Blueberry Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blueberry Hill Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	15% 7% 2 6% 4 4 4 4 6% 7% 40%	149404
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 42%	142334
Transparency	Day	41% 5% 12% 23% 17% 3	152933
	Night	41% 5% 12% 23% 17% 3	146894
Seeing	Day	21% 14% 29% 22% 10% 3	17570
	Night	19% 12% 31% 28% 9% 1	26738

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	13% 7% 2 6% 4 3 3 4 5% 5% 48%	4661
	11th to 20th	Day	15% 7% 2 7% 4 3 3 2 4 5% 47%	4687
	21st to end	Day	16% 7% 2 6% 4 4 4 3 5% 6% 42%	5780
February	1st to 10th	Day	14% 8% 3 7% 5% 4 4 3 6% 6% 41%	1890
	11th to 20th	Day	17% 8% 3 6% 5% 4 5% 3 4 6% 40%	2043
	21st to end	Day	17% 9% 4 5% 5% 3 4 3 5% 5% 41%	1730
March	1st to 10th	Day	22% 7% 2 5% 3 3 3 3 3 5% 46%	4576
	11th to 20th	Day	11% 8% 3 6% 4 4 5% 4 6% 6% 45%	4991
	21st to end	Day	17% 6% 2 6% 3 4 4 3 5% 6% 43%	5996
April	1st to 10th	Day	17% 8% 2 6% 4 4 5% 4 5% 6% 40%	2520
	11th to 20th	Day	15% 6% 2 6% 4 4 4 4 6% 7% 42%	2494
	21st to end	Day	16% 6% 2 6% 5% 4 4 4 6% 7% 40%	2520
May	1st to 10th	Day	22% 8% 3 6% 4 4 4 3 5% 7% 34%	7518
	11th to 20th	Day	20% 7% 2 6% 3 4 4 3 5% 7% 39%	8180
	21st to end	Day	24% 7% 3 6% 3 4 4 3 5% 7% 34%	9060
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 9% 40%	2940
	11th to 20th	Day	12% 7% 2 6% 5% 5% 4 4 6% 9% 40%	2940
	21st to end	Day	11% 6% 2 6% 4 5% 6% 5% 6% 8% 40%	2940
July	1st to 10th	Day	12% 6% 2 7% 5% 5% 6% 5% 7% 9% 36%	8280
	11th to 20th	Day	9% 7% 2 7% 5% 5% 6% 5% 8% 11% 34%	8280
	21st to end	Day	10% 7% 3 7% 6% 5% 5% 5% 7% 10% 36%	8979
August	1st to 10th	Day	14% 7% 3 8% 6% 6% 6% 5% 8% 9% 28%	5925
	11th to 20th	Day	12% 6% 2 6% 5% 5% 6% 5% 7% 10% 36%	5488
	21st to end	Day	10% 6% 2 6% 4 5% 5% 6% 7% 10% 39%	6080
September	1st to 10th	Day	13% 6% 2 6% 3 5% 5% 3 5% 8% 44%	2520
	11th to 20th	Day	12% 6% 2 5% 3 4 4 4 5% 7% 48%	2457
	21st to end	Day	19% 8% 3 7% 4 5% 4 3 5% 5% 37%	2310
October	1st to 10th	Day	17% 6% 3 5% 3 3 3 3 4 7% 45%	2153
	11th to 20th	Day	17% 5% 2 4 3 3 4 3 4 6% 49%	2030
	21st to end	Day	17% 5% 2 5% 3 3 4 3 4 6% 48%	2095
November	1st to 10th	Day	13% 6% 2 4 4 3 3 3 4 7% 52%	1851
	11th to 20th	Day	15% 6% 2 5% 4 4 4 3 5% 6% 46%	1890
	21st to end	Day	15% 6% 2 6% 4 4 4 3 4 7% 45%	1806
December	1st to 10th	Day	13% 7% 2 6% 4 3 4 3 6% 8% 45%	2680
	11th to 20th	Day	20% 10% 3 4 3 3 3 2 3 5% 45%	3066
	21st to end	Day	16% 6% 2 5% 4 4 5% 4 6% 7% 41%	4048
January	1st to 10th	Night	12% 8% 3 5% 4 3 4 3 5% 6% 48%	8234
	11th to 20th	Night	15% 7% 2 6% 3 3 3 3 4 5% 48%	8195
	21st to end	Night	15% 8% 2 5% 3 4 4 3 4 5% 45%	8401
February	1st to 10th	Night	15% 9% 3 6% 4 3 4 3 4 6% 42%	2730
	11th to 20th	Night	18% 8% 3 6% 4 4 3 3 5% 7% 40%	2461
	21st to end	Night	17% 8% 3 7% 3 3 3 3 5% 5% 42%	2076
March	1st to 10th	Night	18% 9% 2 5% 4 3 3 2 4 5% 45%	5484
