Overstreet Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Overstreet Observatory 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	14% 6% 2 6% 4 4 4 4 6% 8% 43%	100999
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 43%	97642
Transparency	Day	39% 4 12% 21% 18% 5%	100812
	Night	41% 4 10% 20% 20% 6%	97499
Seeing	Day	17% 8% 22% 25% 19% 9%	15093
	Night	17% 8% 22% 30% 20% 3	25141

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% 4 1 4 2 3 3 3 5% 7% 51%	2836
	11th to 20th	Day	15% 7% 2 4 3 3 3 3 3 3 53%	2840
	21st to end	Day	27% 5% 1 4 2 2 3 2 4 4 45%	3124
February	1st to 10th	Day	16% 5% 2 6% 4 4 4 4 6% 7% 43%	1771
	11th to 20th	Day	21% 6% 2 5% 4 3 4 3 3 5% 45%	1848
	21st to end	Day	20% 6% 2 6% 4 4 4 3 3 5% 41%	1570
March	1st to 10th	Day	25% 6% 2 6% 3 4 4 2 4 5% 41%	3424
	11th to 20th	Day	12% 4 1 5% 4 3 3 4 4 6% 52%	3480
	21st to end	Day	21% 5% 2 4 4 4 4 4 4 5% 42%	3815
April	1st to 10th	Day	11% 5% 1 5% 4 3 4 4 6% 8% 48%	2090
	11th to 20th	Day	13% 5% 1 5% 5% 4 4 3 5% 7% 47%	2225
	21st to end	Day	12% 5% 1 4 4 4 4 3 6% 7% 49%	2470
May	1st to 10th	Day	11% 7% 2 6% 5% 5% 5% 4 7% 8% 38%	4082
	11th to 20th	Day	17% 7% 3 7% 5% 4 4 3 5% 8% 37%	4081
	21st to end	Day	7% 5% 2 5% 4 5% 5% 4 7% 10% 45%	4476
June	1st to 10th	Day	10% 5% 2 7% 6% 5% 6% 5% 7% 10% 36%	2470
	11th to 20th	Day	6% 4 2 6% 5% 6% 7% 5% 8% 11% 41%	2470
	21st to end	Day	7% 5% 2 7% 5% 5% 7% 6% 7% 9% 41%	2470
July	1st to 10th	Day	7% 6% 2 8% 6% 6% 7% 5% 8% 10% 35%	4120
	11th to 20th	Day	7% 5% 2 6% 5% 6% 6% 5% 8% 11% 38%	4120
	21st to end	Day	10% 8% 3 10% 6% 7% 7% 6% 9% 11% 22%	4466
August	1st to 10th	Day	17% 7% 3 7% 5% 5% 5% 4 7% 10% 28%	4104
	11th to 20th	Day	9% 5% 2 6% 4 6% 6% 5% 8% 12% 37%	4044
	21st to end	Day	6% 4 1 5% 4 4 5% 4 6% 8% 53%	4189
September	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	2090
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 5% 8% 42%	2090
	21st to end	Day	16% 6% 3 5% 4 4 4 3 5% 6% 42%	2090
October	1st to 10th	Day	15% 5% 1 4 3 2 2 3 4 5% 56%	2033
	11th to 20th	Day	12% 4 2 4 2 3 2 2 3 6% 60%	1890
	21st to end	Day	13% 4 2 5% 3 3 3 3 3 5% 57%	1980
November	1st to 10th	Day	14% 4 2 3 3 3 3 2 4 6% 56%	1834
	11th to 20th	Day	15% 5% 2 4 3 3 4 3 4 5% 53%	1710
	21st to end	Day	16% 5% 2 4 3 3 4 3 5% 5% 50%	1710
December	1st to 10th	Day	20% 7% 2 6% 4 3 3 3 5% 6% 40%	1710
	11th to 20th	Day	18% 5% 2 4 3 3 2 3 4 5% 50%	2153
	21st to end	Day	21% 6% 2 4 4 3 3 3 6% 6% 43%	3124
January	1st to 10th	Night	18% 5% 2 4 3 4 3 3 4 5% 51%	4109
	11th to 20th	Night	13% 7% 2 5% 4 4 4 3 5% 5% 49%	4120
	21st to end	Night	25% 7% 2 4 3 2 2 2 3 4 45%	4532
February	1st to 10th	Night	16% 7% 2 4 4 4 3 3 4 5% 48%	2404
	11th to 20th	Night	21% 7% 2 5% 4 3 3 2 4 4 44%	2218
	21st to end	Night	17% 7% 2 5% 4 3 4 3 5% 6% 43%	1884
March	1st to 10th	Night	23% 7% 2 6% 4 4 3 3 5% 6% 37%	3428
