Millbrook
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Millbrook clear sky chart from 2005-11-06 to 2026-08-30.

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% 10% 4 8% 5% 5% 5% 4 6% 7% 34%	144490
	Night	13% 9% 4 9% 5% 5% 5% 4 6% 7% 34%	137696
Transparency	Day	47% 7% 13% 21% 11% 1	148074
	Night	46% 7% 13% 21% 12% 1	142199
Seeing	Day	20% 14% 31% 21% 10% 3	16729
	Night	19% 12% 31% 27% 10% 1	25976

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	14% 12% 4 11% 6% 5% 5% 4 5% 6% 27%	4661
	11th to 20th	Day	7% 19% 8% 17% 9% 7% 5% 3 4 4 15%	4863
	21st to end	Day	12% 15% 7% 13% 8% 6% 5% 3 5% 5% 20%	5780
February	1st to 10th	Day	13% 17% 5% 12% 6% 5% 4 4 4 5% 25%	1890
	11th to 20th	Day	11% 14% 7% 14% 8% 5% 5% 3 4 5% 25%	1843
	21st to end	Day	16% 15% 5% 10% 7% 5% 4 3 4 5% 26%	1620
March	1st to 10th	Day	17% 12% 5% 10% 6% 4 3 3 4 6% 31%	4991
	11th to 20th	Day	11% 12% 5% 10% 6% 5% 5% 4 6% 7% 30%	5090
	21st to end	Day	17% 11% 3 8% 4 4 4 3 4 5% 37%	5620
April	1st to 10th	Day	17% 13% 4 8% 5% 5% 4 3 5% 5% 31%	2520
	11th to 20th	Day	19% 10% 3 7% 5% 4 3 3 4 6% 36%	2518
	21st to end	Day	19% 9% 3 8% 4 5% 5% 3 5% 5% 33%	2730
May	1st to 10th	Day	18% 8% 3 9% 5% 5% 5% 4 6% 6% 31%	7820
	11th to 20th	Day	13% 10% 4 8% 5% 4 4 3 5% 8% 36%	8221
	21st to end	Day	17% 7% 2 6% 4 3 4 3 5% 8% 40%	9060
June	1st to 10th	Day	16% 7% 2 7% 5% 5% 4 5% 6% 8% 36%	2940
	11th to 20th	Day	12% 6% 3 7% 4 5% 5% 4 7% 10% 39%	2940
	21st to end	Day	15% 7% 2 7% 4 5% 6% 4 6% 9% 37%	2940
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 5% 8% 10% 38%	8280
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 7% 10% 47%	8280
	21st to end	Day	8% 4 2 6% 5% 5% 6% 5% 7% 10% 43%	8703
August	1st to 10th	Day	15% 8% 3 7% 4 4 5% 4 5% 7% 37%	4364
	11th to 20th	Day	12% 6% 3 7% 4 4 5% 4 8% 10% 37%	4304
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 10% 40%	4537
September	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 3 5% 8% 43%	2400
	11th to 20th	Day	11% 5% 2 5% 3 4 5% 4 6% 8% 48%	2353
	21st to end	Day	14% 6% 2 8% 4 4 5% 4 5% 7% 39%	2115
October	1st to 10th	Day	18% 7% 2 7% 5% 4 4 5% 5% 6% 37%	2000
	11th to 20th	Day	19% 9% 4 8% 5% 4 5% 3 5% 7% 31%	2000
	21st to end	Day	22% 12% 4 8% 5% 4 4 3 6% 6% 27%	2200
November	1st to 10th	Day	15% 10% 3 7% 5% 4 5% 4 6% 6% 35%	1753
	11th to 20th	Day	14% 11% 4 10% 6% 5% 6% 4 7% 7% 28%	1680
	21st to end	Day	16% 16% 4 10% 6% 5% 4 3 6% 7% 24%	1680
December	1st to 10th	Day	16% 17% 6% 13% 8% 5% 5% 4 5% 5% 18%	2680
	11th to 20th	Day	16% 17% 5% 10% 6% 4 4 3 5% 5% 26%	3066
	21st to end	Day	21% 17% 5% 13% 6% 4 4 3 4 5% 18%	4048
January	1st to 10th	Night	13% 12% 5% 13% 6% 6% 5% 4 6% 6% 24%	8234
	11th to 20th	Night	9% 15% 6% 14% 9% 7% 6% 4 5% 7% 19%	8019
	21st to end	Night	11% 11% 6% 11% 6% 5% 6% 4 6% 7% 28%	8401
February	1st to 10th	Night	11% 14% 4 11% 7% 5% 6% 4 5% 6% 26%	2730
