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 2026-07-31.

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% 6% 2 6% 4 4 4 4 6% 8% 43%	138230
	Night	15% 6% 2 6% 4 4 4 3 5% 7% 44%	134794
Transparency	Day	39% 5% 11% 21% 18% 6%	141928
	Night	39% 4 10% 19% 21% 7%	139168
Seeing	Day	17% 9% 23% 25% 18% 9%	17465
	Night	17% 8% 23% 30% 19% 3	27502

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	15% 6% 2 5% 3 3 3 2 4 5% 52%	5258
	11th to 20th	Day	12% 5% 2 3 3 2 3 2 3 3 63%	5280
	21st to end	Day	21% 7% 2 6% 4 4 3 2 4 5% 41%	5780
February	1st to 10th	Day	15% 5% 3 6% 4 4 4 4 5% 6% 44%	1958
	11th to 20th	Day	22% 6% 2 5% 4 3 4 3 4 5% 44%	2048
	21st to end	Day	19% 6% 2 6% 4 4 4 3 4 5% 43%	1730
March	1st to 10th	Day	21% 6% 2 6% 4 4 4 3 4 5% 41%	5033
	11th to 20th	Day	10% 5% 2 6% 4 3 3 3 4 6% 54%	5090
	21st to end	Day	18% 6% 2 4 3 3 4 3 4 5% 47%	5586
April	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 6% 8% 47%	2310
	11th to 20th	Day	12% 5% 1 5% 4 4 4 3 5% 7% 50%	2462
	21st to end	Day	12% 5% 2 4 4 4 5% 3 6% 7% 49%	2730
May	1st to 10th	Day	13% 7% 2 7% 5% 5% 5% 4 7% 8% 38%	7616
	11th to 20th	Day	14% 6% 3 7% 5% 4 5% 4 6% 8% 39%	7626
	21st to end	Day	16% 8% 3 7% 5% 5% 5% 4 6% 8% 33%	8391
June	1st to 10th	Day	10% 5% 2 7% 6% 5% 6% 5% 7% 10% 36%	2730
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 5% 8% 12% 40%	2730
	21st to end	Day	6% 4 2 7% 5% 5% 6% 6% 7% 9% 42%	2730
July	1st to 10th	Day	6% 5% 2 7% 5% 6% 6% 5% 8% 10% 39%	6030
	11th to 20th	Day	6% 5% 2 6% 5% 6% 7% 6% 8% 13% 37%	6030
	21st to end	Day	8% 7% 3 8% 6% 7% 7% 6% 9% 12% 27%	6554
August	1st to 10th	Day	18% 7% 3 7% 5% 5% 5% 4 7% 10% 28%	4234
	11th to 20th	Day	9% 5% 2 6% 5% 6% 6% 5% 8% 11% 37%	4174
	21st to end	Day	7% 4 1 5% 4 4 5% 5% 6% 8% 52%	4321
September	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 4 7% 9% 45%	2200
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 5% 8% 43%	2200
	21st to end	Day	16% 6% 3 5% 4 4 4 3 5% 6% 42%	2200
October	1st to 10th	Day	14% 5% 1 4 3 2 3 3 4 6% 55%	2140
	11th to 20th	Day	12% 5% 2 4 2 3 2 2 3 5% 61%	2000
	21st to end	Day	14% 4 2 5% 3 3 3 2 4 5% 58%	2200
November	1st to 10th	Day	14% 5% 2 4 3 3 3 2 4 6% 55%	2031
	11th to 20th	Day	14% 5% 2 4 2 3 4 3 3 6% 54%	1890
	21st to end	Day	15% 5% 2 4 3 3 4 3 4 5% 51%	1890
December	1st to 10th	Day	17% 6% 2 5% 3 3 3 2 4 5% 52%	3020
	11th to 20th	Day	20% 7% 2 5% 4 3 3 3 5% 6% 42%	3463
	21st to end	Day	22% 5% 2 4 3 2 2 3 5% 6% 46%	4565
January	1st to 10th	Night	15% 5% 2 4 2 3 2 2 3 4 57%	7635
	11th to 20th	Night	12% 6% 2 4 3 3 3 2 4 4 57%	7680
	21st to end	Night	20% 7% 2 5% 3 3 3 2 4 5% 46%	8401
February	1st to 10th	Night	15% 7% 2 5% 4 4 3 3 4 5% 48%	2656
	11th to 20th	Night	21% 7% 2 5% 4 4 3 2 4 4 44%	2458
	21st to end	Night	17% 7% 2 5% 4 3 4 3 5% 6% 44%	2076
March	1st to 10th	Night	19% 6% 2 5% 3 4 3 3 5% 7% 43%	5039
