Mill Ridge
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mill Ridge 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	9% 8% 4 8% 6% 5% 6% 4 7% 8% 35%	138146
	Night	10% 9% 4 9% 5% 5% 5% 4 6% 8% 36%	134883
Transparency	Day	43% 10% 12% 19% 13% 3	141857
	Night	42% 10% 12% 19% 14% 3	139244
Seeing	Day	19% 16% 25% 21% 13% 5%	17301
	Night	18% 13% 22% 28% 16% 2	27333

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% 11% 5% 12% 6% 5% 4 3 5% 7% 34%	5258
	11th to 20th	Day	6% 9% 6% 13% 5% 5% 4 3 4 5% 41%	5280
	21st to end	Day	10% 12% 6% 9% 6% 4 4 4 4 6% 35%	5780
February	1st to 10th	Day	11% 10% 5% 10% 6% 5% 4 3 5% 7% 34%	1990
	11th to 20th	Day	13% 12% 6% 10% 5% 5% 5% 3 4 6% 32%	2048
	21st to end	Day	13% 12% 3 10% 4 4 5% 3 6% 5% 35%	1730
March	1st to 10th	Day	12% 9% 5% 9% 5% 5% 4 3 5% 6% 35%	4898
	11th to 20th	Day	9% 8% 3 7% 4 4 4 4 5% 7% 46%	5090
	21st to end	Day	11% 9% 3 8% 5% 4 4 4 5% 7% 41%	5586
April	1st to 10th	Day	9% 10% 3 5% 5% 5% 4 4 7% 9% 40%	2310
	11th to 20th	Day	13% 8% 2 6% 4 5% 4 4 6% 7% 42%	2539
	21st to end	Day	11% 8% 3 7% 5% 4 6% 4 6% 8% 39%	2730
May	1st to 10th	Day	9% 8% 4 10% 6% 6% 6% 5% 7% 8% 32%	7616
	11th to 20th	Day	10% 8% 3 8% 6% 7% 6% 5% 8% 8% 32%	7626
	21st to end	Day	10% 8% 3 10% 7% 7% 7% 6% 9% 9% 25%	8391
June	1st to 10th	Day	9% 7% 2 7% 6% 7% 7% 6% 8% 9% 32%	2730
	11th to 20th	Day	6% 6% 3 9% 6% 7% 7% 6% 9% 10% 32%	2730
	21st to end	Day	8% 6% 3 7% 6% 7% 8% 5% 9% 10% 32%	2730
July	1st to 10th	Day	5% 5% 3 7% 7% 6% 7% 6% 8% 10% 36%	6030
	11th to 20th	Day	8% 6% 3 9% 8% 8% 8% 5% 9% 10% 27%	6030
	21st to end	Day	9% 8% 4 12% 8% 7% 7% 6% 8% 10% 21%	6554
August	1st to 10th	Day	10% 6% 3 8% 7% 6% 8% 6% 9% 11% 26%	4234
	11th to 20th	Day	7% 6% 3 8% 7% 6% 7% 6% 9% 11% 30%	4174
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 4 6% 8% 46%	4330
September	1st to 10th	Day	8% 7% 3 9% 5% 5% 6% 4 7% 8% 37%	2200
	11th to 20th	Day	12% 8% 3 9% 6% 5% 5% 4 5% 8% 35%	2200
	21st to end	Day	13% 7% 3 7% 4 5% 6% 5% 6% 7% 37%	2200
October	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 5% 7% 46%	2175
	11th to 20th	Day	10% 7% 3 6% 3 3 3 4 5% 6% 50%	2000
	21st to end	Day	12% 8% 3 5% 3 3 4 3 5% 6% 49%	2200
November	1st to 10th	Day	9% 7% 2 5% 4 4 4 3 5% 6% 51%	1929
	11th to 20th	Day	9% 9% 2 7% 4 4 4 3 6% 7% 45%	1890
	21st to end	Day	10% 10% 4 6% 4 4 4 3 6% 7% 43%	1890
December	1st to 10th	Day	9% 9% 3 9% 5% 4 4 5% 6% 7% 40%	3020
	11th to 20th	Day	10% 11% 5% 9% 5% 6% 6% 5% 7% 7% 31%	3463
	21st to end	Day	11% 9% 6% 8% 4 5% 6% 4 6% 7% 33%	4565
January	1st to 10th	Night	10% 10% 5% 14% 7% 5% 4 3 5% 8% 31%	7635
	11th to 20th	Night	9% 8% 5% 13% 6% 5% 4 3 4 6% 37%	7680
	21st to end	Night	10% 12% 4 9% 5% 4 5% 3 5% 6% 36%	8401
February	1st to 10th	Night	10% 11% 4 9% 5% 5% 4 4 6% 6% 36%	2627
	11th to 20th	Night	15% 12% 4 11% 5% 4 4 3 5% 5% 32%	2458
	21st to end	Night	13% 12% 4 10% 5% 4 4 4 6% 5% 33%	2076