	11th to 20th	Night	12% 12% 5% 11% 7% 5% 4 4 6% 6% 28%	2663
	21st to end	Night	12% 13% 5% 9% 6% 5% 5% 4 4 7% 30%	2185
March	1st to 10th	Night	14% 11% 4 10% 6% 6% 4 3 5% 6% 32%	5067
	11th to 20th	Night	10% 8% 3 8% 5% 6% 6% 5% 7% 8% 36%	5089
	21st to end	Night	18% 8% 3 7% 4 4 4 3 4 6% 39%	5553
April	1st to 10th	Night	19% 9% 4 7% 5% 4 4 3 6% 7% 32%	2100
	11th to 20th	Night	16% 8% 3 7% 4 5% 5% 3 5% 7% 37%	2070
	21st to end	Night	16% 9% 3 7% 5% 4 5% 4 7% 6% 36%	1890
May	1st to 10th	Night	20% 8% 3 7% 4 4 4 4 6% 6% 34%	5063
	11th to 20th	Night	12% 8% 3 7% 4 4 5% 4 6% 7% 40%	4639
	21st to end	Night	17% 6% 2 6% 4 4 4 3 5% 8% 43%	5114
June	1st to 10th	Night	12% 6% 3 6% 4 5% 5% 5% 7% 8% 39%	1680
	11th to 20th	Night	9% 5% 2 6% 5% 4 4 5% 7% 9% 44%	1680
	21st to end	Night	11% 8% 2 7% 5% 4 4 3 6% 8% 41%	1680
July	1st to 10th	Night	6% 5% 2 7% 5% 4 5% 4 6% 10% 45%	4680
	11th to 20th	Night	7% 4 2 5% 4 4 5% 4 6% 9% 50%	4680
	21st to end	Night	9% 4 2 6% 4 4 4 4 5% 8% 50%	5357
August	1st to 10th	Night	9% 5% 2 6% 4 4 4 3 5% 8% 48%	3005
	11th to 20th	Night	12% 5% 2 6% 3 4 3 3 5% 8% 50%	2965
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 6% 9% 49%	3525
September	1st to 10th	Night	9% 6% 2 6% 5% 4 4 4 6% 7% 48%	2000
	11th to 20th	Night	10% 4 1 4 4 3 4 4 5% 7% 54%	2044
	21st to end	Night	15% 7% 2 7% 4 5% 4 3 4 6% 44%	2285
October	1st to 10th	Night	16% 6% 2 7% 5% 4 4 4 6% 7% 41%	2400
	11th to 20th	Night	18% 8% 3 7% 6% 4 5% 4 5% 7% 33%	2400
	21st to end	Night	20% 9% 3 8% 4 4 5% 4 6% 6% 32%	2640
November	1st to 10th	Night	15% 9% 3 7% 5% 4 5% 3 6% 7% 36%	2760
	11th to 20th	Night	15% 11% 4 9% 5% 5% 5% 5% 6% 7% 29%	2940
	21st to end	Night	18% 13% 4 9% 6% 5% 6% 5% 6% 6% 25%	2940
December	1st to 10th	Night	14% 15% 5% 11% 6% 6% 6% 4 7% 7% 20%	4720
	11th to 20th	Night	14% 16% 5% 11% 6% 5% 5% 4 5% 6% 23%	5348
	21st to end	Night	22% 15% 5% 13% 6% 5% 5% 3 4 5% 16%	7150

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	54% 7% 13% 17% 9% 1	4662
	11th to 20th	Day	70% 5% 10% 9% 6% 1	4894
	21st to end	Day	62% 6% 11% 12% 8% 1	5780
February	1st to 10th	Day	62% 5% 10% 16% 5% 1	1890
	11th to 20th	Day	61% 5% 9% 15% 9% 1	1852
	21st to end	Day	60% 6% 10% 16% 8% 1	1620
March	1st to 10th	Day	53% 5% 12% 14% 15% 1	4994
	11th to 20th	Day	51% 5% 17% 16% 8% 2	5090
	21st to end	Day	49% 5% 7% 19% 18% 2	5620
April	1st to 10th	Day	55% 5% 10% 17% 12% 2	2520
	11th to 20th	Day	50% 5% 9% 20% 16% 1	2518
	21st to end	Day	51% 6% 11% 18% 12% 2	2730
May	1st to 10th	Day	49% 7% 12% 18% 11% 3	7829
	11th to 20th	Day	46% 7% 11% 20% 15% 1	8225
	21st to end	Day	41% 6% 12% 27% 13% 2	9061
June	1st to 10th	Day	44% 7% 13% 21% 15% 1	2940
	11th to 20th	Day	39% 8% 13% 25% 14% 1	2940
	21st to end	Day	42% 8% 14% 22% 13% 1	2940
July	1st to 10th	Day	37% 8% 17% 25% 13% 1	8210
	11th to 20th	Day	28% 6% 14% 36% 14% 1	8205
	21st to end	Day	33% 8% 17% 31% 12%	8510
