Star
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Star clear sky chart from 2005-11-06 to 2024-10-20.

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% 5% 5% 4 6% 7% 41%	100270
	Night	12% 7% 2 6% 4 4 4 3 5% 7% 45%	98396
Transparency	Day	39% 8% 12% 19% 17% 5%	100085
	Night	37% 7% 11% 19% 19% 7%	98252
Seeing	Day	14% 9% 21% 25% 22% 9%	14928
	Night	14% 8% 23% 30% 21% 4	24610

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	16% 13% 2 6% 3 3 3 3 4 6% 41%	2847
	11th to 20th	Day	16% 7% 2 6% 3 3 3 3 4 6% 48%	2850
	21st to end	Day	23% 9% 2 4 3 3 3 3 4 5% 40%	3272
February	1st to 10th	Day	17% 11% 4 5% 4 4 3 2 4 5% 40%	1900
	11th to 20th	Day	21% 7% 3 6% 4 3 3 3 4 4 42%	1848
	21st to end	Day	17% 8% 4 7% 5% 4 3 3 4 5% 39%	1570
March	1st to 10th	Day	24% 10% 3 9% 4 4 4 3 5% 7% 28%	3216
	11th to 20th	Day	16% 7% 3 7% 5% 5% 5% 4 7% 7% 34%	3480
	21st to end	Day	14% 8% 3 8% 5% 5% 4 4 5% 5% 39%	4025
April	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 4 5% 6% 44%	2280
	11th to 20th	Day	16% 7% 3 6% 5% 4 4 2 5% 7% 41%	2254
	21st to end	Day	8% 6% 2 6% 4 4 5% 4 6% 8% 47%	2280
May	1st to 10th	Day	11% 5% 2 6% 5% 5% 6% 4 7% 9% 39%	3765
	11th to 20th	Day	13% 7% 2 7% 4 4 4 4 5% 6% 44%	3764
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 7% 10% 43%	4337
June	1st to 10th	Day	10% 5% 2 7% 6% 6% 6% 6% 8% 10% 35%	2578
	11th to 20th	Day	5% 5% 3 6% 6% 6% 7% 6% 8% 9% 39%	2660
	21st to end	Day	6% 5% 2 8% 6% 7% 7% 6% 8% 9% 36%	2635
July	1st to 10th	Day	7% 6% 3 9% 7% 8% 7% 6% 9% 10% 29%	4120
	11th to 20th	Day	9% 5% 2 7% 6% 7% 7% 7% 10% 12% 28%	4120
	21st to end	Day	12% 8% 4 10% 7% 6% 7% 5% 8% 9% 25%	4247
August	1st to 10th	Day	3 3 1 4 3 4 4 3 5% 7% 62%	3785
	11th to 20th	Day	5% 4 2 6% 5% 4 6% 4 6% 11% 47%	3729
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 6% 8% 43%	4180
September	1st to 10th	Day	9% 5% 2 6% 6% 6% 7% 5% 6% 8% 39%	2280
	11th to 20th	Day	14% 6% 2 6% 5% 4 5% 4 5% 7% 42%	2220
	21st to end	Day	10% 4 2 5% 3 3 4 3 5% 8% 52%	2090
October	1st to 10th	Day	7% 4 1 5% 4 3 4 3 5% 6% 57%	1996
	11th to 20th	Day	10% 4 2 6% 3 3 3 3 4 6% 55%	1890
	21st to end	Day	13% 5% 1 5% 3 2 3 2 4 5% 56%	1980
November	1st to 10th	Day	10% 5% 2 6% 4 4 4 3 4 5% 55%	1852
	11th to 20th	Day	11% 7% 2 5% 4 3 4 3 4 5% 52%	1710
	21st to end	Day	16% 6% 2 6% 4 4 3 3 4 5% 48%	1710
December	1st to 10th	Day	16% 8% 3 5% 4 3 4 3 5% 7% 42%	1710
	11th to 20th	Day	14% 11% 3 8% 5% 3 4 2 3 5% 42%	2153
	21st to end	Day	15% 6% 3 6% 4 4 4 4 6% 7% 42%	2937
January	1st to 10th	Night	17% 12% 2 4 2 3 3 3 4 5% 44%	4108
	11th to 20th	Night	17% 7% 3 6% 4 3 3 2 3 4 48%	4120
	21st to end	Night	26% 10% 2 4 2 2 2 2 3 4 43%	4373
February	1st to 10th	Night	16% 10% 3 6% 4 3 3 4 4 6% 41%	2280
	11th to 20th	Night	21% 8% 2 4 4 3 3 3 4 5% 44%	2218
	21st to end	Night	21% 8% 2 6% 4 4 3 2 5% 5% 39%	1884
