	Day	14% 4 16% 23% 24% 19%	671
	11th to 20th	Day	8% 3 18% 33% 26% 12%	668
	21st to end	Day	10% 3 16% 31% 26% 13%	547
August	1st to 10th	Day	11% 2 14% 29% 25% 18%	486
	11th to 20th	Day	14% 5% 18% 29% 24% 11%	479
	21st to end	Day	8% 2 13% 29% 31% 17%	539
September	1st to 10th	Day	13% 2 16% 28% 31% 10%	490
	11th to 20th	Day	10% 2 14% 29% 31% 13%	490
	21st to end	Day	17% 7% 19% 23% 25% 9%	490
October	1st to 10th	Day	10% 5% 23% 35% 20% 7%	490
	11th to 20th	Day	13% 15% 32% 26% 12% 3	468
	21st to end	Day	24% 11% 39% 21% 5%	330
November	1st to 10th	Day	14% 14% 33% 26% 9% 4	300
	11th to 20th	Day	25% 15% 32% 20% 7% 1	300
	21st to end	Day	25% 14% 32% 21% 7%	300
December	1st to 10th	Day	23% 23% 30% 17% 6% 1	300
	11th to 20th	Day	22% 17% 36% 19% 5% 1	300
	21st to end	Day	32% 19% 32% 13% 3	330
January	1st to 10th	Night	20% 18% 33% 26% 4	981
	11th to 20th	Night	26% 14% 35% 21% 5%	950
	21st to end	Night	23% 15% 33% 27% 2	842
February	1st to 10th	Night	27% 10% 36% 25% 3	760
	11th to 20th	Night	24% 15% 37% 23% 1	743
	21st to end	Night	27% 13% 38% 20% 2	628
March	1st to 10th	Night	22% 14% 32% 27% 5%	760
	11th to 20th	Night	22% 10% 38% 23% 7%	760
	21st to end	Night	23% 11% 30% 30% 6% 1	784
April	1st to 10th	Night	15% 10% 36% 31% 9%	570
	11th to 20th	Night	18% 10% 31% 29% 12% 1	563
	21st to end	Night	19% 6% 34% 32% 8% 1	570
May	1st to 10th	Night	18% 5% 32% 30% 14% 1	570
	11th to 20th	Night	15% 4 21% 36% 19% 4	561
	21st to end	Night	9% 3 20% 37% 26% 6%	617
June	1st to 10th	Night	10% 1 21% 38% 26% 3	570
	11th to 20th	Night	7% 3 16% 44% 26% 3	570
	21st to end	Night	5% 1 19% 41% 30% 4	570
July	1st to 10th	Night	12% 1 11% 31% 35% 10%	555
	11th to 20th	Night	7% 1 11% 34% 41% 6%	552
	21st to end	Night	9% 2 11% 37% 34% 6%	624
August	1st to 10th	Night	8% 1 10% 35% 41% 4	567
	11th to 20th	Night	6% 1 11% 41% 35% 5%	564
	21st to end	Night	7% 1 7% 33% 45% 7%	627
September	1st to 10th	Night	13% 2 10% 31% 39% 5%	604
	11th to 20th	Night	11% 3 14% 29% 38% 5%	760
	21st to end	Night	15% 5% 19% 28% 29% 4	760
October	1st to 10th	Night	9% 4 20% 35% 27% 5%	760
	11th to 20th	Night	15% 7% 27% 31% 18% 2	760
	21st to end	Night	21% 8% 30% 28% 12%	836
November	1st to 10th	Night	16% 8% 29% 30% 17% 1	780
	11th to 20th	Night	18% 12% 32% 28% 10% 1	800
	21st to end	Night	22% 12% 27% 29% 9% 1	800
December	1st to 10th	Night	25% 9% 33% 28% 5%	800
	11th to 20th	Night	25% 10% 32% 25% 8%	805
	21st to end	Night	29% 13% 29% 24% 5%	1100

Page generated at 2025-01-04 03:24:00. Version ac07

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