	11th to 20th	Night	12% 10% 3 5% 3 3 3 3 4 6% 48%	5183
	21st to end	Night	16% 7% 3 7% 4 4 4 3 5% 5% 42%	5177
April	1st to 10th	Night	17% 8% 3 6% 4 4 3 3 4 6% 41%	2100
	11th to 20th	Night	15% 7% 3 7% 4 4 4 3 4 7% 42%	2075
	21st to end	Night	17% 6% 2 5% 3 3 4 3 5% 7% 45%	2100
May	1st to 10th	Night	18% 8% 3 7% 4 4 5% 4 6% 9% 32%	5379
	11th to 20th	Night	19% 7% 3 7% 5% 4 4 3 5% 8% 36%	4722
	21st to end	Night	25% 8% 3 7% 4 4 4 3 4 7% 32%	5114
June	1st to 10th	Night	13% 5% 2 6% 4 4 4 3 6% 8% 44%	1680
	11th to 20th	Night	10% 8% 2 7% 6% 5% 5% 4 6% 7% 41%	1680
	21st to end	Night	8% 6% 2 5% 5% 6% 6% 5% 8% 10% 39%	1680
July	1st to 10th	Night	13% 6% 3 7% 5% 5% 5% 5% 6% 10% 35%	4680
	11th to 20th	Night	9% 7% 3 8% 5% 5% 5% 5% 8% 11% 33%	4680
	21st to end	Night	12% 7% 3 7% 4 4 5% 4 6% 9% 39%	5084
August	1st to 10th	Night	18% 6% 3 7% 5% 5% 6% 4 7% 8% 31%	4175
	11th to 20th	Night	13% 7% 2 7% 5% 4 5% 3 5% 8% 41%	4557
	21st to end	Night	10% 6% 3 5% 4 4 5% 4 5% 8% 46%	5045
September	1st to 10th	Night	13% 7% 2 6% 4 5% 4 4 5% 6% 45%	2100
	11th to 20th	Night	10% 6% 2 5% 3 4 3 3 5% 7% 52%	2163
	21st to end	Night	21% 7% 2 6% 4 3 4 3 5% 6% 39%	2310
October	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 6% 6% 46%	2463
	11th to 20th	Night	15% 5% 2 4 3 3 4 3 4 6% 50%	2437
	21st to end	Night	17% 6% 2 5% 3 3 3 3 4 6% 48%	2743
November	1st to 10th	Night	11% 5% 2 6% 4 4 3 3 4 7% 53%	2667
	11th to 20th	Night	15% 6% 3 5% 4 3 3 3 4 5% 48%	2730
	21st to end	Night	13% 6% 2 5% 4 4 4 4 6% 7% 45%	2811
December	1st to 10th	Night	13% 7% 2 5% 3 4 3 3 5% 6% 48%	4720
	11th to 20th	Night	22% 7% 2 5% 3 3 3 3 4 7% 41%	5348
	21st to end	Night	18% 8% 3 6% 4 4 4 3 4 6% 42%	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	38% 3 9% 27% 20% 3	4662
	11th to 20th	Day	41% 4 7% 23% 21% 6%	4717
	21st to end	Day	42% 4 12% 27% 14% 2	5780
February	1st to 10th	Day	43% 4 13% 25% 13% 2	1890
	11th to 20th	Day	45% 5% 10% 23% 15% 2	2052
	21st to end	Day	46% 5% 9% 23% 13% 5%	1730
March	1st to 10th	Day	42% 4 9% 23% 15% 7%	4579
	11th to 20th	Day	37% 5% 10% 27% 16% 5%	4991
	21st to end	Day	41% 4 9% 17% 23% 6%	5996
April	1st to 10th	Day	42% 6% 10% 20% 16% 5%	2520
	11th to 20th	Day	38% 6% 14% 22% 16% 3	2494
	21st to end	Day	41% 6% 10% 20% 18% 4	2520
May	1st to 10th	Day	49% 5% 13% 17% 14% 1	7529
	11th to 20th	Day	45% 5% 10% 25% 14% 1	8184
	21st to end	Day	49% 6% 9% 22% 13% 2	9061
June	1st to 10th	Day	39% 8% 12% 22% 17% 2	2940
	11th to 20th	Day	40% 8% 12% 24% 13% 3	2940
	21st to end	Day	39% 8% 13% 22% 16% 2	2940
July	1st to 10th	Day	43% 8% 14% 19% 15% 1	8210
	11th to 20th	Day	41% 6% 16% 23% 13% 1	8205
	21st to end	Day	40% 6% 11% 22% 21% 1	8781
August	1st to 10th	Day	50% 7% 17% 18% 7%	5912
	11th to 20th	Day	40% 7% 17% 25% 11% 1	5524
	21st to end	Day	37% 6% 13% 26% 16% 2	6080