March	1st to 10th	Night	14% 12% 4 8% 5% 5% 5% 4 5% 6% 33%	5173
	11th to 20th	Night	8% 8% 3 6% 3 4 4 4 5% 7% 48%	5090
	21st to end	Night	9% 7% 3 7% 5% 4 4 4 7% 6% 42%	5588
April	1st to 10th	Night	10% 9% 3 6% 4 3 4 4 6% 8% 41%	2310
	11th to 20th	Night	10% 9% 3 6% 4 4 3 4 4 8% 44%	2046
	21st to end	Night	10% 8% 2 8% 6% 4 4 4 6% 6% 42%	1890
May	1st to 10th	Night	10% 8% 3 9% 5% 5% 5% 6% 9% 11% 30%	5237
	11th to 20th	Night	11% 9% 4 9% 6% 5% 5% 5% 6% 8% 32%	5234
	21st to end	Night	11% 11% 4 10% 7% 6% 6% 5% 7% 8% 25%	5761
June	1st to 10th	Night	8% 6% 3 7% 6% 5% 6% 5% 8% 12% 35%	1890
	11th to 20th	Night	4 6% 2 7% 5% 6% 5% 5% 9% 14% 37%	1890
	21st to end	Night	7% 6% 3 6% 5% 5% 5% 4 8% 12% 39%	1890
July	1st to 10th	Night	7% 6% 3 7% 5% 6% 7% 5% 8% 12% 34%	4148
	11th to 20th	Night	6% 5% 4 9% 7% 6% 7% 6% 10% 12% 29%	4147
	21st to end	Night	8% 8% 4 11% 8% 7% 7% 6% 8% 10% 22%	4517
August	1st to 10th	Night	7% 6% 3 9% 6% 5% 6% 5% 9% 12% 32%	2915
	11th to 20th	Night	7% 7% 3 10% 6% 5% 6% 4 8% 8% 35%	2875
	21st to end	Night	5% 4 2 6% 5% 4 5% 5% 8% 10% 47%	3530
September	1st to 10th	Night	9% 7% 2 8% 5% 5% 5% 5% 7% 8% 40%	2200
	11th to 20th	Night	11% 8% 4 8% 5% 5% 5% 4 6% 9% 36%	2200
	21st to end	Night	11% 7% 3 8% 5% 4 4 3 5% 6% 43%	2200
October	1st to 10th	Night	13% 7% 2 6% 4 3 4 3 5% 7% 48%	2224
	11th to 20th	Night	11% 7% 2 5% 4 4 4 3 5% 6% 48%	2400
	21st to end	Night	10% 10% 2 6% 3 4 3 3 4 6% 49%	2640
November	1st to 10th	Night	10% 7% 3 6% 4 4 4 4 5% 6% 47%	2586
	11th to 20th	Night	9% 10% 3 7% 4 3 4 3 5% 7% 45%	2730
	21st to end	Night	10% 10% 3 7% 4 3 3 4 5% 7% 44%	2730
December	1st to 10th	Night	9% 8% 4 12% 6% 5% 5% 4 5% 6% 36%	4380
	11th to 20th	Night	13% 11% 6% 13% 5% 4 4 3 4 6% 30%	4952
	21st to end	Night	11% 9% 5% 11% 5% 5% 5% 4 5% 8% 31%	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	46% 8% 11% 21% 13% 2	5259
	11th to 20th	Day	42% 6% 6% 14% 27% 5%	5280
	21st to end	Day	46% 8% 8% 19% 16% 4	5780
February	1st to 10th	Day	50% 7% 9% 16% 15% 3	1990
	11th to 20th	Day	53% 6% 9% 15% 14% 3	2058
	21st to end	Day	48% 6% 10% 15% 15% 5%	1730
March	1st to 10th	Day	47% 7% 9% 14% 16% 8%	4898
	11th to 20th	Day	37% 5% 8% 17% 20% 13%	5090
	21st to end	Day	42% 8% 8% 28% 12% 2	5586
April	1st to 10th	Day	39% 9% 12% 21% 16% 3	2310
	11th to 20th	Day	39% 9% 9% 20% 19% 4	2539
	21st to end	Day	41% 9% 12% 23% 13% 2	2730
May	1st to 10th	Day	45% 12% 12% 20% 10% 1	7651
	11th to 20th	Day	45% 11% 15% 19% 9%	7630
	21st to end	Day	50% 13% 12% 14% 10% 1	8405
June	1st to 10th	Day	43% 15% 16% 17% 9% 1	2730
	11th to 20th	Day	42% 14% 16% 15% 11% 1	2730
	21st to end	Day	42% 13% 15% 18% 10% 2	2730
July	1st to 10th	Day	39% 15% 16% 21% 9%	5965
	11th to 20th	Day	47% 15% 19% 15% 4 1	5961
	21st to end	Day	54% 15% 14% 11% 6%	6370
August	1st to 10th	Day	46% 15% 17% 19% 3	4221
	11th to 20th	Day	39% 14% 21% 16% 9% 1	4213