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 5% 7% 52%	3480
	21st to end	Night	20% 5% 3 5% 4 4 4 3 5% 5% 43%	3817
April	1st to 10th	Night	12% 6% 2 4 3 4 4 4 5% 6% 49%	2090
	11th to 20th	Night	12% 5% 2 5% 4 3 2 3 5% 8% 50%	1918
	21st to end	Night	12% 5% 2 6% 4 4 4 3 5% 5% 51%	1710
May	1st to 10th	Night	13% 7% 3 7% 5% 3 4 3 6% 10% 39%	2813
	11th to 20th	Night	18% 6% 2 6% 4 4 4 4 5% 8% 39%	2812
	21st to end	Night	9% 4 2 6% 5% 4 5% 5% 7% 11% 41%	3079
June	1st to 10th	Night	11% 6% 2 6% 7% 5% 6% 5% 9% 10% 34%	1710
	11th to 20th	Night	10% 6% 3 5% 4 6% 5% 5% 7% 10% 38%	1710
	21st to end	Night	8% 5% 2 7% 6% 6% 7% 5% 9% 10% 35%	1710
July	1st to 10th	Night	10% 7% 3 9% 8% 6% 6% 6% 8% 9% 28%	2840
	11th to 20th	Night	8% 6% 3 6% 5% 6% 5% 7% 10% 14% 31%	2840
	21st to end	Night	12% 10% 4 10% 6% 7% 6% 6% 8% 10% 20%	3087
August	1st to 10th	Night	17% 8% 4 9% 5% 5% 6% 5% 8% 10% 24%	2825
	11th to 20th	Night	11% 8% 3 10% 6% 7% 6% 4 8% 10% 28%	2785
	21st to end	Night	6% 5% 2 6% 5% 5% 5% 5% 7% 8% 46%	3429
September	1st to 10th	Night	11% 6% 2 7% 4 5% 5% 4 6% 8% 43%	2090
	11th to 20th	Night	17% 8% 2 6% 5% 5% 4 5% 6% 7% 35%	2090
	21st to end	Night	16% 6% 3 6% 4 4 4 3 5% 6% 43%	2090
October	1st to 10th	Night	15% 4 2 5% 4 3 3 3 4 6% 51%	2147
	11th to 20th	Night	14% 4 2 5% 3 3 3 3 4 6% 54%	2275
	21st to end	Night	13% 5% 1 4 3 2 3 3 3 5% 58%	2376
November	1st to 10th	Night	14% 6% 2 4 3 3 3 3 3 5% 55%	2240
	11th to 20th	Night	15% 5% 2 4 4 2 3 3 4 6% 52%	2470
	21st to end	Night	15% 6% 1 4 4 4 4 3 6% 5% 48%	2470
December	1st to 10th	Night	20% 7% 2 5% 4 3 4 4 4 5% 43%	2470
	11th to 20th	Night	17% 6% 2 5% 3 2 3 2 4 5% 52%	3042
	21st to end	Night	21% 5% 2 5% 4 4 4 4 5% 7% 40%	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	35% 2 5% 27% 24% 7%	2837
	11th to 20th	Day	37% 2 5% 17% 33% 5%	2840
	21st to end	Day	44% 2 3 19% 26% 7%	3124
February	1st to 10th	Day	40% 3 7% 22% 22% 5%	1771
	11th to 20th	Day	44% 3 6% 20% 22% 6%	1858
	21st to end	Day	47% 2 6% 17% 21% 7%	1570
March	1st to 10th	Day	48% 2 6% 19% 15% 10%	3424
	11th to 20th	Day	34% 3 7% 15% 29% 12%	3480
	21st to end	Day	45% 3 6% 22% 22% 3	3815
April	1st to 10th	Day	33% 4 10% 21% 26% 6%	2090
	11th to 20th	Day	37% 3 8% 21% 22% 10%	2225
	21st to end	Day	34% 3 10% 28% 20% 5%	2470
May	1st to 10th	Day	43% 3 16% 21% 14% 3	4120
	11th to 20th	Day	46% 3 12% 23% 15% 1	4085
	21st to end	Day	34% 4 16% 25% 19% 2	4489
June	1st to 10th	Day	41% 6% 17% 25% 10% 2	2470
	11th to 20th	Day	34% 8% 19% 25% 13% 1	2470
	21st to end	Day	36% 9% 16% 21% 15% 3	2470
July	1st to 10th	Day	41% 9% 20% 20% 9% 2	4055
	11th to 20th	Day	37% 9% 28% 19% 5% 1	4051
	21st to end	Day	52% 10% 17% 14% 6% 1	4324
August	1st to 10th	Day	49% 7% 16% 23% 5%	4091
	11th to 20th	Day	37% 8% 20% 20% 14% 2	4083
	21st to end	Day	29% 6% 15% 28% 19% 3	4189