	11th to 20th	Night	10% 4 1 4 3 3 3 3 6% 8% 55%	5090
	21st to end	Night	16% 5% 2 5% 4 4 4 3 5% 6% 47%	5588
April	1st to 10th	Night	13% 6% 2 4 4 4 4 4 5% 6% 48%	2310
	11th to 20th	Night	11% 5% 2 4 4 3 2 3 5% 8% 52%	2122
	21st to end	Night	11% 5% 2 6% 4 4 4 3 5% 6% 51%	1890
May	1st to 10th	Night	13% 6% 3 8% 5% 4 5% 4 7% 10% 36%	5237
	11th to 20th	Night	17% 6% 2 6% 5% 4 4 4 6% 8% 38%	5234
	21st to end	Night	19% 8% 3 7% 5% 4 5% 4 6% 8% 32%	5761
June	1st to 10th	Night	11% 6% 2 7% 6% 5% 6% 5% 9% 9% 34%	1890
	11th to 20th	Night	10% 6% 3 6% 4 6% 5% 5% 7% 10% 38%	1890
	21st to end	Night	7% 5% 2 7% 6% 6% 7% 5% 9% 11% 37%	1890
July	1st to 10th	Night	9% 7% 3 9% 7% 6% 7% 6% 8% 10% 29%	4148
	11th to 20th	Night	8% 6% 3 7% 6% 7% 7% 6% 10% 13% 27%	4147
	21st to end	Night	12% 10% 3 10% 6% 7% 6% 5% 8% 10% 23%	4517
August	1st to 10th	Night	17% 9% 4 8% 5% 5% 6% 5% 7% 10% 24%	2915
	11th to 20th	Night	11% 8% 3 10% 6% 7% 6% 4 8% 9% 27%	2875
	21st to end	Night	6% 6% 2 6% 5% 5% 5% 5% 7% 8% 45%	3538
September	1st to 10th	Night	11% 5% 2 7% 4 5% 5% 4 6% 8% 43%	2200
	11th to 20th	Night	16% 7% 2 6% 5% 5% 4 5% 6% 8% 36%	2200
	21st to end	Night	16% 6% 3 6% 4 4 4 3 5% 6% 43%	2200
October	1st to 10th	Night	14% 5% 2 5% 4 3 3 3 4 6% 51%	2260
	11th to 20th	Night	13% 4 2 5% 3 3 3 3 4 6% 54%	2400
	21st to end	Night	14% 4 1 4 3 2 3 3 3 4 58%	2640
November	1st to 10th	Night	14% 6% 2 4 3 3 3 3 3 5% 54%	2482
	11th to 20th	Night	14% 5% 2 4 4 2 3 3 4 6% 53%	2730
	21st to end	Night	15% 6% 1 4 4 4 3 3 5% 5% 49%	2730
December	1st to 10th	Night	17% 6% 2 4 3 3 3 3 4 5% 53%	4380
	11th to 20th	Night	19% 7% 2 5% 3 3 4 3 4 5% 44%	4952
	21st to end	Night	21% 5% 2 4 3 3 4 3 4 6% 45%	6633

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	37% 2 8% 24% 24% 5%	5259
	11th to 20th	Day	29% 1 4 12% 39% 14%	5280
	21st to end	Day	48% 2 3 16% 20% 11%	5780
February	1st to 10th	Day	40% 3 7% 23% 22% 5%	1958
	11th to 20th	Day	44% 3 6% 20% 21% 6%	2058
	21st to end	Day	45% 2 6% 16% 23% 7%	1730
March	1st to 10th	Day	47% 2 6% 16% 19% 11%	5033
	11th to 20th	Day	32% 3 6% 14% 26% 20%	5090
	21st to end	Day	42% 3 5% 23% 22% 5%	5586
April	1st to 10th	Day	34% 4 10% 22% 25% 6%	2310
	11th to 20th	Day	35% 3 8% 23% 23% 9%	2462
	21st to end	Day	35% 3 10% 27% 20% 4	2730
May	1st to 10th	Day	43% 4 15% 24% 13% 2	7651
	11th to 20th	Day	43% 5% 15% 22% 13% 2	7630
	21st to end	Day	49% 6% 12% 17% 14% 2	8405
June	1st to 10th	Day	41% 6% 16% 24% 10% 2	2730
	11th to 20th	Day	34% 8% 20% 25% 13% 1	2730
	21st to end	Day	35% 9% 18% 22% 14% 3	2730
July	1st to 10th	Day	38% 9% 18% 24% 10% 2	5965
	11th to 20th	Day	35% 12% 31% 17% 4 1	5961
	21st to end	Day	46% 10% 17% 20% 6% 1	6370
August	1st to 10th	Day	50% 7% 16% 22% 5%	4221
	11th to 20th	Day	37% 8% 20% 20% 14% 2	4213
	21st to end	Day	29% 6% 15% 27% 19% 3	4321
