Sanders Preserve
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sanders Preserve clear sky chart from 2005-11-06 to 2025-11-03.

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	16% 8% 3 8% 5% 5% 5% 4 6% 7% 33%	126871
	Night	17% 8% 3 8% 5% 5% 5% 4 5% 7% 34%	123183
Transparency	Day	48% 6% 12% 22% 12% 1	130347
	Night	49% 5% 12% 20% 12% 1	127794
Seeing	Day	22% 16% 28% 21% 10% 3	16069
	Night	21% 14% 30% 26% 8% 1	25093

By Month

Percentage Cloud Cover NE (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	11% 10% 4 15% 7% 7% 6% 5% 6% 6% 24%	3597
	11th to 20th	Day	14% 11% 5% 12% 6% 5% 5% 4 6% 6% 25%	3570
	21st to end	Day	16% 10% 3 8% 6% 5% 5% 4 6% 6% 31%	4118
February	1st to 10th	Day	14% 11% 3 9% 7% 7% 7% 5% 6% 6% 25%	1800
	11th to 20th	Day	12% 10% 5% 10% 7% 6% 6% 5% 6% 6% 26%	1903
	21st to end	Day	18% 10% 4 9% 6% 5% 4 4 5% 6% 29%	1650
March	1st to 10th	Day	19% 10% 4 8% 5% 5% 6% 4 5% 5% 30%	4476
	11th to 20th	Day	16% 8% 3 8% 5% 4 4 4 6% 7% 37%	4745
	21st to end	Day	19% 8% 3 8% 6% 5% 4 3 4 5% 34%	5940
April	1st to 10th	Day	18% 9% 3 8% 5% 5% 4 3 4 7% 34%	2400
	11th to 20th	Day	17% 8% 2 7% 5% 5% 5% 4 5% 7% 35%	2374
	21st to end	Day	18% 7% 2 8% 5% 5% 5% 5% 6% 6% 33%	2482
May	1st to 10th	Day	31% 8% 3 7% 4 4 5% 4 5% 5% 25%	6060
	11th to 20th	Day	15% 8% 3 9% 6% 5% 6% 4 6% 7% 31%	6318
	21st to end	Day	18% 7% 2 8% 5% 5% 5% 4 6% 9% 31%	6949
June	1st to 10th	Day	16% 6% 2 7% 6% 5% 5% 5% 7% 8% 33%	2800
	11th to 20th	Day	16% 5% 2 6% 5% 5% 5% 5% 8% 9% 33%	2800
	21st to end	Day	15% 6% 3 7% 5% 4 5% 4 7% 8% 35%	2800
July	1st to 10th	Day	9% 6% 3 8% 5% 5% 6% 5% 8% 9% 36%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 10% 38%	6360
	21st to end	Day	13% 5% 2 6% 4 4 5% 5% 6% 8% 42%	6926
August	1st to 10th	Day	15% 5% 3 6% 5% 5% 5% 4 7% 8% 36%	4536
	11th to 20th	Day	13% 5% 3 7% 4 6% 6% 5% 7% 9% 35%	3864
	21st to end	Day	11% 5% 2 6% 5% 5% 6% 5% 7% 9% 40%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 5% 6% 4 6% 7% 45%	2400
	11th to 20th	Day	12% 5% 2 4 3 3 4 3 6% 7% 49%	2319
	21st to end	Day	15% 6% 2 6% 4 4 5% 3 6% 8% 41%	2200
October	1st to 10th	Day	19% 7% 2 7% 5% 5% 5% 4 5% 6% 36%	2070
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 3 6% 7% 38%	2000
	21st to end	Day	22% 8% 2 7% 5% 4 5% 3 5% 6% 32%	1997
November	1st to 10th	Day	14% 7% 3 6% 4 4 3 4 6% 7% 42%	1779
	11th to 20th	Day	15% 7% 3 6% 5% 5% 5% 4 6% 7% 37%	1800
	21st to end	Day	18% 9% 3 6% 5% 5% 5% 5% 6% 8% 31%	1620
December	1st to 10th	Day	18% 13% 5% 10% 6% 6% 5% 3 6% 6% 22%	2600
	11th to 20th	Day	21% 9% 3 7% 5% 5% 4 4 4 6% 30%	2986
	21st to end	Day	22% 13% 5% 10% 5% 4 4 3 5% 6% 23%	3960
January	1st to 10th	Night	13% 10% 4 11% 7% 6% 5% 5% 6% 7% 27%	6349
	11th to 20th	Night	11% 13% 5% 10% 6% 6% 6% 5% 6% 7% 25%	6315
	21st to end	Night	16% 10% 3 10% 6% 6% 5% 4 6% 6% 28%	6835
February	1st to 10th	Night	14% 11% 4 10% 7% 6% 5% 5% 6% 6% 27%	2600