September	1st to 10th	Day	38% 6% 14% 19% 20% 3	2520
	11th to 20th	Day	35% 5% 11% 24% 23% 3	2457
	21st to end	Day	49% 6% 8% 17% 18% 2	2310
October	1st to 10th	Day	36% 3 11% 18% 28% 5%	3439
	11th to 20th	Day	26% 3 10% 23% 31% 7%	3270
	21st to end	Day	26% 3 13% 26% 27% 5%	3359
November	1st to 10th	Day	33% 4 13% 27% 20% 3	1851
	11th to 20th	Day	38% 4 9% 23% 22% 3	1890
	21st to end	Day	39% 5% 13% 30% 12% 2	1806
December	1st to 10th	Day	38% 4 16% 26% 14% 2	2680
	11th to 20th	Day	43% 4 9% 29% 12% 3	3066
	21st to end	Day	40% 6% 12% 23% 17% 2	4048
January	1st to 10th	Night	37% 3 12% 24% 18% 5%	8234
	11th to 20th	Night	39% 3 9% 22% 19% 7%	8240
	21st to end	Night	41% 3 8% 25% 20% 3	8401
February	1st to 10th	Night	44% 4 14% 23% 13% 3	2730
	11th to 20th	Night	44% 4 9% 20% 18% 5%	2463
	21st to end	Night	43% 5% 8% 23% 17% 5%	2076
March	1st to 10th	Night	42% 3 8% 23% 13% 10%	5494
	11th to 20th	Night	38% 5% 14% 25% 14% 5%	5184
	21st to end	Night	44% 4 8% 16% 24% 4	5177
April	1st to 10th	Night	44% 5% 10% 19% 18% 5%	2100
	11th to 20th	Night	42% 5% 10% 20% 19% 3	2075
	21st to end	Night	39% 6% 11% 20% 20% 5%	2100
May	1st to 10th	Night	48% 6% 12% 20% 12% 2	5385
	11th to 20th	Night	47% 5% 12% 22% 13% 1	4723
	21st to end	Night	51% 6% 10% 19% 12% 1	5115
June	1st to 10th	Night	38% 8% 12% 22% 19% 2	1680
	11th to 20th	Night	41% 8% 13% 24% 13% 2	1680
	21st to end	Night	36% 8% 16% 22% 15% 2	1680
July	1st to 10th	Night	44% 8% 14% 21% 12% 1	4640
	11th to 20th	Night	42% 6% 19% 22% 10% 1	4632
	21st to end	Night	41% 5% 15% 21% 17% 1	4967
August	1st to 10th	Night	49% 7% 19% 16% 9% 1	4165
	11th to 20th	Night	41% 5% 14% 29% 9%	4587
	21st to end	Night	36% 6% 11% 27% 19% 1	5045
September	1st to 10th	Night	39% 7% 12% 20% 19% 2	2100
	11th to 20th	Night	32% 5% 12% 24% 23% 4	2163
	21st to end	Night	45% 6% 9% 21% 18% 2	2310
October	1st to 10th	Night	36% 5% 11% 23% 22% 3	3957
	11th to 20th	Night	25% 3 10% 27% 29% 6%	3936
	21st to end	Night	27% 3 11% 31% 25% 4	4429
November	1st to 10th	Night	33% 4 12% 25% 22% 5%	2667
	11th to 20th	Night	40% 4 9% 22% 21% 5%	2730
	21st to end	Night	38% 5% 12% 26% 18% 2	2811
December	1st to 10th	Night	37% 4 12% 30% 14% 3	4720
	11th to 20th	Night	44% 3 11% 23% 14% 4	5348
	21st to end	Night	44% 5% 11% 20% 17% 3	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	28% 22% 34% 12% 4	220
	11th to 20th	Day	36% 25% 25% 12% 1	226
	21st to end	Day	29% 26% 27% 13% 4 1	363
February	1st to 10th	Day	25% 27% 33% 12% 2 1	330
	11th to 20th	Day	32% 21% 33% 12% 2	531
	21st to end	Day	29% 21% 32% 14% 3	446
March	1st to 10th	Day	28% 20% 31% 17% 4	540
	11th to 20th	Day	24% 24% 34% 12% 4 1	540
	21st to end	Day	28% 22% 30% 15% 4 1	594
April	1st to 10th	Day	27% 21% 33% 12% 5% 1	540
	11th to 20th	Day	24% 19% 33% 16% 8% 2	532
	21st to end	Day	27% 17% 30% 19% 4 2	540
