Quarry Road Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Quarry Road Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	18% 11% 4 8% 5% 4 4 4 5% 7% 30%	131778
	Night	19% 13% 4 10% 6% 4 4 3 4 5% 27%	131046
Transparency	Day	52% 8% 12% 18% 9% 1	135259
	Night	57% 7% 11% 16% 8% 1	135652
Seeing	Day	26% 20% 29% 17% 6% 2	16619
	Night	25% 17% 28% 22% 7% 1	25688

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% 15% 8% 17% 7% 5% 4 4 5% 4 18%	4661
	11th to 20th	Day	10% 17% 7% 18% 8% 5% 5% 3 3 5% 20%	4650
	21st to end	Day	11% 15% 6% 11% 8% 6% 6% 4 5% 5% 24%	5124
February	1st to 10th	Day	13% 14% 4 12% 8% 5% 5% 3 5% 5% 25%	1863
	11th to 20th	Day	12% 15% 6% 13% 6% 5% 4 3 4 6% 25%	2024
	21st to end	Day	16% 15% 5% 11% 7% 4 4 3 5% 5% 25%	1730
March	1st to 10th	Day	23% 15% 6% 11% 4 4 3 3 3 4 24%	4576
	11th to 20th	Day	15% 17% 4 8% 5% 4 4 3 5% 6% 30%	4773
	21st to end	Day	23% 11% 4 9% 4 4 4 3 4 5% 30%	6102
April	1st to 10th	Day	23% 15% 3 6% 3 4 3 3 5% 6% 30%	2520
	11th to 20th	Day	20% 12% 2 7% 5% 4 3 3 4 6% 34%	2494
	21st to end	Day	23% 11% 3 7% 5% 4 4 3 5% 5% 32%	2685
May	1st to 10th	Day	33% 10% 3 8% 4 4 4 3 4 5% 21%	6152
	11th to 20th	Day	18% 10% 3 7% 6% 5% 5% 4 6% 7% 28%	6402
	21st to end	Day	20% 9% 3 7% 5% 4 4 4 6% 8% 30%	6949
June	1st to 10th	Day	21% 8% 3 7% 5% 5% 5% 4 5% 7% 29%	2800
	11th to 20th	Day	18% 6% 2 6% 5% 5% 4 4 6% 9% 34%	2800
	21st to end	Day	19% 9% 3 7% 5% 5% 5% 5% 6% 7% 30%	2800
July	1st to 10th	Day	13% 6% 2 7% 5% 4 6% 5% 7% 10% 34%	6360
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 4 7% 10% 35%	6360
	21st to end	Day	15% 5% 2 5% 4 4 5% 4 7% 9% 41%	6926
August	1st to 10th	Day	17% 7% 2 6% 4 5% 5% 4 6% 9% 35%	4549
	11th to 20th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 8% 34%	3990
	21st to end	Day	15% 5% 2 6% 4 4 5% 4 6% 9% 41%	4312
September	1st to 10th	Day	14% 5% 1 5% 4 4 4 3 6% 8% 46%	2400
	11th to 20th	Day	14% 6% 2 5% 2 4 3 3 5% 7% 48%	2284
	21st to end	Day	18% 6% 2 7% 4 3 4 4 5% 7% 39%	2200
October	1st to 10th	Day	22% 9% 3 8% 4 4 4 3 4 5% 35%	2120
	11th to 20th	Day	18% 10% 3 7% 4 4 3 3 6% 7% 35%	1951
	21st to end	Day	26% 12% 3 6% 4 4 4 3 4 7% 27%	1980
November	1st to 10th	Day	17% 12% 3 8% 5% 3 4 3 5% 7% 33%	1851
	11th to 20th	Day	18% 16% 3 8% 5% 4 4 3 5% 5% 29%	1890
	21st to end	Day	18% 15% 4 10% 4 5% 4 4 5% 6% 26%	1706
December	1st to 10th	Day	19% 18% 6% 11% 6% 4 4 4 5% 6% 17%	2680
	11th to 20th	Day	23% 16% 5% 10% 4 4 3 3 4 5% 24%	3066
	21st to end	Day	21% 18% 5% 10% 4 5% 5% 3 4 4 19%	4048
January	1st to 10th	Night	15% 18% 8% 18% 6% 4 3 2 3 4 20%	8234