March	1st to 10th	Night	21% 12% 3 6% 4 3 3 4 5% 6% 33%	3651
	11th to 20th	Night	12% 7% 3 6% 5% 4 5% 4 7% 8% 40%	3480
	21st to end	Night	16% 6% 2 6% 5% 4 5% 4 5% 6% 40%	3604
April	1st to 10th	Night	12% 5% 2 6% 4 4 4 4 6% 7% 46%	1900
	11th to 20th	Night	13% 5% 2 5% 3 4 4 3 6% 8% 46%	1875
	21st to end	Night	8% 5% 2 4 3 4 5% 3 6% 8% 53%	1900
May	1st to 10th	Night	10% 6% 2 5% 4 4 4 3 6% 10% 46%	3130
	11th to 20th	Night	16% 6% 3 6% 4 4 4 3 5% 7% 43%	3129
	21st to end	Night	7% 4 3 7% 5% 5% 6% 4 7% 10% 43%	3214
June	1st to 10th	Night	7% 6% 2 6% 5% 4 5% 5% 7% 10% 42%	1595
	11th to 20th	Night	4 4 2 3 5% 5% 5% 5% 6% 11% 51%	1520
	21st to end	Night	5% 5% 2 5% 4 5% 5% 5% 8% 11% 45%	1536
July	1st to 10th	Night	5% 6% 3 8% 6% 5% 5% 4 8% 11% 38%	2850
	11th to 20th	Night	7% 4 2 8% 5% 6% 7% 7% 10% 13% 31%	2850
	21st to end	Night	10% 8% 4 9% 6% 6% 5% 4 6% 11% 31%	3307
August	1st to 10th	Night	2 2 1 3 3 3 4 4 6% 9% 61%	3144
	11th to 20th	Night	3 3 1 5% 3 5% 5% 5% 9% 13% 47%	3100
	21st to end	Night	7% 5% 2 7% 5% 5% 6% 4 6% 8% 46%	3476
September	1st to 10th	Night	7% 4 2 6% 5% 5% 6% 5% 6% 8% 44%	1900
	11th to 20th	Night	9% 6% 2 6% 4 4 4 4 6% 7% 47%	1939
	21st to end	Night	8% 4 2 5% 3 4 4 4 5% 7% 54%	2090
October	1st to 10th	Night	6% 2 1 5% 4 3 3 3 5% 6% 62%	2178
	11th to 20th	Night	10% 5% 2 5% 3 3 4 3 4 6% 57%	2275
	21st to end	Night	14% 6% 2 4 3 3 3 2 3 4 56%	2376
November	1st to 10th	Night	11% 5% 1 3 3 3 3 3 5% 5% 58%	2224
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 4 6% 54%	2470
	21st to end	Night	14% 6% 2 5% 3 4 3 3 5% 6% 49%	2470
December	1st to 10th	Night	17% 8% 2 6% 4 3 3 3 5% 5% 44%	2470
	11th to 20th	Night	15% 8% 3 6% 4 4 4 3 4 5% 45%	3042
	21st to end	Night	18% 11% 2 6% 4 4 3 2 3 6% 41%	4718

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	44% 4 4 18% 24% 6%	2848
	11th to 20th	Day	36% 5% 7% 22% 23% 7%	2850
	21st to end	Day	47% 4 6% 14% 23% 7%	3272
February	1st to 10th	Day	48% 5% 7% 14% 16% 10%	1900
	11th to 20th	Day	45% 5% 6% 16% 21% 8%	1858
	21st to end	Day	46% 6% 8% 14% 17% 9%	1570
March	1st to 10th	Day	55% 6% 8% 12% 14% 5%	3216
	11th to 20th	Day	45% 7% 9% 18% 16% 5%	3480
	21st to end	Day	46% 6% 7% 16% 19% 7%	4025
April	1st to 10th	Day	38% 6% 10% 19% 17% 8%	2280
	11th to 20th	Day	42% 5% 8% 17% 21% 7%	2254
	21st to end	Day	33% 8% 10% 22% 24% 3	2280
May	1st to 10th	Day	37% 11% 14% 21% 14% 3	3800
	11th to 20th	Day	39% 7% 13% 19% 20% 2	3768
	21st to end	Day	32% 10% 18% 27% 12% 1	4350
June	1st to 10th	Day	41% 11% 14% 20% 11% 2	2578
	11th to 20th	Day	37% 11% 16% 21% 13% 3	2660
	21st to end	Day	39% 13% 17% 19% 11% 2	2635
July	1st to 10th	Day	45% 13% 19% 16% 6% 1	4056
	11th to 20th	Day	40% 14% 27% 15% 3 1	4051
	21st to end	Day	53% 11% 12% 18% 6%	4112
August	1st to 10th	Day	21% 7% 13% 27% 28% 3	3773
	11th to 20th	Day	30% 11% 17% 29% 12% 2	3765