	21st to end	Day	33% 11% 14% 32% 9% 1	4330
September	1st to 10th	Day	41% 12% 12% 17% 15% 4	2200
	11th to 20th	Day	46% 9% 11% 19% 14% 1	2200
	21st to end	Day	44% 9% 13% 16% 14% 4	2200
October	1st to 10th	Day	28% 6% 9% 19% 26% 12%	3539
	11th to 20th	Day	26% 4 7% 25% 20% 18%	3240
	21st to end	Day	28% 6% 9% 26% 19% 13%	3538
November	1st to 10th	Day	33% 6% 9% 19% 26% 7%	1929
	11th to 20th	Day	36% 7% 7% 22% 23% 5%	1890
	21st to end	Day	40% 6% 9% 22% 18% 5%	1890
December	1st to 10th	Day	41% 9% 9% 17% 22% 3	3020
	11th to 20th	Day	49% 8% 10% 17% 14% 2	3460
	21st to end	Day	46% 9% 13% 21% 9% 2	4565
January	1st to 10th	Night	51% 8% 11% 18% 10% 2	7636
	11th to 20th	Night	46% 7% 6% 14% 22% 5%	7680
	21st to end	Night	47% 7% 8% 17% 18% 3	8401
February	1st to 10th	Night	47% 7% 10% 18% 16% 2	2627
	11th to 20th	Night	53% 6% 8% 16% 12% 4	2459
	21st to end	Night	50% 8% 9% 16% 13% 4	2076
March	1st to 10th	Night	48% 8% 9% 14% 16% 5%	5173
	11th to 20th	Night	33% 7% 9% 20% 22% 10%	5090
	21st to end	Night	40% 8% 13% 25% 12% 3	5588
April	1st to 10th	Night	38% 8% 15% 22% 14% 2	2310
	11th to 20th	Night	38% 8% 10% 22% 19% 3	2046
	21st to end	Night	39% 9% 13% 26% 12% 1	1890
May	1st to 10th	Night	42% 13% 15% 21% 8% 1	5264
	11th to 20th	Night	45% 11% 16% 21% 7%	5235
	21st to end	Night	51% 14% 12% 16% 6%	5771
June	1st to 10th	Night	38% 14% 21% 18% 10%	1890
	11th to 20th	Night	33% 15% 22% 20% 9% 1	1890
	21st to end	Night	35% 13% 19% 20% 11% 2	1890
July	1st to 10th	Night	40% 14% 17% 22% 6%	4105
	11th to 20th	Night	43% 18% 22% 14% 3 1	4096
	21st to end	Night	53% 17% 14% 11% 6%	4387
August	1st to 10th	Night	41% 14% 23% 18% 4	2904
	11th to 20th	Night	39% 15% 21% 17% 7%	2902
	21st to end	Night	29% 10% 18% 32% 9% 1	3530
September	1st to 10th	Night	40% 10% 15% 19% 13% 3	2200
	11th to 20th	Night	43% 10% 14% 17% 15% 1	2200
	21st to end	Night	39% 9% 12% 21% 16% 3	2200
October	1st to 10th	Night	31% 7% 12% 18% 22% 11%	3640
	11th to 20th	Night	25% 6% 9% 21% 23% 16%	3899
	21st to end	Night	30% 6% 6% 24% 20% 14%	4252
November	1st to 10th	Night	36% 8% 10% 20% 21% 5%	2586
	11th to 20th	Night	38% 6% 7% 21% 22% 6%	2730
	21st to end	Night	40% 7% 7% 24% 18% 4	2730
December	1st to 10th	Night	43% 10% 10% 15% 20% 2	4380
	11th to 20th	Night	55% 7% 8% 15% 12% 2	4954
	21st to end	Night	46% 10% 15% 17% 11% 1	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	35% 29% 27% 8% 1	330
	11th to 20th	Day	42% 23% 24% 11% 1	330
	21st to end	Day	34% 31% 27% 7% 2	363
February	1st to 10th	Day	27% 30% 32% 9% 1	330
	11th to 20th	Day	42% 31% 22% 5%	330
	21st to end	Day	38% 27% 28% 5% 1	273
March	1st to 10th	Day	28% 24% 29% 13% 6%	466
	11th to 20th	Day	29% 26% 26% 14% 4 1	540
	21st to end	Day	27% 25% 29% 15% 4	594
April	1st to 10th	Day	25% 22% 27% 17% 8% 1	540
	11th to 20th	Day	21% 22% 26% 20% 9% 2	532
	21st to end	Day	20% 18% 29% 20% 10% 3	540
