Flint Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Flint Hill Observatory clear sky chart from 2005-11-06 to 2024-11-03.

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	7% 4 2 4 3 3 3 3 5% 7% 59%	100889
	Night	7% 4 1 4 3 3 3 3 5% 7% 61%	98226
Transparency	Day	26% 5% 9% 22% 24% 13%	100703
	Night	23% 5% 10% 22% 25% 15%	98083
Seeing	Day	9% 6% 25% 33% 21% 6%	14910
	Night	8% 7% 35% 37% 12% 1	24833

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	9% 5% 2 4 2 2 2 2 3 5% 63%	2847
	11th to 20th	Day	6% 5% 2 2 2 2 2 2 4 7% 66%	2982
	21st to end	Day	9% 6% 2 5% 3 3 3 3 4 6% 56%	3487
February	1st to 10th	Day	9% 6% 3 4 3 3 3 3 3 7% 57%	1900
	11th to 20th	Day	10% 6% 3 5% 2 3 3 3 4 6% 55%	1849
	21st to end	Day	10% 5% 1 5% 3 2 3 2 5% 5% 59%	1570
March	1st to 10th	Day	9% 6% 2 5% 4 4 4 3 4 5% 53%	3130
	11th to 20th	Day	18% 6% 2 6% 3 3 3 4 5% 5% 45%	3476
	21st to end	Day	5% 5% 2 5% 4 3 4 3 6% 8% 56%	3814
April	1st to 10th	Day	7% 4 1 4 3 3 3 3 5% 7% 60%	2210
	11th to 20th	Day	9% 6% 2 4 3 3 4 4 4 6% 55%	2254
	21st to end	Day	9% 4 2 4 3 2 3 3 3 6% 62%	2280
May	1st to 10th	Day	10% 6% 2 5% 3 4 4 3 5% 6% 52%	4068
	11th to 20th	Day	11% 7% 2 5% 3 3 3 2 4 6% 54%	4082
	21st to end	Day	7% 4 1 4 3 4 4 4 6% 8% 55%	4477
June	1st to 10th	Day	3 2 1 3 3 3 4 4 5% 9% 62%	2470
	11th to 20th	Day	4 2 1 3 3 3 4 4 5% 9% 62%	2470
	21st to end	Day	5% 4 1 3 3 4 5% 4 7% 8% 56%	2470
July	1st to 10th	Day	4 3 1 4 5% 4 5% 4 6% 7% 57%	4120
	11th to 20th	Day	2 2 1 3 3 3 3 3 6% 10% 66%	4120
	21st to end	Day	5% 3 1 4 3 4 4 3 5% 8% 60%	4379
August	1st to 10th	Day	1 1 1 2 3 3 3 3 4 7% 73%	3785
	11th to 20th	Day	4 2 1 3 2 2 3 3 5% 8% 68%	3726
	21st to end	Day	4 3 2 5% 4 5% 5% 5% 7% 9% 52%	4180
September	1st to 10th	Day	11% 4 1 5% 4 4 4 3 6% 7% 50%	2157
	11th to 20th	Day	10% 4 1 5% 5% 4 4 3 6% 8% 50%	2090
	21st to end	Day	10% 3 1 4 3 3 4 3 5% 6% 58%	2090
October	1st to 10th	Day	5% 3 1 4 3 3 3 3 4 6% 65%	2090
	11th to 20th	Day	8% 4 2 3 3 3 2 3 4 5% 64%	2090
	21st to end	Day	10% 4 1 4 2 2 2 2 4 4 65%	2109
November	1st to 10th	Day	8% 5% 1 4 2 3 3 2 5% 7% 61%	1691
	11th to 20th	Day	6% 4 2 3 3 2 4 3 5% 6% 63%	1710
	21st to end	Day	10% 5% 2 4 3 3 3 3 5% 6% 56%	1710
December	1st to 10th	Day	12% 5% 2 5% 2 3 3 2 5% 5% 56%	1710
	11th to 20th	Day	8% 5% 1 4 4 4 4 3 5% 6% 55%	2161
	21st to end	Day	9% 5% 1 4 2 2 2 2 3 5% 66%	3135
January	1st to 10th	Night	6% 4 2 5% 2 2 3 3 4 5% 64%	4108
	11th to 20th	Night	7% 4 1 2 1 2 2 2 3 6% 68%	4015
	21st to end	Night	11% 6% 1 4 3 3 3 2 4 6% 56%	4180
February	1st to 10th	Night	8% 5% 2 4 3 3 3 3 4 6% 59%	2280
	11th to 20th	Night	11% 6% 2 4 3 3 3 2 4 6% 56%	2218
	21st to end	Night	10% 5% 2 4 3 2 3 3 5% 5% 59%	1884
March	1st to 10th	Night	6% 5% 1 4 3 3 4 3 5% 7% 58%	3734