May	1st to 10th	Day	31% 17% 29% 14% 7% 1	543
	11th to 20th	Day	23% 12% 36% 20% 8% 2	567
	21st to end	Day	24% 8% 25% 23% 16% 4	617
June	1st to 10th	Day	16% 11% 28% 27% 15% 2	570
	11th to 20th	Day	17% 12% 31% 24% 13% 3	570
	21st to end	Day	12% 10% 30% 31% 14% 4	570
July	1st to 10th	Day	13% 5% 22% 33% 21% 7%	565
	11th to 20th	Day	8% 6% 27% 32% 22% 6%	564
	21st to end	Day	11% 8% 27% 34% 15% 5%	620
August	1st to 10th	Day	12% 5% 27% 31% 18% 7%	566
	11th to 20th	Day	14% 4 23% 31% 21% 7%	559
	21st to end	Day	12% 5% 27% 29% 17% 9%	627
September	1st to 10th	Day	16% 7% 31% 22% 16% 8%	570
	11th to 20th	Day	14% 6% 27% 31% 14% 8%	570
	21st to end	Day	26% 11% 24% 24% 11% 5%	570
October	1st to 10th	Day	22% 10% 26% 22% 12% 7%	570
	11th to 20th	Day	22% 14% 26% 24% 11% 4	542
	21st to end	Day	18% 14% 35% 20% 10% 2	525
November	1st to 10th	Day	17% 14% 35% 21% 11% 2	425
	11th to 20th	Day	25% 17% 31% 20% 7%	430
	21st to end	Day	24% 15% 34% 18% 8% 1	346
December	1st to 10th	Day	26% 24% 35% 9% 7%	220
	11th to 20th	Day	35% 17% 31% 13% 4	220
	21st to end	Day	31% 15% 30% 17% 5% 1	242
January	1st to 10th	Night	21% 19% 35% 21% 4	840
	11th to 20th	Night	25% 17% 33% 22% 3	840
	21st to end	Night	20% 18% 38% 23% 2	924
February	1st to 10th	Night	21% 16% 37% 24% 2	840
	11th to 20th	Night	26% 20% 38% 16% 1	823
	21st to end	Night	23% 21% 35% 19% 2	692
March	1st to 10th	Night	23% 17% 35% 21% 4	840
	11th to 20th	Night	22% 18% 36% 19% 5%	840
	21st to end	Night	22% 12% 41% 19% 5%	924
April	1st to 10th	Night	22% 19% 36% 18% 5%	840
	11th to 20th	Night	19% 14% 38% 25% 4	831
	21st to end	Night	20% 13% 34% 28% 5%	840
May	1st to 10th	Night	20% 13% 35% 25% 7%	672
	11th to 20th	Night	19% 11% 37% 28% 4	447
	21st to end	Night	19% 5% 24% 38% 12% 1	455
June	1st to 10th	Night	16% 5% 30% 39% 10% 1	420
	11th to 20th	Night	13% 5% 28% 40% 13%	420
	21st to end	Night	10% 5% 26% 46% 13%	420
July	1st to 10th	Night	10% 5% 21% 44% 18% 1	410
	11th to 20th	Night	7% 4 21% 47% 19% 2	408
	21st to end	Night	10% 3 25% 44% 17%	460
August	1st to 10th	Night	11% 2 17% 45% 22% 2	641
	11th to 20th	Night	13% 4 19% 39% 23% 2	830
	21st to end	Night	13% 4 19% 40% 22% 2	924
September	1st to 10th	Night	16% 3 24% 37% 17% 3	840
	11th to 20th	Night	13% 5% 21% 34% 23% 4	840
	21st to end	Night	25% 5% 21% 33% 16% 1	840
October	1st to 10th	Night	20% 6% 28% 27% 16% 3	840
	11th to 20th	Night	18% 10% 27% 32% 13% 1	813
	21st to end	Night	20% 10% 35% 25% 10%	880
November	1st to 10th	Night	15% 9% 36% 28% 11% 1	820
	11th to 20th	Night	20% 16% 29% 27% 8%	840
	21st to end	Night	18% 15% 34% 25% 7% 1	840
December	1st to 10th	Night	25% 14% 34% 20% 6% 1	840
	11th to 20th	Night	21% 16% 33% 24% 5%	840
	21st to end	Night	21% 15% 34% 27% 2	924

Page generated at 2026-10-11 05:06:31. Version ac07

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