September	1st to 10th	Day	35% 6% 14% 22% 19% 4	2090
	11th to 20th	Day	41% 5% 12% 18% 21% 3	2090
	21st to end	Day	43% 3 9% 21% 18% 6%	2090
October	1st to 10th	Day	32% 3 9% 21% 29% 7%	2033
	11th to 20th	Day	29% 2 8% 24% 27% 10%	1890
	21st to end	Day	32% 2 5% 22% 29% 10%	1980
November	1st to 10th	Day	32% 3 7% 19% 30% 10%	1834
	11th to 20th	Day	35% 3 5% 23% 24% 9%	1710
	21st to end	Day	37% 3 7% 22% 23% 8%	1710
December	1st to 10th	Day	46% 3 6% 22% 17% 6%	1710
	11th to 20th	Day	38% 2 7% 20% 23% 10%	2150
	21st to end	Day	43% 2 5% 24% 23% 3	3124
January	1st to 10th	Night	37% 3 5% 20% 28% 7%	4109
	11th to 20th	Night	38% 3 5% 18% 30% 7%	4120
	21st to end	Night	46% 2 3 17% 25% 7%	4532
February	1st to 10th	Night	40% 2 9% 20% 23% 7%	2404
	11th to 20th	Night	46% 2 4 17% 23% 7%	2219
	21st to end	Night	42% 2 9% 17% 21% 8%	1884
March	1st to 10th	Night	50% 2 8% 17% 15% 8%	3428
	11th to 20th	Night	32% 3 7% 16% 27% 15%	3480
	21st to end	Night	45% 2 5% 20% 20% 8%	3817
April	1st to 10th	Night	35% 3 9% 19% 26% 7%	2090
	11th to 20th	Night	35% 2 7% 22% 24% 9%	1918
	21st to end	Night	35% 3 9% 26% 22% 5%	1710
May	1st to 10th	Night	42% 4 14% 23% 16% 2	2840
	11th to 20th	Night	45% 4 13% 23% 15% 1	2813
	21st to end	Night	34% 5% 13% 29% 16% 1	3087
June	1st to 10th	Night	42% 7% 16% 22% 11% 2	1710
	11th to 20th	Night	39% 6% 18% 22% 12% 2	1710
	21st to end	Night	40% 9% 16% 17% 14% 4	1710
July	1st to 10th	Night	49% 9% 18% 15% 7% 1	2795
	11th to 20th	Night	39% 11% 25% 20% 4 1	2786
	21st to end	Night	56% 10% 16% 13% 4 1	2988
August	1st to 10th	Night	52% 8% 19% 16% 5%	2814
	11th to 20th	Night	49% 8% 18% 16% 9%	2812
	21st to end	Night	34% 6% 13% 25% 19% 2	3429
September	1st to 10th	Night	38% 7% 14% 20% 17% 4	2090
	11th to 20th	Night	46% 6% 11% 18% 17% 2	2090
	21st to end	Night	42% 4 10% 19% 19% 6%	2090
October	1st to 10th	Night	35% 3 9% 19% 28% 5%	2147
	11th to 20th	Night	32% 3 8% 24% 22% 11%	2275
	21st to end	Night	32% 1 4 21% 29% 12%	2376
November	1st to 10th	Night	33% 4 6% 19% 26% 12%	2240
	11th to 20th	Night	37% 2 4 18% 27% 12%	2470
	21st to end	Night	38% 3 4 21% 27% 7%	2470
December	1st to 10th	Night	44% 3 7% 18% 21% 6%	2470
	11th to 20th	Night	37% 2 7% 18% 27% 9%	3044
	21st to end	Night	45% 3 6% 22% 20% 3	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% 16% 33% 24% 4	270
	11th to 20th	Day	28% 15% 34% 18% 5%	270
	21st to end	Day	25% 17% 37% 16% 5%	297
February	1st to 10th	Day	23% 18% 33% 19% 6% 1	270
	11th to 20th	Day	35% 16% 32% 14% 3	270
	21st to end	Day	28% 20% 35% 14% 3	225
March	1st to 10th	Day	21% 13% 35% 22% 8% 1	449
	11th to 20th	Day	20% 14% 36% 23% 5% 1	460
	21st to end	Day	22% 14% 36% 21% 7% 1	506
April	1st to 10th	Day	18% 14% 33% 21% 12% 2	460
	11th to 20th	Day	16% 13% 32% 24% 13% 2	452
	21st to end	Day	16% 11% 27% 25% 14% 6%	460