August	1st to 10th	Day	44% 8% 13% 21% 12% 1	4351
	11th to 20th	Day	39% 7% 16% 29% 9%	4343
	21st to end	Day	35% 8% 15% 31% 10% 1	4537
September	1st to 10th	Day	38% 8% 13% 30% 10% 1	2400
	11th to 20th	Day	32% 8% 13% 31% 15% 1	2353
	21st to end	Day	41% 10% 14% 25% 10% 1	2115
October	1st to 10th	Day	37% 7% 14% 28% 13% 1	3260
	11th to 20th	Day	41% 7% 16% 21% 14% 1	3240
	21st to end	Day	49% 7% 16% 20% 8%	3538
November	1st to 10th	Day	47% 7% 14% 20% 10% 1	1753
	11th to 20th	Day	52% 8% 14% 15% 9% 1	1680
	21st to end	Day	59% 7% 13% 15% 6%	1680
December	1st to 10th	Day	67% 6% 14% 10% 3	2680
	11th to 20th	Day	59% 8% 10% 14% 10%	3066
	21st to end	Day	68% 6% 9% 10% 7%	4048
January	1st to 10th	Night	57% 7% 13% 14% 9%	8234
	11th to 20th	Night	60% 8% 11% 13% 8% 1	8063
	21st to end	Night	52% 7% 11% 15% 14% 1	8401
February	1st to 10th	Night	58% 7% 10% 18% 7% 1	2730
	11th to 20th	Night	55% 7% 10% 18% 10%	2665
	21st to end	Night	53% 6% 12% 20% 8% 1	2185
March	1st to 10th	Night	51% 5% 11% 17% 13% 4	5077
	11th to 20th	Night	42% 7% 18% 21% 10% 1	5090
	21st to end	Night	45% 5% 10% 19% 18% 2	5553
April	1st to 10th	Night	50% 6% 10% 20% 13% 1	2100
	11th to 20th	Night	46% 6% 10% 19% 17% 2	2070
	21st to end	Night	46% 8% 11% 19% 15% 2	1890
May	1st to 10th	Night	49% 6% 10% 16% 15% 3	5070
	11th to 20th	Night	40% 8% 13% 22% 15% 2	4640
	21st to end	Night	40% 6% 12% 25% 16% 2	5115
June	1st to 10th	Night	40% 9% 10% 23% 16% 3	1680
	11th to 20th	Night	34% 7% 14% 23% 20% 2	1680
	21st to end	Night	39% 7% 13% 25% 14% 2	1680
July	1st to 10th	Night	33% 6% 19% 25% 15% 1	4640
	11th to 20th	Night	29% 6% 14% 33% 18% 1	4632
	21st to end	Night	31% 7% 14% 30% 18% 1	5235
August	1st to 10th	Night	34% 6% 14% 27% 16% 2	2994
	11th to 20th	Night	33% 5% 17% 31% 13% 1	2992
	21st to end	Night	30% 7% 13% 34% 14% 1	3525
September	1st to 10th	Night	34% 8% 15% 29% 13% 1	2000
	11th to 20th	Night	28% 7% 13% 33% 18% 1	2044
	21st to end	Night	41% 7% 10% 28% 13% 1	2285
October	1st to 10th	Night	31% 6% 11% 29% 23% 1	3920
	11th to 20th	Night	41% 5% 15% 28% 10%	3899
	21st to end	Night	40% 8% 16% 26% 9% 1	4252
November	1st to 10th	Night	45% 7% 14% 21% 12% 1	2760
	11th to 20th	Night	50% 8% 13% 17% 10% 1	2940
	21st to end	Night	56% 8% 15% 15% 5%	2940
December	1st to 10th	Night	58% 7% 17% 13% 4	4720
	11th to 20th	Night	59% 9% 10% 14% 9% 1	5348
	21st to end	Night	67% 8% 10% 9% 6%	7150

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% 25% 30% 15% 5% 1	220
	11th to 20th	Day	25% 29% 29% 12% 4 2	259
	21st to end	Day	25% 23% 32% 16% 3	363
February	1st to 10th	Day	28% 24% 32% 13% 3	330
	11th to 20th	Day	25% 28% 37% 10% 1	330
	21st to end	Day	34% 20% 37% 9%	336
March	1st to 10th	Day	24% 20% 37% 16% 3	540
	11th to 20th	Day	28% 20% 31% 17% 3 1	540
	21st to end	Day	33% 15% 33% 14% 5% 1	594
April	1st to 10th	Day	29% 20% 32% 14% 6% 1	540
	11th to 20th	Day	31% 17% 28% 14% 7% 3	532