	11th to 20th	Night	11% 10% 4 10% 6% 6% 4 5% 6% 7% 30%	2382
	21st to end	Night	17% 9% 3 9% 6% 5% 5% 4 6% 6% 31%	1980
March	1st to 10th	Night	19% 10% 3 9% 5% 5% 5% 5% 6% 7% 28%	5364
	11th to 20th	Night	16% 9% 3 6% 5% 5% 5% 4 5% 8% 36%	5209
	21st to end	Night	20% 9% 4 9% 5% 4 4 3 5% 5% 32%	4991
April	1st to 10th	Night	20% 8% 3 7% 5% 5% 5% 3 5% 6% 32%	2000
	11th to 20th	Night	19% 8% 3 7% 5% 4 5% 4 5% 5% 35%	1975
	21st to end	Night	20% 6% 2 6% 4 5% 4 4 5% 6% 39%	1915
May	1st to 10th	Night	29% 9% 3 7% 5% 4 5% 3 5% 6% 25%	3922
	11th to 20th	Night	15% 9% 3 9% 6% 5% 5% 4 6% 7% 30%	3575
	21st to end	Night	18% 8% 3 8% 5% 6% 5% 4 6% 8% 30%	3926
June	1st to 10th	Night	18% 6% 2 6% 5% 5% 4 4 5% 7% 39%	1600
	11th to 20th	Night	15% 6% 2 6% 4 4 5% 3 7% 8% 40%	1600
	21st to end	Night	13% 5% 2 6% 4 5% 5% 3 6% 8% 43%	1600
July	1st to 10th	Night	9% 6% 3 8% 5% 5% 6% 4 6% 8% 40%	3600
	11th to 20th	Night	12% 7% 3 8% 5% 5% 5% 4 6% 8% 37%	3600
	21st to end	Night	11% 6% 2 6% 4 4 4 4 6% 7% 47%	3926
August	1st to 10th	Night	18% 6% 3 5% 4 5% 5% 4 6% 8% 36%	2610
	11th to 20th	Night	17% 7% 3 5% 4 5% 5% 3 6% 7% 39%	3152
	21st to end	Night	13% 5% 2 5% 4 3 3 4 6% 9% 46%	3586
September	1st to 10th	Night	13% 5% 2 6% 4 4 4 4 5% 8% 46%	2000
	11th to 20th	Night	13% 5% 2 5% 4 3 4 3 4 7% 51%	2080
	21st to end	Night	15% 5% 2 5% 5% 4 4 4 5% 7% 44%	2200
October	1st to 10th	Night	20% 6% 3 7% 4 4 4 3 5% 6% 38%	2326
	11th to 20th	Night	15% 6% 2 7% 5% 5% 4 3 5% 7% 41%	2400
	21st to end	Night	22% 7% 2 6% 4 4 4 4 5% 8% 33%	2841
November	1st to 10th	Night	14% 7% 3 7% 4 5% 4 3 5% 7% 42%	2564
	11th to 20th	Night	17% 6% 2 7% 5% 5% 5% 4 5% 7% 37%	2600
	21st to end	Night	15% 9% 2 6% 5% 5% 4 4 6% 7% 36%	2776
December	1st to 10th	Night	16% 10% 4 9% 5% 5% 5% 5% 6% 8% 27%	4580
	11th to 20th	Night	23% 8% 3 7% 4 4 4 4 5% 5% 31%	5208
	21st to end	Night	25% 13% 4 8% 4 4 4 3 4 5% 27%	6996

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	57% 5% 10% 21% 6% 1	3598
	11th to 20th	Day	57% 6% 10% 14% 12% 2	3600
	21st to end	Day	52% 6% 9% 19% 13% 1	4118
February	1st to 10th	Day	57% 7% 12% 15% 8% 1	1800
	11th to 20th	Day	56% 7% 9% 18% 9% 2	1912
	21st to end	Day	55% 6% 11% 17% 11% 1	1650
March	1st to 10th	Day	55% 4 10% 15% 12% 4	4479
	11th to 20th	Day	47% 6% 17% 19% 8% 4	4745
	21st to end	Day	52% 5% 6% 16% 19% 3	5940
April	1st to 10th	Day	52% 5% 8% 21% 12% 2	2400
	11th to 20th	Day	48% 6% 10% 21% 14% 2	2374
	21st to end	Day	50% 6% 10% 18% 13% 3	2482
May	1st to 10th	Day	61% 6% 8% 16% 8% 1	6071
	11th to 20th	Day	51% 7% 13% 20% 9% 1	6322
	21st to end	Day	48% 7% 13% 23% 9% 1	6949
June	1st to 10th	Day	46% 8% 13% 21% 11% 1	2800
	11th to 20th	Day	44% 8% 15% 20% 12% 1	2800
	21st to end	Day	44% 7% 14% 22% 12% 1	2800
July	1st to 10th	Day	43% 6% 14% 23% 13% 1	6290
	11th to 20th	Day	40% 5% 15% 26% 13% 1	6285
	21st to end	Day	39% 4 13% 30% 13%	6728