	11th to 20th	Night	12% 21% 8% 18% 8% 5% 4 3 4 4 13%	8235
	21st to end	Night	13% 14% 6% 14% 9% 7% 6% 4 4 5% 18%	9057
February	1st to 10th	Night	15% 15% 5% 13% 7% 6% 4 3 5% 5% 22%	2756
	11th to 20th	Night	13% 16% 5% 13% 8% 5% 4 4 5% 5% 23%	2481
	21st to end	Night	15% 16% 5% 11% 6% 4 3 3 4 4 28%	2076
March	1st to 10th	Night	22% 14% 4 9% 6% 5% 4 3 3 5% 25%	5484
	11th to 20th	Night	16% 16% 4 9% 5% 4 4 3 4 6% 30%	5402
	21st to end	Night	21% 16% 4 10% 5% 3 3 2 3 4 28%	5072
April	1st to 10th	Night	23% 13% 4 7% 4 4 4 3 4 5% 30%	2100
	11th to 20th	Night	25% 12% 4 7% 4 4 3 3 4 5% 30%	2075
	21st to end	Night	22% 10% 4 7% 5% 3 3 3 4 5% 34%	1933
May	1st to 10th	Night	33% 11% 3 7% 4 4 3 3 5% 6% 21%	4050
	11th to 20th	Night	21% 10% 4 9% 5% 5% 4 3 5% 6% 28%	3623
	21st to end	Night	22% 9% 2 7% 4 5% 4 4 6% 7% 30%	3926
June	1st to 10th	Night	21% 8% 2 8% 5% 4 5% 3 6% 7% 32%	1600
	11th to 20th	Night	20% 7% 2 5% 4 4 4 4 5% 8% 37%	1600
	21st to end	Night	20% 7% 3 7% 5% 4 4 4 6% 7% 33%	1600
July	1st to 10th	Night	14% 6% 2 7% 5% 5% 5% 5% 7% 8% 35%	3600
	11th to 20th	Night	19% 7% 2 7% 5% 5% 5% 4 6% 7% 35%	3600
	21st to end	Night	12% 7% 3 7% 5% 4 4 4 5% 7% 42%	3926
August	1st to 10th	Night	21% 8% 2 6% 5% 4 4 3 4 6% 37%	2586
	11th to 20th	Night	16% 7% 3 7% 4 5% 4 3 6% 8% 37%	3022
	21st to end	Night	13% 6% 2 5% 4 4 4 4 6% 8% 42%	3586
September	1st to 10th	Night	16% 6% 2 5% 3 3 4 3 5% 7% 46%	2000
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 4 6% 51%	2115
	21st to end	Night	17% 6% 2 5% 5% 4 4 4 5% 7% 42%	2200
October	1st to 10th	Night	23% 8% 3 7% 4 4 4 3 5% 6% 34%	2276
	11th to 20th	Night	21% 9% 3 7% 5% 4 5% 4 4 6% 32%	2447
	21st to end	Night	25% 12% 3 7% 5% 4 5% 3 4 6% 27%	2860
November	1st to 10th	Night	20% 12% 4 8% 5% 4 4 3 4 4 32%	2667
	11th to 20th	Night	20% 14% 4 8% 5% 4 3 3 4 5% 31%	2730
	21st to end	Night	19% 16% 6% 8% 5% 4 4 3 6% 6% 25%	2909
December	1st to 10th	Night	19% 24% 9% 13% 6% 4 4 2 3 4 12%	4720
	11th to 20th	Night	25% 16% 4 9% 5% 3 3 3 3 4 24%	5348
	21st to end	Night	23% 17% 5% 10% 6% 6% 4 3 3 4 19%	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	66% 7% 11% 13% 3	4662
	11th to 20th	Day	68% 6% 7% 11% 7% 1	4680
	21st to end	Day	60% 7% 10% 13% 9% 1	5124
February	1st to 10th	Day	62% 6% 10% 15% 6% 2	1863
	11th to 20th	Day	60% 8% 9% 16% 7% 1	2033
	21st to end	Day	59% 8% 10% 12% 10% 1	1730
March	1st to 10th	Day	64% 6% 9% 10% 8% 4	4579
	11th to 20th	Day	53% 7% 15% 14% 8% 2	4773
	21st to end	Day	55% 7% 7% 12% 16% 2	6102
April	1st to 10th	Day	57% 5% 9% 18% 10% 2	2520
	11th to 20th	Day	52% 7% 9% 17% 14% 1	2494
	21st to end	Day	55% 6% 9% 17% 11% 2	2685
