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 2025-03-27.

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% 7% 44%	109521
	Night	15% 6% 2 5% 4 4 4 3 5% 7% 45%	109831
Transparency	Day	38% 4 11% 21% 20% 6%	113263
	Night	39% 4 9% 19% 22% 7%	114228
Seeing	Day	17% 8% 22% 24% 19% 9%	15613
	Night	17% 8% 22% 30% 19% 3	25773

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% 5% 1 4 3 3 3 2 4 5% 56%	4056
	11th to 20th	Day	12% 5% 2 4 3 2 3 2 3 3 60%	4060
	21st to end	Day	20% 6% 2 6% 3 3 3 2 4 4 46%	4466
February	1st to 10th	Day	15% 5% 2 6% 3 4 4 4 6% 7% 44%	1865
	11th to 20th	Day	22% 6% 2 5% 4 3 4 3 3 5% 44%	1948
	21st to end	Day	19% 6% 2 6% 4 4 4 3 4 5% 43%	1650
March	1st to 10th	Day	24% 6% 2 6% 3 4 4 2 4 5% 41%	3534
	11th to 20th	Day	11% 4 1 5% 4 3 3 4 5% 7% 53%	3590
	21st to end	Day	21% 5% 2 4 4 4 4 4 4 5% 43%	3881
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 4 5% 60%	1900
	21st to end	Day	13% 4 2 5% 3 3 3 3 4 5% 58%	2090
November	1st to 10th	Day	15% 5% 2 4 3 3 3 2 4 6% 54%	1932
	11th to 20th	Day	15% 5% 2 4 3 3 4 3 4 6% 53%	1800
	21st to end	Day	16% 4 2 4 3 3 4 3 5% 5% 51%	1800
December	1st to 10th	Day	16% 6% 2 4 3 3 3 2 4 5% 53%	2930
	11th to 20th	Day	20% 7% 2 6% 4 4 3 4 5% 6% 41%	3373
	21st to end	Day	23% 5% 2 4 3 2 2 3 5% 6% 46%	4466
January	1st to 10th	Night	14% 4 2 3 2 3 3 2 3 4 60%	5889
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 4 4 56%	5900
	21st to end	Night	19% 7% 2 4 3 2 3 2 4 5% 49%	6490
February	1st to 10th	Night	15% 6% 2 5% 4 4 3 3 4 5% 48%	2530
	11th to 20th	Night	22% 7% 2 5% 4 3 3 3 4 4 44%	2338
	21st to end	Night	16% 7% 2 5% 4 3 4 3 5% 6% 45%	1980
March	1st to 10th	Night	23% 7% 2 6% 4 4 3 3 4 6% 38%	3538
	11th to 20th	Night	11% 5% 2 4 3 3 4 3 5% 7% 52%	3590
	21st to end	Night	19% 5% 3 5% 4 4 4 3 4 5% 44%	3890
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%	2280
	21st to end	Night	13% 4 1 4 3 2 3 3 3 4 59%	2508
November	1st to 10th	Night	14% 6% 2 4 3 3 3 3 3 5% 53%	2361
	11th to 20th	Night	15% 5% 2 4 4 2 3 3 4 6% 52%	2600
	21st to end	Night	15% 6% 1 4 4 4 4 3 5% 5% 49%	2600
December	1st to 10th	Night	16% 5% 1 4 3 2 3 3 4 5% 53%	4250
	11th to 20th	Night	20% 7% 2 5% 3 3 4 3 4 5% 43%	4822
	21st to end	Night	21% 5% 2 4 3 3 4 3 4 6% 45%	6490