September	1st to 10th	Day	35% 6% 15% 22% 19% 4	2200
	11th to 20th	Day	40% 5% 12% 19% 21% 3	2200
	21st to end	Day	43% 3 10% 21% 17% 6%	2200
October	1st to 10th	Day	28% 3 7% 20% 32% 10%	3491
	11th to 20th	Day	20% 1 5% 17% 31% 26%	3240
	21st to end	Day	28% 2 4 24% 26% 16%	3538
November	1st to 10th	Day	34% 3 7% 19% 28% 9%	2031
	11th to 20th	Day	33% 3 5% 24% 25% 9%	1890
	21st to end	Day	35% 3 8% 23% 24% 7%	1890
December	1st to 10th	Day	38% 2 5% 19% 27% 10%	3020
	11th to 20th	Day	45% 3 8% 18% 20% 7%	3460
	21st to end	Day	41% 1 7% 28% 20% 2	4565
January	1st to 10th	Night	33% 2 8% 23% 27% 6%	7636
	11th to 20th	Night	33% 2 4 16% 36% 10%	7680
	21st to end	Night	43% 2 4 17% 24% 10%	8401
February	1st to 10th	Night	39% 2 8% 21% 22% 7%	2656
	11th to 20th	Night	46% 2 5% 18% 23% 7%	2459
	21st to end	Night	41% 2 8% 17% 22% 8%	2076
March	1st to 10th	Night	42% 2 7% 15% 27% 8%	5039
	11th to 20th	Night	28% 3 7% 14% 30% 18%	5090
	21st to end	Night	40% 2 6% 22% 21% 9%	5588
April	1st to 10th	Night	36% 3 10% 20% 25% 7%	2310
	11th to 20th	Night	32% 2 7% 24% 25% 9%	2122
	21st to end	Night	35% 3 9% 27% 21% 4	1890
May	1st to 10th	Night	44% 4 14% 22% 14% 2	5264
	11th to 20th	Night	45% 4 14% 23% 13% 1	5235
	21st to end	Night	49% 6% 12% 20% 12% 2	5771
June	1st to 10th	Night	42% 6% 16% 21% 12% 2	1890
	11th to 20th	Night	39% 7% 18% 22% 12% 1	1890
	21st to end	Night	38% 9% 17% 18% 14% 4	1890
July	1st to 10th	Night	47% 9% 18% 19% 6% 1	4105
	11th to 20th	Night	43% 13% 26% 15% 3 1	4096
	21st to end	Night	54% 10% 16% 15% 5% 1	4387
August	1st to 10th	Night	53% 8% 18% 15% 5%	2904
	11th to 20th	Night	50% 8% 18% 16% 9%	2902
	21st to end	Night	34% 6% 13% 25% 19% 2	3538
September	1st to 10th	Night	38% 7% 14% 20% 17% 4	2200
	11th to 20th	Night	45% 7% 12% 18% 18% 2	2200
	21st to end	Night	42% 4 11% 19% 18% 6%	2200
October	1st to 10th	Night	31% 4 10% 18% 31% 7%	3689
	11th to 20th	Night	23% 2 5% 19% 27% 24%	3899
	21st to end	Night	30% 1 4 21% 27% 17%	4252
November	1st to 10th	Night	34% 4 6% 19% 26% 11%	2482
	11th to 20th	Night	35% 2 5% 18% 29% 12%	2730
	21st to end	Night	37% 3 4 22% 27% 8%	2730
December	1st to 10th	Night	36% 3 6% 19% 29% 8%	4380
	11th to 20th	Night	44% 3 9% 16% 19% 8%	4954
	21st to end	Night	42% 2 7% 26% 20% 2	6633

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	24% 18% 34% 22% 4	330
	11th to 20th	Day	27% 18% 32% 18% 4	330
	21st to end	Day	25% 18% 37% 16% 4	363
February	1st to 10th	Day	22% 18% 35% 19% 5% 1	330
	11th to 20th	Day	38% 15% 32% 12% 3	330
	21st to end	Day	29% 19% 33% 16% 3	273
March	1st to 10th	Day	22% 14% 34% 22% 7% 1	528
	11th to 20th	Day	21% 16% 34% 24% 4 1	540
	21st to end	Day	21% 15% 36% 22% 6% 1	594
April	1st to 10th	Day	17% 15% 32% 22% 11% 2	540
	11th to 20th	Day	15% 13% 34% 24% 12% 2	532
	21st to end	Day	15% 11% 27% 27% 13% 6%	540
May	1st to 10th	Day	12% 15% 34% 20% 14% 4	543