	11th to 20th	Night	16% 5% 2 4 3 3 4 3 6% 7% 48%	3481
	21st to end	Night	4 4 2 5% 4 4 4 4 6% 7% 55%	3817
April	1st to 10th	Night	5% 4 2 4 3 3 3 3 6% 7% 61%	1965
	11th to 20th	Night	8% 4 2 4 3 3 3 3 5% 7% 58%	1877
	21st to end	Night	6% 4 1 3 2 2 3 2 4 5% 67%	1900
May	1st to 10th	Night	8% 5% 2 5% 4 4 3 3 4 7% 55%	2833
	11th to 20th	Night	8% 4 2 4 2 2 3 3 5% 7% 60%	2823
	21st to end	Night	4 3 2 4 3 3 4 4 7% 9% 58%	3090
June	1st to 10th	Night	3 2 1 2 3 4 3 4 7% 10% 62%	1710
	11th to 20th	Night	2 2 1 2 3 2 3 4 5% 10% 67%	1710
	21st to end	Night	3 2 1 3 2 3 3 5% 4 8% 66%	1710
July	1st to 10th	Night	4 2 1 3 3 4 3 3 5% 8% 63%	2850
	11th to 20th	Night	1 1 2 1 1 2 3 5% 8% 75%	2850
	21st to end	Night	3 2 1 4 3 3 3 3 5% 8% 65%	3179
August	1st to 10th	Night	1 2 1 3 2 2 3 3 4 7% 73%	3155
	11th to 20th	Night	3 1 1 3 2 3 4 3 6% 9% 65%	3113
	21st to end	Night	3 3 1 3 3 4 5% 4 6% 12% 55%	3487
September	1st to 10th	Night	8% 3 1 4 4 4 4 3 6% 9% 54%	2023
	11th to 20th	Night	9% 3 1 4 3 4 5% 4 6% 9% 54%	2090
	21st to end	Night	9% 3 1 3 2 3 3 3 5% 7% 61%	2090
October	1st to 10th	Night	4 2 1 3 3 3 3 3 5% 7% 67%	2090
	11th to 20th	Night	9% 3 1 3 2 2 3 2 3 5% 65%	2090
	21st to end	Night	8% 4 1 4 2 2 2 2 4 4 67%	2464
November	1st to 10th	Night	9% 3 1 3 2 2 3 2 4 6% 65%	2432
	11th to 20th	Night	4 4 1 4 4 3 2 3 5% 6% 64%	2470
	21st to end	Night	10% 4 2 4 3 3 3 3 5% 7% 55%	2470
December	1st to 10th	Night	9% 4 1 4 3 2 4 2 4 6% 60%	2470
	11th to 20th	Night	7% 5% 2 5% 4 4 4 3 5% 7% 55%	3036
	21st to end	Night	8% 4 1 4 3 3 3 3 4 6% 61%	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	26% 2 5% 15% 27% 24%	2848
	11th to 20th	Day	21% 3 7% 18% 33% 19%	2982
	21st to end	Day	30% 4 7% 17% 30% 12%	3487
February	1st to 10th	Day	30% 3 6% 16% 24% 22%	1900
	11th to 20th	Day	31% 4 6% 20% 21% 18%	1859
	21st to end	Day	28% 3 4 13% 30% 22%	1570
March	1st to 10th	Day	33% 4 5% 11% 22% 25%	3130
	11th to 20th	Day	41% 7% 5% 12% 20% 15%	3476
	21st to end	Day	26% 5% 9% 16% 24% 20%	3814
April	1st to 10th	Day	24% 4 8% 19% 23% 21%	2210
	11th to 20th	Day	31% 4 7% 15% 23% 19%	2254
	21st to end	Day	27% 3 7% 19% 26% 18%	2280
May	1st to 10th	Day	32% 6% 8% 12% 20% 22%	4106
	11th to 20th	Day	32% 5% 8% 18% 24% 14%	4086
	21st to end	Day	26% 7% 13% 22% 16% 16%	4489
June	1st to 10th	Day	19% 6% 13% 32% 24% 7%	2470
	11th to 20th	Day	19% 5% 12% 27% 26% 10%	2470
	21st to end	Day	24% 8% 13% 24% 22% 9%	2470
July	1st to 10th	Day	24% 7% 15% 37% 16% 1	4056
	11th to 20th	Day	15% 5% 15% 37% 23% 4	4051
	21st to end	Day	21% 6% 12% 30% 27% 3	4241
August	1st to 10th	Day	13% 4 10% 34% 36% 3	3772
	11th to 20th	Day	17% 4 13% 42% 22% 2	3762
	21st to end	Day	26% 9% 16% 23% 23% 4	4179
September	1st to 10th	Day	34% 6% 9% 23% 19% 10%	2157