August	1st to 10th	Day	44% 7% 16% 21% 11% 1	4522
	11th to 20th	Day	44% 5% 16% 27% 7%	3900
	21st to end	Day	38% 6% 14% 32% 11% 1	4312
September	1st to 10th	Day	37% 7% 13% 29% 14% 1	2400
	11th to 20th	Day	34% 6% 12% 28% 19% 1	2319
	21st to end	Day	40% 7% 12% 25% 14% 2	2200
October	1st to 10th	Day	44% 4 7% 26% 17% 1	3369
	11th to 20th	Day	38% 4 12% 28% 17% 1	3240
	21st to end	Day	37% 5% 15% 27% 15% 1	3197
November	1st to 10th	Day	40% 7% 15% 24% 13% 1	1779
	11th to 20th	Day	45% 6% 11% 21% 15% 2	1800
	21st to end	Day	51% 6% 18% 17% 7% 1	1620
December	1st to 10th	Day	63% 4 12% 17% 4	2600
	11th to 20th	Day	56% 4 13% 17% 9% 1	2986
	21st to end	Day	61% 4 9% 15% 11% 1	3960
January	1st to 10th	Night	57% 5% 11% 16% 10% 1	6349
	11th to 20th	Night	57% 5% 11% 14% 12% 1	6360
	21st to end	Night	55% 5% 10% 13% 14% 3	6835
February	1st to 10th	Night	55% 8% 12% 18% 7% 1	2600
	11th to 20th	Night	52% 7% 12% 18% 10% 1	2384
	21st to end	Night	53% 6% 14% 18% 8% 2	1980
March	1st to 10th	Night	55% 4 11% 14% 14% 2	5374
	11th to 20th	Night	47% 5% 18% 20% 8% 2	5210
	21st to end	Night	54% 5% 5% 19% 16% 2	4991
April	1st to 10th	Night	52% 5% 11% 20% 11% 1	2000
	11th to 20th	Night	49% 6% 10% 21% 13% 1	1975
	21st to end	Night	46% 6% 10% 18% 17% 3	1915
May	1st to 10th	Night	60% 5% 12% 14% 9% 1	3928
	11th to 20th	Night	51% 7% 11% 21% 8% 1	3576
	21st to end	Night	51% 8% 12% 17% 11% 1	3927
June	1st to 10th	Night	43% 7% 12% 23% 14% 1	1600
	11th to 20th	Night	40% 7% 16% 24% 12% 1	1600
	21st to end	Night	38% 8% 15% 25% 14% 1	1600
July	1st to 10th	Night	43% 4 15% 23% 15% 1	3560
	11th to 20th	Night	45% 5% 14% 21% 16%	3552
	21st to end	Night	37% 4 11% 34% 13% 1	3809
August	1st to 10th	Night	44% 7% 16% 24% 9% 1	2600
	11th to 20th	Night	45% 5% 17% 23% 10%	3182
	21st to end	Night	36% 4 13% 36% 11% 1	3586
September	1st to 10th	Night	37% 6% 12% 28% 16% 1	2000
	11th to 20th	Night	34% 5% 12% 28% 21% 1	2080
	21st to end	Night	40% 6% 12% 24% 17% 1	2200
October	1st to 10th	Night	47% 3 10% 25% 14% 1	3807
	11th to 20th	Night	34% 4 13% 27% 22% 1	3899
	21st to end	Night	41% 4 14% 29% 11%	4591
November	1st to 10th	Night	42% 6% 12% 23% 15% 2	2564
	11th to 20th	Night	46% 6% 12% 22% 13% 2	2600
	21st to end	Night	47% 6% 17% 22% 8% 1	2776
December	1st to 10th	Night	56% 4 12% 23% 5%	4580
	11th to 20th	Night	54% 5% 13% 14% 12% 1	5208
	21st to end	Night	60% 5% 9% 14% 10% 1	6996

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	29% 20% 31% 17% 4 1	200
	11th to 20th	Day	29% 24% 33% 13% 2 1	200
	21st to end	Day	22% 26% 34% 13% 5%	255
February	1st to 10th	Day	34% 27% 27% 11% 1	300
	11th to 20th	Day	30% 27% 30% 12% 1	451
	21st to end	Day	28% 21% 34% 14% 2	414
March	1st to 10th	Day	30% 21% 33% 13% 2	500
	11th to 20th	Day	28% 25% 30% 12% 5% 1	500
	21st to end	Day	32% 24% 22% 15% 5% 1	550
April	1st to 10th	Day	33% 24% 23% 14% 6% 1	500