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	32% 2 5% 24% 31% 7%	4057
	11th to 20th	Day	31% 2 4 15% 38% 10%	4060
	21st to end	Day	43% 2 3 18% 24% 11%	4466
February	1st to 10th	Day	40% 3 7% 23% 21% 5%	1865
	11th to 20th	Day	45% 3 6% 19% 21% 6%	1958
	21st to end	Day	45% 2 6% 16% 23% 7%	1650
March	1st to 10th	Day	48% 2 6% 19% 16% 10%	3534
	11th to 20th	Day	33% 3 7% 15% 28% 13%	3590
	21st to end	Day	45% 3 5% 22% 22% 3	3881
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	28% 3 7% 20% 33% 11%	3384
	11th to 20th	Day	19% 1 5% 17% 31% 26%	3140
	21st to end	Day	27% 2 4 24% 26% 16%	3428
November	1st to 10th	Day	34% 2 7% 19% 28% 9%	1932
	11th to 20th	Day	34% 3 5% 24% 25% 9%	1800
	21st to end	Day	36% 3 8% 23% 24% 8%	1800
December	1st to 10th	Day	37% 2 4 19% 28% 10%	2930
	11th to 20th	Day	45% 3 7% 18% 19% 7%	3370
	21st to end	Day	41% 1 7% 28% 21% 2	4466
January	1st to 10th	Night	31% 2 4 20% 35% 8%	5889
	11th to 20th	Night	33% 2 4 16% 36% 8%	5900
	21st to end	Night	40% 2 3 17% 29% 9%	6490
February	1st to 10th	Night	39% 2 9% 21% 22% 6%	2530
	11th to 20th	Night	46% 2 5% 17% 23% 7%	2339
	21st to end	Night	40% 2 9% 17% 23% 9%	1980
March	1st to 10th	Night	49% 2 8% 16% 17% 8%	3538
	11th to 20th	Night	32% 3 7% 16% 27% 15%	3590
	21st to end	Night	44% 2 6% 20% 20% 8%	3890
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	31% 4 10% 17% 31% 7%	3576
	11th to 20th	Night	22% 2 5% 19% 27% 25%	3779
	21st to end	Night	29% 1 4 21% 27% 17%	4120
November	1st to 10th	Night	34% 4 7% 19% 25% 11%	2361
	11th to 20th	Night	36% 2 4 18% 28% 11%	2600
	21st to end	Night	37% 3 4 22% 27% 7%	2600
December	1st to 10th	Night	35% 3 5% 19% 30% 8%	4250
	11th to 20th	Night	44% 4 8% 17% 19% 8%	4824
	21st to end	Night	43% 2 7% 26% 20% 2	6490

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% 18% 34% 22% 4	300
	11th to 20th	Day	27% 18% 33% 18% 4	300
	21st to end	Day	24% 18% 37% 17% 4	330
February	1st to 10th	Day	21% 19% 35% 19% 6% 1	300
	11th to 20th	Day	37% 16% 31% 13% 3	300
	21st to end	Day	26% 20% 35% 17% 3	249
March	1st to 10th	Day	22% 14% 34% 21% 7% 1	489
	11th to 20th	Day	21% 15% 35% 24% 4 1	500
	21st to end	Day	22% 15% 36% 20% 6% 1	530
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% 25% 28% 22% 6%	490
	21st to end	Day	17% 8% 19% 29% 19% 6%	539
November	1st to 10th	Day	22% 7% 23% 26% 17% 4	470
	11th to 20th	Day	20% 13% 34% 24% 8% 2	300
	21st to end	Day	13% 10% 32% 30% 12% 3	300
December	1st to 10th	Day	28% 12% 33% 22% 5%	300
	11th to 20th	Day	25% 10% 33% 23% 8% 1	300
	21st to end	Day	28% 10% 32% 20% 8% 2	330
January	1st to 10th	Night	22% 16% 31% 26% 5%	800
	11th to 20th	Night	23% 15% 30% 28% 5%	800
	21st to end	Night	21% 15% 35% 27% 2	880
February	1st to 10th	Night	21% 13% 30% 33% 4	800
	11th to 20th	Night	26% 14% 34% 24% 2	783
	21st to end	Night	21% 14% 36% 26% 3	660
March	1st to 10th	Night	19% 14% 30% 31% 6%	800
	11th to 20th	Night	19% 14% 33% 26% 8% 1	800
	21st to end	Night	19% 12% 35% 26% 8%	863
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	760
	21st to end	Night	15% 7% 22% 33% 21% 2	836
November	1st to 10th	Night	17% 6% 23% 33% 19% 3	780
	11th to 20th	Night	18% 9% 28% 32% 13% 1	800
	21st to end	Night	17% 9% 27% 33% 13% 1	800
December	1st to 10th	Night	23% 12% 23% 34% 8%	800
	11th to 20th	Night	20% 10% 28% 32% 10%	800
	21st to end	Night	27% 10% 23% 32% 8%	880

Page generated at 2025-03-24 09:04:27. Version ac07

Copyright © 2025 Attilla Danko.