	11th to 20th	Day	33% 6% 11% 21% 21% 7%	2090
	21st to end	Day	27% 5% 10% 21% 22% 15%	2090
October	1st to 10th	Day	21% 5% 10% 24% 25% 15%	2090
	11th to 20th	Day	24% 4 7% 17% 26% 21%	2090
	21st to end	Day	25% 2 4 17% 32% 19%	2109
November	1st to 10th	Day	25% 4 8% 25% 23% 16%	1691
	11th to 20th	Day	23% 3 10% 19% 26% 19%	1710
	21st to end	Day	30% 4 5% 18% 27% 15%	1710
December	1st to 10th	Day	30% 4 7% 20% 26% 13%	1710
	11th to 20th	Day	29% 4 8% 16% 24% 18%	2159
	21st to end	Day	25% 2 3 18% 30% 21%	3135
January	1st to 10th	Night	24% 3 5% 16% 30% 22%	4108
	11th to 20th	Night	19% 2 5% 21% 29% 24%	4015
	21st to end	Night	29% 4 7% 17% 29% 15%	4180
February	1st to 10th	Night	28% 3 7% 15% 26% 22%	2280
	11th to 20th	Night	32% 3 6% 16% 22% 22%	2218
	21st to end	Night	27% 4 6% 13% 25% 24%	1884
March	1st to 10th	Night	24% 5% 7% 14% 28% 21%	3734
	11th to 20th	Night	36% 6% 7% 16% 19% 15%	3481
	21st to end	Night	26% 6% 10% 18% 21% 18%	3817
April	1st to 10th	Night	22% 4 10% 22% 18% 23%	1965
	11th to 20th	Night	26% 6% 9% 19% 19% 21%	1877
	21st to end	Night	21% 5% 10% 23% 27% 14%	1900
May	1st to 10th	Night	29% 7% 10% 16% 21% 18%	2860
	11th to 20th	Night	24% 6% 11% 23% 21% 15%	2824
	21st to end	Night	23% 7% 14% 23% 18% 15%	3099
June	1st to 10th	Night	15% 7% 17% 33% 24% 5%	1710
	11th to 20th	Night	14% 5% 13% 31% 27% 9%	1710
	21st to end	Night	18% 6% 15% 27% 26% 8%	1710
July	1st to 10th	Night	19% 5% 18% 38% 17% 2	2805
	11th to 20th	Night	9% 4 13% 41% 28% 6%	2796
	21st to end	Night	16% 5% 13% 37% 27% 2	3078
August	1st to 10th	Night	13% 3 11% 29% 40% 5%	3144
	11th to 20th	Night	15% 6% 16% 41% 20% 2	3143
	21st to end	Night	20% 9% 22% 24% 21% 3	3487
September	1st to 10th	Night	26% 7% 12% 23% 21% 11%	2023
	11th to 20th	Night	26% 7% 15% 22% 23% 7%	2090
	21st to end	Night	23% 4 12% 24% 20% 17%	2090
October	1st to 10th	Night	17% 5% 11% 25% 23% 19%	2090
	11th to 20th	Night	25% 2 8% 17% 25% 22%	2090
	21st to end	Night	23% 3 6% 17% 28% 22%	2464
November	1st to 10th	Night	23% 3 9% 22% 24% 19%	2432
	11th to 20th	Night	21% 4 7% 22% 25% 20%	2470
	21st to end	Night	29% 4 6% 17% 26% 17%	2470
December	1st to 10th	Night	26% 4 8% 21% 25% 16%	2470
	11th to 20th	Night	30% 3 6% 18% 23% 20%	3037
	21st to end	Night	26% 3 6% 16% 29% 21%	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	9% 14% 45% 23% 7% 1	270
	11th to 20th	Day	14% 12% 33% 30% 8% 3	270
	21st to end	Day	13% 11% 36% 29% 9% 1	297
February	1st to 10th	Day	16% 12% 41% 21% 8% 1	270
	11th to 20th	Day	17% 12% 45% 23% 3 1	270
	21st to end	Day	14% 11% 52% 19% 3	225
March	1st to 10th	Day	14% 11% 45% 25% 5%	270
	11th to 20th	Day	17% 11% 39% 23% 9% 1	270
	21st to end	Day	10% 17% 47% 21% 4 1	297
April	1st to 10th	Day	11% 16% 43% 25% 6%	390
	11th to 20th	Day	13% 16% 42% 23% 5% 1	452