	11th to 20th	Day	29% 25% 24% 15% 6% 2	492
	21st to end	Day	28% 21% 31% 13% 6% 1	500
May	1st to 10th	Day	32% 17% 26% 16% 7% 2	503
	11th to 20th	Day	18% 18% 33% 23% 7% 1	527
	21st to end	Day	18% 13% 29% 24% 13% 3	573
June	1st to 10th	Day	17% 16% 31% 21% 12% 3	530
	11th to 20th	Day	16% 14% 29% 25% 14% 2	530
	21st to end	Day	16% 14% 34% 24% 11% 2	530
July	1st to 10th	Day	13% 9% 26% 30% 17% 5%	525
	11th to 20th	Day	11% 9% 28% 33% 16% 3	524
	21st to end	Day	14% 8% 34% 29% 12% 3	576
August	1st to 10th	Day	11% 8% 27% 29% 20% 5%	526
	11th to 20th	Day	14% 8% 25% 30% 17% 6%	519
	21st to end	Day	12% 13% 26% 27% 18% 5%	583
September	1st to 10th	Day	16% 9% 26% 24% 17% 9%	530
	11th to 20th	Day	13% 10% 22% 22% 21% 13%	530
	21st to end	Day	18% 10% 25% 25% 15% 7%	530
October	1st to 10th	Day	23% 13% 25% 18% 15% 5%	530
	11th to 20th	Day	18% 19% 28% 22% 11% 2	530
	21st to end	Day	27% 14% 28% 23% 7% 1	470
November	1st to 10th	Day	24% 13% 27% 22% 12% 1	401
	11th to 20th	Day	25% 21% 28% 20% 6% 1	400
	21st to end	Day	27% 25% 25% 19% 5%	220
December	1st to 10th	Day	28% 21% 33% 14% 6%	200
	11th to 20th	Day	30% 23% 34% 12% 3	200
	21st to end	Day	25% 22% 33% 15% 4 1	220
January	1st to 10th	Night	22% 22% 34% 20% 2	800
	11th to 20th	Night	24% 23% 31% 20% 3	800
	21st to end	Night	18% 20% 37% 22% 2	880
February	1st to 10th	Night	24% 18% 37% 21% 1	800
	11th to 20th	Night	22% 22% 38% 17% 1	783
	21st to end	Night	26% 19% 33% 20% 2	660
March	1st to 10th	Night	24% 21% 35% 18% 2	800
	11th to 20th	Night	24% 23% 32% 18% 3	800
	21st to end	Night	24% 20% 31% 20% 4	880
April	1st to 10th	Night	26% 20% 31% 20% 3	800
	11th to 20th	Night	25% 17% 34% 19% 5%	791
	21st to end	Night	22% 20% 30% 23% 5%	715
May	1st to 10th	Night	25% 17% 30% 21% 6% 1	546
	11th to 20th	Night	18% 11% 35% 27% 7% 1	394
	21st to end	Night	16% 8% 29% 36% 11%	433
June	1st to 10th	Night	18% 7% 31% 34% 9% 1	400
	11th to 20th	Night	17% 6% 28% 38% 11%	400
	21st to end	Night	14% 6% 29% 39% 12%	400
July	1st to 10th	Night	12% 5% 25% 41% 17%	390
	11th to 20th	Night	13% 5% 22% 45% 15% 1	388
	21st to end	Night	13% 5% 26% 42% 14% 1	438
August	1st to 10th	Night	13% 3 22% 41% 19% 1	409
	11th to 20th	Night	15% 4 24% 34% 19% 4	757
	21st to end	Night	16% 6% 23% 36% 18% 2	880
September	1st to 10th	Night	13% 7% 23% 38% 18% 1	800
	11th to 20th	Night	16% 5% 23% 30% 23% 4	800
	21st to end	Night	18% 6% 27% 32% 16% 2	800
October	1st to 10th	Night	21% 10% 24% 29% 15% 2	800
	11th to 20th	Night	18% 14% 29% 31% 8%	800
	21st to end	Night	27% 11% 30% 27% 5%	880
November	1st to 10th	Night	19% 13% 30% 30% 8%	789
	11th to 20th	Night	21% 19% 29% 24% 7% 1	800
	21st to end	Night	22% 17% 31% 25% 5%	800
December	1st to 10th	Night	25% 18% 34% 21% 3	800
	11th to 20th	Night	24% 21% 32% 20% 3	800
	21st to end	Night	29% 20% 28% 20% 3	880

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