	21st to end	Day	26% 14% 36% 15% 6% 1	540
May	1st to 10th	Day	23% 13% 34% 20% 7% 4	543
	11th to 20th	Day	18% 13% 33% 23% 11% 3	567
	21st to end	Day	16% 13% 31% 23% 13% 5%	617
June	1st to 10th	Day	17% 11% 35% 25% 10% 2	570
	11th to 20th	Day	14% 13% 30% 26% 14% 3	570
	21st to end	Day	18% 10% 32% 25% 11% 3	570
July	1st to 10th	Day	10% 7% 28% 32% 17% 5%	565
	11th to 20th	Day	13% 7% 28% 29% 16% 7%	564
	21st to end	Day	9% 6% 33% 34% 13% 5%	620
August	1st to 10th	Day	11% 6% 30% 30% 16% 7%	566
	11th to 20th	Day	10% 7% 27% 28% 19% 8%	559
	21st to end	Day	12% 7% 30% 27% 18% 7%	619
September	1st to 10th	Day	13% 10% 25% 25% 18% 9%	530
	11th to 20th	Day	10% 5% 26% 23% 22% 15%	530
	21st to end	Day	16% 11% 30% 23% 15% 5%	530
October	1st to 10th	Day	20% 11% 30% 18% 15% 5%	530
	11th to 20th	Day	22% 17% 34% 19% 7% 2	530
	21st to end	Day	27% 18% 29% 21% 5%	583
November	1st to 10th	Day	27% 17% 28% 21% 8%	320
	11th to 20th	Day	25% 22% 33% 14% 5% 1	220
	21st to end	Day	24% 22% 36% 14% 4	220
December	1st to 10th	Day	28% 19% 36% 13% 3 1	220
	11th to 20th	Day	27% 26% 33% 11% 3	220
	21st to end	Day	29% 22% 32% 13% 3	242
January	1st to 10th	Night	22% 21% 35% 16% 5%	840
	11th to 20th	Night	27% 16% 33% 19% 5% 1	840
	21st to end	Night	20% 15% 35% 25% 5%	924
February	1st to 10th	Night	21% 17% 39% 21% 2	840
	11th to 20th	Night	20% 18% 38% 20% 3	823
	21st to end	Night	20% 20% 37% 20% 3	692
March	1st to 10th	Night	21% 16% 37% 23% 4	840
	11th to 20th	Night	21% 17% 35% 22% 4 1	840
	21st to end	Night	20% 15% 35% 22% 7%	924
April	1st to 10th	Night	24% 13% 34% 24% 4	840
	11th to 20th	Night	20% 14% 34% 23% 8% 1	826
	21st to end	Night	20% 10% 32% 28% 10%	630
May	1st to 10th	Night	20% 8% 29% 29% 12% 2	564
	11th to 20th	Night	15% 9% 31% 34% 10%	414
	21st to end	Night	14% 7% 26% 37% 15% 1	455
June	1st to 10th	Night	13% 6% 31% 37% 13%	420
	11th to 20th	Night	9% 8% 28% 38% 16%	420
	21st to end	Night	14% 5% 27% 39% 15%	420
July	1st to 10th	Night	6% 4 22% 48% 19% 1	410
	11th to 20th	Night	8% 3 24% 40% 24% 1	408
	21st to end	Night	10% 2 27% 41% 18% 2	554
August	1st to 10th	Night	11% 3 16% 41% 27% 2	627
	11th to 20th	Night	9% 2 19% 39% 27% 4	624
	21st to end	Night	11% 3 25% 35% 24% 2	817
September	1st to 10th	Night	11% 5% 25% 36% 21% 3	800
	11th to 20th	Night	11% 4 21% 31% 27% 6%	800
	21st to end	Night	18% 8% 22% 30% 19% 4	800
October	1st to 10th	Night	18% 7% 28% 27% 17% 3	800
	11th to 20th	Night	21% 12% 33% 25% 9% 1	800
	21st to end	Night	23% 13% 32% 26% 5%	880
November	1st to 10th	Night	22% 14% 28% 26% 10% 1	820
	11th to 20th	Night	20% 16% 36% 20% 7% 1	840
	21st to end	Night	24% 15% 34% 21% 6%	840
December	1st to 10th	Night	24% 18% 34% 19% 4	840
	11th to 20th	Night	23% 18% 35% 19% 4	840
	21st to end	Night	30% 16% 34% 18% 2	924

Page generated at 2026-08-27 04:07:12. Version ac07

Copyright © 2026 Attilla Danko.