May	1st to 10th	Day	23% 18% 30% 17% 9% 4	543
	11th to 20th	Day	19% 15% 26% 24% 11% 5%	567
	21st to end	Day	17% 12% 26% 23% 14% 8%	617
June	1st to 10th	Day	11% 11% 29% 25% 17% 7%	570
	11th to 20th	Day	8% 7% 29% 26% 22% 8%	570
	21st to end	Day	9% 9% 28% 27% 16% 10%	570
July	1st to 10th	Day	9% 5% 18% 29% 24% 15%	565
	11th to 20th	Day	10% 6% 16% 29% 26% 14%	564
	21st to end	Day	10% 6% 20% 28% 24% 12%	616
August	1st to 10th	Day	16% 8% 20% 25% 20% 12%	526
	11th to 20th	Day	11% 7% 20% 29% 23% 12%	519
	21st to end	Day	10% 4 17% 30% 24% 15%	583
September	1st to 10th	Day	9% 6% 18% 33% 23% 11%	530
	11th to 20th	Day	12% 7% 20% 24% 27% 10%	530
	21st to end	Day	19% 6% 22% 29% 19% 4	530
October	1st to 10th	Day	15% 9% 25% 26% 19% 5%	530
	11th to 20th	Day	16% 16% 25% 25% 16% 2	530
	21st to end	Day	21% 16% 24% 27% 10% 2	583
November	1st to 10th	Day	16% 20% 28% 25% 9% 2	407
	11th to 20th	Day	24% 21% 35% 16% 4	330
	21st to end	Day	26% 22% 33% 14% 5% 1	330
December	1st to 10th	Day	28% 31% 30% 9% 2	330
	11th to 20th	Day	28% 21% 32% 15% 5%	330
	21st to end	Day	26% 25% 34% 11% 3	363
January	1st to 10th	Night	26% 21% 28% 21% 3	840
	11th to 20th	Night	28% 20% 28% 20% 4	840
	21st to end	Night	23% 21% 33% 21% 2	924
February	1st to 10th	Night	22% 18% 30% 26% 3	840
	11th to 20th	Night	30% 23% 30% 16% 1	823
	21st to end	Night	27% 24% 31% 17% 1	692
March	1st to 10th	Night	22% 20% 30% 24% 4	840
	11th to 20th	Night	25% 18% 29% 20% 7%	840
	21st to end	Night	21% 18% 33% 22% 6%	924
April	1st to 10th	Night	22% 17% 29% 25% 7%	840
	11th to 20th	Night	18% 15% 28% 29% 9% 1	692
	21st to end	Night	15% 13% 25% 36% 10% 1	630
May	1st to 10th	Night	15% 13% 27% 29% 15% 1	630
	11th to 20th	Night	14% 8% 25% 32% 19% 2	621
	21st to end	Night	15% 5% 14% 35% 27% 4	683
June	1st to 10th	Night	9% 4 16% 41% 27% 4	630
	11th to 20th	Night	6% 5% 14% 37% 29% 7%	630
	21st to end	Night	8% 3 13% 39% 33% 4	630
July	1st to 10th	Night	11% 2 11% 30% 40% 6%	615
	11th to 20th	Night	9% 3 9% 29% 42% 8%	612
	21st to end	Night	9% 5% 8% 31% 38% 9%	687
August	1st to 10th	Night	10% 3 10% 34% 39% 5%	597
	11th to 20th	Night	11% 5% 11% 32% 36% 6%	594
	21st to end	Night	11% 3 9% 33% 37% 7%	695
September	1st to 10th	Night	11% 6% 12% 33% 32% 6%	800
	11th to 20th	Night	15% 5% 15% 31% 29% 5%	800
	21st to end	Night	16% 6% 17% 34% 25% 2	800
October	1st to 10th	Night	16% 7% 18% 30% 23% 6%	800
	11th to 20th	Night	15% 12% 22% 33% 16% 2	800
	21st to end	Night	17% 16% 22% 32% 12% 2	880
November	1st to 10th	Night	16% 11% 22% 33% 15% 3	820
	11th to 20th	Night	20% 15% 24% 30% 9% 1	840
	21st to end	Night	20% 17% 24% 28% 10% 1	840
December	1st to 10th	Night	23% 18% 28% 25% 5%	840
	11th to 20th	Night	21% 17% 27% 28% 6% 1	840
	21st to end	Night	25% 19% 28% 25% 3	924

Page generated at 2026-07-28 03:25:21. Version ac07

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