	11th to 20th	Day	17% 8% 17% 26% 24% 8%	567
	21st to end	Day	18% 5% 19% 23% 24% 12%	617
June	1st to 10th	Day	12% 5% 16% 30% 26% 12%	570
	11th to 20th	Day	10% 3 15% 31% 27% 15%	570
	21st to end	Day	9% 4 15% 29% 27% 16%	570
July	1st to 10th	Day	7% 3 9% 27% 30% 24%	565
	11th to 20th	Day	8% 2 6% 28% 34% 21%	564
	21st to end	Day	9% 2 11% 31% 31% 17%	616
August	1st to 10th	Day	22% 2 11% 23% 26% 16%	526
	11th to 20th	Day	12% 2 10% 29% 28% 19%	519
	21st to end	Day	13% 2 9% 22% 32% 22%	583
September	1st to 10th	Day	10% 4 12% 29% 29% 17%	530
	11th to 20th	Day	18% 2 14% 18% 30% 17%	530
	21st to end	Day	21% 4 19% 25% 24% 7%	530
October	1st to 10th	Day	18% 4 19% 28% 22% 9%	530
	11th to 20th	Day	13% 6% 25% 29% 22% 6%	530
	21st to end	Day	19% 9% 20% 28% 18% 6%	583
November	1st to 10th	Day	21% 7% 25% 27% 16% 4	509
	11th to 20th	Day	18% 15% 35% 23% 8% 2	330
	21st to end	Day	14% 12% 31% 29% 12% 2	330
December	1st to 10th	Day	30% 11% 34% 21% 5%	330
	11th to 20th	Day	25% 10% 33% 23% 8% 1	330
	21st to end	Day	27% 10% 32% 21% 8% 2	363
January	1st to 10th	Night	22% 15% 31% 26% 5%	840
	11th to 20th	Night	23% 15% 31% 27% 5%	840
	21st to end	Night	21% 15% 36% 26% 2	924
February	1st to 10th	Night	20% 14% 29% 33% 4	840
	11th to 20th	Night	26% 15% 34% 23% 2	823
	21st to end	Night	23% 14% 35% 25% 3	692
March	1st to 10th	Night	18% 15% 30% 31% 6%	840
	11th to 20th	Night	19% 15% 33% 26% 7% 1	840
	21st to end	Night	19% 11% 35% 27% 7%	924
April	1st to 10th	Night	16% 13% 33% 28% 9%	840
	11th to 20th	Night	14% 11% 30% 30% 14% 1	801
	21st to end	Night	14% 5% 28% 35% 17% 1	630
May	1st to 10th	Night	12% 8% 29% 34% 17% 1	630
	11th to 20th	Night	15% 6% 20% 33% 23% 2	621
	21st to end	Night	18% 4 12% 29% 32% 5%	683
June	1st to 10th	Night	14% 2 12% 34% 33% 6%	630
	11th to 20th	Night	12% 2 15% 30% 31% 10%	630
	21st to end	Night	8% 2 11% 35% 37% 7%	630
July	1st to 10th	Night	10% 1 8% 30% 39% 12%	615
	11th to 20th	Night	11% 2 7% 26% 44% 11%	612
	21st to end	Night	11% 2 9% 31% 39% 9%	687
August	1st to 10th	Night	17% 3 7% 26% 37% 11%	597
	11th to 20th	Night	16% 1 7% 28% 40% 8%	594
	21st to end	Night	12% 2 9% 24% 44% 8%	755
September	1st to 10th	Night	12% 3 10% 30% 39% 7%	800
	11th to 20th	Night	19% 2 12% 27% 34% 7%	800
	21st to end	Night	18% 4 16% 32% 28% 3	800
October	1st to 10th	Night	16% 4 17% 30% 29% 5%	800
	11th to 20th	Night	17% 6% 23% 31% 21% 2	800
	21st to end	Night	16% 7% 23% 33% 20% 2	880
November	1st to 10th	Night	17% 6% 23% 33% 18% 3	820
	11th to 20th	Night	17% 9% 29% 31% 13% 1	840
	21st to end	Night	17% 10% 28% 33% 12% 1	840
December	1st to 10th	Night	24% 11% 23% 34% 8%	840
	11th to 20th	Night	20% 10% 28% 32% 10%	840
	21st to end	Night	26% 10% 24% 32% 8%	924

Page generated at 2026-07-28 03:25:22. Version ac07

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