May	1st to 10th	Day	12% 14% 34% 21% 15% 5%	463
	11th to 20th	Day	16% 6% 17% 25% 25% 9%	487
	21st to end	Day	16% 5% 18% 23% 26% 13%	529
June	1st to 10th	Day	12% 4 14% 31% 26% 12%	490
	11th to 20th	Day	10% 3 11% 31% 28% 17%	490
	21st to end	Day	9% 4 15% 30% 25% 17%	490
July	1st to 10th	Day	9% 3 8% 26% 30% 25%	485
	11th to 20th	Day	8% 2 7% 29% 33% 21%	484
	21st to end	Day	8% 2 8% 33% 31% 17%	532
August	1st to 10th	Day	19% 2 11% 23% 28% 17%	486
	11th to 20th	Day	13% 1 9% 29% 28% 19%	479
	21st to end	Day	12% 1 8% 22% 33% 23%	539
September	1st to 10th	Day	10% 3 11% 27% 30% 18%	490
	11th to 20th	Day	19% 3 14% 18% 30% 16%	490
	21st to end	Day	21% 3 18% 25% 26% 7%	490
October	1st to 10th	Day	18% 4 19% 27% 22% 10%	490
	11th to 20th	Day	13% 7% 26% 27% 22% 6%	486
	21st to end	Day	18% 9% 20% 30% 18% 6%	495
November	1st to 10th	Day	21% 6% 22% 27% 18% 5%	432
	11th to 20th	Day	19% 13% 34% 24% 9% 1	270
	21st to end	Day	13% 9% 32% 31% 13% 3	270
December	1st to 10th	Day	29% 10% 33% 23% 5%	270
	11th to 20th	Day	25% 10% 32% 24% 9% 1	270
	21st to end	Day	27% 10% 32% 20% 8% 2	297
January	1st to 10th	Night	22% 15% 31% 27% 5%	760
	11th to 20th	Night	23% 14% 30% 28% 5%	760
	21st to end	Night	21% 14% 34% 28% 2	836
February	1st to 10th	Night	21% 13% 29% 33% 4	760
	11th to 20th	Night	26% 14% 34% 24% 2	743
	21st to end	Night	22% 14% 36% 25% 3	628
March	1st to 10th	Night	19% 13% 30% 31% 6%	760
	11th to 20th	Night	20% 14% 32% 26% 8% 1	760
	21st to end	Night	20% 11% 34% 27% 8%	836
April	1st to 10th	Night	16% 14% 33% 28% 9%	760
	11th to 20th	Night	15% 11% 29% 29% 15% 1	724
	21st to end	Night	15% 5% 28% 34% 18% 1	570
May	1st to 10th	Night	11% 8% 28% 34% 18% 1	570
	11th to 20th	Night	15% 5% 20% 33% 24% 2	561
	21st to end	Night	16% 2 12% 30% 34% 6%	617
June	1st to 10th	Night	14% 2 12% 34% 33% 6%	570
	11th to 20th	Night	12% 2 14% 29% 33% 11%	570
	21st to end	Night	8% 2 11% 34% 37% 7%	570
July	1st to 10th	Night	11% 1 7% 30% 39% 11%	555
	11th to 20th	Night	11% 2 7% 26% 44% 11%	552
	21st to end	Night	10% 2 8% 32% 40% 9%	624
August	1st to 10th	Night	16% 3 7% 26% 38% 11%	567
	11th to 20th	Night	15% 1 7% 29% 40% 8%	564
	21st to end	Night	12% 2 8% 24% 45% 9%	717
September	1st to 10th	Night	12% 3 9% 29% 40% 8%	760
	11th to 20th	Night	20% 2 12% 26% 34% 7%	760
	21st to end	Night	18% 4 16% 32% 29% 3	760
October	1st to 10th	Night	16% 4 17% 29% 29% 5%	760
	11th to 20th	Night	17% 6% 22% 31% 22% 3	759
	21st to end	Night	16% 7% 22% 33% 20% 2	792
November	1st to 10th	Night	17% 6% 23% 33% 19% 3	740
	11th to 20th	Night	18% 9% 28% 32% 13% 1	760
	21st to end	Night	18% 8% 26% 34% 13% 1	760
December	1st to 10th	Night	24% 11% 22% 35% 8%	760
	11th to 20th	Night	19% 10% 27% 32% 11%	760
	21st to end	Night	27% 10% 24% 32% 8%	836

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