	21st to end	Day	38% 9% 13% 20% 16% 3	4179
September	1st to 10th	Day	39% 12% 14% 16% 14% 6%	2280
	11th to 20th	Day	41% 9% 10% 15% 18% 7%	2220
	21st to end	Day	30% 6% 12% 24% 19% 9%	2090
October	1st to 10th	Day	28% 5% 10% 18% 26% 14%	1996
	11th to 20th	Day	30% 5% 7% 17% 28% 14%	1890
	21st to end	Day	31% 4 6% 16% 30% 13%	1980
November	1st to 10th	Day	32% 6% 6% 16% 29% 11%	1852
	11th to 20th	Day	34% 4 7% 15% 27% 14%	1710
	21st to end	Day	38% 4 8% 17% 23% 10%	1710
December	1st to 10th	Day	41% 7% 9% 17% 18% 9%	1710
	11th to 20th	Day	46% 4 7% 13% 24% 7%	2150
	21st to end	Day	39% 6% 8% 20% 24% 3	2937
January	1st to 10th	Night	42% 4 6% 14% 25% 9%	4108
	11th to 20th	Night	39% 5% 6% 18% 21% 11%	4120
	21st to end	Night	47% 3 4 16% 23% 6%	4373
February	1st to 10th	Night	45% 5% 7% 15% 19% 9%	2280
	11th to 20th	Night	43% 4 6% 15% 22% 10%	2219
	21st to end	Night	48% 5% 7% 13% 15% 12%	1884
March	1st to 10th	Night	50% 7% 9% 13% 13% 7%	3651
	11th to 20th	Night	40% 7% 11% 18% 18% 6%	3480
	21st to end	Night	42% 7% 10% 16% 20% 4	3604
April	1st to 10th	Night	36% 8% 10% 18% 19% 10%	1900
	11th to 20th	Night	35% 6% 10% 20% 19% 9%	1875
	21st to end	Night	27% 6% 11% 23% 26% 6%	1900
May	1st to 10th	Night	32% 9% 13% 25% 18% 4	3160
	11th to 20th	Night	38% 8% 12% 24% 17% 1	3130
	21st to end	Night	31% 11% 17% 24% 15% 2	3222
June	1st to 10th	Night	34% 11% 16% 23% 15% 1	1595
	11th to 20th	Night	25% 10% 20% 27% 14% 4	1520
	21st to end	Night	29% 11% 21% 23% 14% 3	1536
July	1st to 10th	Night	37% 14% 23% 19% 6% 2	2805
	11th to 20th	Night	38% 15% 27% 15% 4 1	2796
	21st to end	Night	46% 11% 18% 19% 5% 1	3202
August	1st to 10th	Night	18% 9% 17% 31% 23% 2	3132
	11th to 20th	Night	25% 11% 21% 31% 10% 2	3130
	21st to end	Night	35% 9% 12% 21% 18% 4	3476
September	1st to 10th	Night	34% 12% 15% 17% 16% 6%	1900
	11th to 20th	Night	34% 8% 14% 16% 19% 9%	1939
	21st to end	Night	29% 6% 10% 25% 20% 10%	2090
October	1st to 10th	Night	24% 6% 11% 18% 24% 18%	2178
	11th to 20th	Night	28% 5% 6% 17% 26% 17%	2275
	21st to end	Night	32% 4 6% 15% 25% 17%	2376
November	1st to 10th	Night	27% 5% 8% 17% 28% 14%	2224
	11th to 20th	Night	32% 5% 7% 15% 27% 15%	2470
	21st to end	Night	36% 4 9% 19% 20% 12%	2470
December	1st to 10th	Night	41% 5% 10% 17% 20% 7%	2470
	11th to 20th	Night	42% 5% 7% 12% 26% 8%	3044
	21st to end	Night	45% 4 7% 17% 23% 5%	4718

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% 24% 33% 14% 3 1	270
	11th to 20th	Day	26% 19% 36% 13% 6% 1	270
	21st to end	Day	27% 21% 35% 13% 3	297
February	1st to 10th	Day	26% 24% 32% 11% 6% 1	270
	11th to 20th	Day	35% 17% 33% 13% 1 1	270
	21st to end	Day	21% 31% 40% 7% 1	225
March	1st to 10th	Day	27% 18% 35% 15% 4	270
	11th to 20th	Day	25% 24% 30% 17% 3	270
	21st to end	Day	24% 16% 31% 24% 5%	418
April	1st to 10th	Day	18% 16% 34% 22% 10%	460
	11th to 20th	Day	22% 15% 29% 22% 10% 2	452