	21st to end	Day	12% 15% 38% 28% 7% 1	460
May	1st to 10th	Day	9% 9% 43% 32% 6% 1	639
	11th to 20th	Day	13% 6% 33% 35% 11% 2	675
	21st to end	Day	9% 4 34% 40% 12% 1	734
June	1st to 10th	Day	4 1 20% 51% 21% 3	680
	11th to 20th	Day	4 1 29% 45% 18% 4	680
	21st to end	Day	3 1 11% 46% 30% 9%	680
July	1st to 10th	Day	5% 1 7% 39% 37% 11%	671
	11th to 20th	Day	2 5% 43% 41% 9%	668
	21st to end	Day	2 1 4 35% 43% 15%	689
August	1st to 10th	Day	2 5% 28% 47% 18%	487
	11th to 20th	Day	8% 1 2 33% 41% 15%	477
	21st to end	Day	4 1 4 30% 41% 19%	539
September	1st to 10th	Day	10% 1 4 27% 38% 19%	367
	11th to 20th	Day	7% 5% 26% 42% 19%	300
	21st to end	Day	13% 2 8% 27% 34% 16%	300
October	1st to 10th	Day	4 3 13% 35% 35% 10%	300
	11th to 20th	Day	10% 3 24% 42% 17% 3	300
	21st to end	Day	14% 8% 27% 29% 18% 3	330
November	1st to 10th	Day	16% 4 30% 36% 13% 1	276
	11th to 20th	Day	14% 11% 28% 28% 14% 5%	270
	21st to end	Day	14% 16% 39% 21% 7% 3	270
December	1st to 10th	Day	17% 6% 31% 33% 11% 3	270
	11th to 20th	Day	10% 13% 38% 30% 9%	270
	21st to end	Day	13% 12% 39% 29% 7%	297
January	1st to 10th	Night	10% 8% 43% 32% 7%	950
	11th to 20th	Night	11% 10% 38% 34% 7%	884
	21st to end	Night	11% 9% 44% 30% 6%	836
February	1st to 10th	Night	10% 10% 50% 24% 6%	760
	11th to 20th	Night	13% 13% 47% 24% 3	744
	21st to end	Night	12% 16% 50% 20% 2	628
March	1st to 10th	Night	11% 15% 49% 20% 4	760
	11th to 20th	Night	13% 11% 46% 27% 4	571
	21st to end	Night	7% 16% 53% 21% 3	627
April	1st to 10th	Night	5% 11% 56% 24% 3	570
	11th to 20th	Night	9% 12% 55% 22% 3	563
	21st to end	Night	8% 11% 55% 22% 4	570
May	1st to 10th	Night	5% 10% 55% 26% 3	570
	11th to 20th	Night	9% 7% 44% 32% 8%	561
	21st to end	Night	5% 5% 45% 37% 8%	617
June	1st to 10th	Night	2 2 34% 51% 10% 1	570
	11th to 20th	Night	2 1 42% 48% 6%	570
	21st to end	Night	3 1 21% 56% 19% 1	570
July	1st to 10th	Night	7% 1 10% 56% 24% 2	555
	11th to 20th	Night	2 1 12% 60% 24% 1	552
	21st to end	Night	1 8% 63% 26% 1	624
August	1st to 10th	Night	1 1 7% 58% 32% 2	567
	11th to 20th	Night	5% 1 6% 55% 30% 4	564
	21st to end	Night	4 1 8% 47% 36% 5%	627
September	1st to 10th	Night	8% 1 9% 47% 32% 3	570
	11th to 20th	Night	9% 1 9% 48% 31% 2	570
	21st to end	Night	11% 1 15% 48% 23% 2	570
October	1st to 10th	Night	5% 1 26% 48% 20%	570
	11th to 20th	Night	10% 4 31% 41% 13% 1	570
	21st to end	Night	10% 7% 32% 36% 13% 2	792
November	1st to 10th	Night	11% 4 33% 37% 14% 1	936
	11th to 20th	Night	6% 6% 37% 35% 14% 1	950
	21st to end	Night	12% 11% 36% 32% 9% 1	950
December	1st to 10th	Night	11% 6% 39% 37% 7% 1	950
	11th to 20th	Night	12% 11% 39% 34% 5%	950
	21st to end	Night	11% 9% 40% 33% 7%	1045

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