	21st to end	Day	11% 12% 33% 28% 15% 1	460
May	1st to 10th	Day	13% 11% 29% 29% 15% 3	463
	11th to 20th	Day	12% 5% 26% 27% 20% 9%	487
	21st to end	Day	7% 4 24% 31% 23% 10%	529
June	1st to 10th	Day	13% 3 13% 34% 26% 10%	598
	11th to 20th	Day	7% 3 13% 35% 32% 10%	680
	21st to end	Day	7% 2 13% 29% 34% 15%	680
July	1st to 10th	Day	5% 2 8% 32% 37% 16%	671
	11th to 20th	Day	7% 2 5% 22% 42% 22%	668
	21st to end	Day	8% 2 9% 23% 40% 18%	609
August	1st to 10th	Day	6% 1 7% 24% 38% 24%	486
	11th to 20th	Day	8% 2 6% 23% 38% 23%	479
	21st to end	Day	13% 2 7% 25% 29% 24%	539
September	1st to 10th	Day	10% 1 11% 27% 36% 16%	490
	11th to 20th	Day	13% 3 11% 20% 36% 17%	490
	21st to end	Day	11% 4 14% 29% 31% 11%	490
October	1st to 10th	Day	9% 6% 21% 38% 17% 10%	396
	11th to 20th	Day	10% 6% 27% 32% 20% 5%	297
	21st to end	Day	21% 9% 25% 29% 11% 4	297
November	1st to 10th	Day	15% 9% 27% 34% 11% 4	270
	11th to 20th	Day	16% 14% 35% 24% 10% 1	270
	21st to end	Day	15% 19% 35% 16% 13% 2	270
December	1st to 10th	Day	24% 14% 35% 19% 7% 1	270
	11th to 20th	Day	21% 16% 37% 20% 6% 1	270
	21st to end	Day	24% 18% 34% 16% 9%	297
January	1st to 10th	Night	24% 15% 30% 26% 5%	950
	11th to 20th	Night	23% 15% 30% 25% 7%	950
	21st to end	Night	24% 16% 32% 25% 3	951
February	1st to 10th	Night	22% 14% 33% 27% 4	760
	11th to 20th	Night	26% 15% 29% 25% 4	743
	21st to end	Night	26% 20% 35% 19% 1	628
March	1st to 10th	Night	20% 15% 30% 30% 5%	717
	11th to 20th	Night	15% 13% 38% 27% 6% 1	570
	21st to end	Night	16% 10% 39% 26% 9%	627
April	1st to 10th	Night	16% 11% 34% 27% 11% 1	570
	11th to 20th	Night	15% 11% 35% 26% 12% 1	563
	21st to end	Night	8% 9% 35% 35% 13%	570
May	1st to 10th	Night	10% 5% 30% 37% 16% 1	570
	11th to 20th	Night	11% 3 24% 39% 20% 3	561
	21st to end	Night	6% 3 20% 34% 31% 7%	617
June	1st to 10th	Night	9% 2 13% 40% 31% 6%	570
	11th to 20th	Night	4 1 9% 36% 41% 8%	570
	21st to end	Night	6% 2 9% 29% 41% 13%	570
July	1st to 10th	Night	6% 1 6% 28% 46% 13%	555
	11th to 20th	Night	6% 1 5% 24% 46% 18%	552
	21st to end	Night	5% 1 6% 28% 47% 14%	624
August	1st to 10th	Night	2 5% 24% 49% 19%	567
	11th to 20th	Night	6% 2 6% 25% 46% 15%	564
	21st to end	Night	11% 2 6% 25% 39% 18%	627
September	1st to 10th	Night	8% 1 10% 29% 45% 8%	570
	11th to 20th	Night	11% 3 11% 28% 41% 7%	609
	21st to end	Night	10% 2 12% 36% 35% 5%	760
October	1st to 10th	Night	7% 2 18% 37% 31% 6%	760
	11th to 20th	Night	12% 6% 23% 35% 22% 3	759
	21st to end	Night	16% 7% 22% 32% 20% 3	792
November	1st to 10th	Night	14% 6% 19% 39% 19% 3	740
	11th to 20th	Night	14% 9% 28% 35% 15% 1	760
	21st to end	Night	17% 8% 30% 30% 14% 1	760
December	1st to 10th	Night	20% 11% 28% 31% 10%	760
	11th to 20th	Night	21% 9% 31% 30% 9%	760
	21st to end	Night	24% 14% 30% 26% 6%	1034

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