May	1st to 10th	Day	66% 7% 7% 14% 6% 1	6163
	11th to 20th	Day	52% 9% 14% 15% 8% 1	6406
	21st to end	Day	50% 9% 12% 21% 8% 1	6949
June	1st to 10th	Day	53% 8% 14% 16% 9% 1	2800
	11th to 20th	Day	47% 8% 12% 19% 12% 2	2800
	21st to end	Day	52% 8% 11% 19% 9% 1	2800
July	1st to 10th	Day	43% 8% 17% 23% 9%	6290
	11th to 20th	Day	42% 9% 15% 24% 9%	6285
	21st to end	Day	40% 8% 14% 31% 7%	6728
August	1st to 10th	Day	46% 8% 16% 21% 8%	4535
	11th to 20th	Day	47% 8% 17% 22% 5%	4029
	21st to end	Day	38% 9% 19% 23% 11% 1	4312
September	1st to 10th	Day	37% 7% 14% 30% 12% 1	2400
	11th to 20th	Day	36% 5% 11% 30% 17% 1	2284
	21st to end	Day	42% 8% 15% 23% 11% 2	2200
October	1st to 10th	Day	46% 7% 9% 25% 12% 1	3458
	11th to 20th	Day	46% 5% 15% 21% 12%	3154
	21st to end	Day	45% 7% 12% 27% 8% 1	3180
November	1st to 10th	Day	51% 7% 13% 19% 9%	1851
	11th to 20th	Day	57% 6% 12% 13% 11% 2	1890
	21st to end	Day	59% 9% 14% 14% 4	1706
December	1st to 10th	Day	67% 9% 11% 10% 3	2680
	11th to 20th	Day	63% 6% 9% 15% 6%	3066
	21st to end	Day	65% 6% 8% 13% 7% 1	4048
January	1st to 10th	Night	69% 5% 12% 10% 3	8234
	11th to 20th	Night	74% 7% 7% 7% 4 1	8280
	21st to end	Night	66% 6% 8% 11% 7% 1	9057
February	1st to 10th	Night	64% 6% 10% 13% 7% 1	2756
	11th to 20th	Night	62% 7% 9% 14% 8% 1	2483
	21st to end	Night	59% 7% 11% 16% 7% 2	2076
March	1st to 10th	Night	62% 6% 8% 12% 10% 1	5494
	11th to 20th	Night	54% 7% 16% 14% 7% 2	5403
	21st to end	Night	59% 7% 6% 13% 13% 2	5072
April	1st to 10th	Night	57% 6% 8% 16% 11% 1	2100
	11th to 20th	Night	56% 6% 9% 17% 11% 1	2075
	21st to end	Night	52% 7% 8% 18% 12% 3	1933
May	1st to 10th	Night	63% 7% 8% 13% 7% 1	4056
	11th to 20th	Night	55% 8% 10% 17% 9% 1	3624
	21st to end	Night	52% 8% 12% 16% 11% 1	3927
June	1st to 10th	Night	50% 9% 12% 17% 12% 1	1600
	11th to 20th	Night	46% 8% 15% 19% 11% 2	1600
	21st to end	Night	49% 8% 13% 21% 9% 1	1600
July	1st to 10th	Night	43% 8% 19% 20% 9%	3560
	11th to 20th	Night	46% 9% 12% 22% 10%	3552
	21st to end	Night	40% 8% 15% 30% 8%	3809
August	1st to 10th	Night	49% 7% 13% 22% 9% 1	2576
	11th to 20th	Night	45% 7% 17% 22% 8%	3049
	21st to end	Night	36% 9% 18% 25% 11% 1	3586
September	1st to 10th	Night	38% 8% 15% 27% 12% 1	2000
	11th to 20th	Night	35% 5% 14% 30% 15% 1	2115
	21st to end	Night	41% 7% 13% 23% 13% 2	2200
October	1st to 10th	Night	53% 7% 10% 20% 9% 1	3718
	11th to 20th	Night	49% 5% 12% 20% 14% 1	3983
	21st to end	Night	51% 6% 13% 23% 6%	4610
November	1st to 10th	Night	55% 7% 11% 17% 8% 2	2667
	11th to 20th	Night	55% 6% 12% 15% 9% 2	2730
	21st to end	Night	60% 7% 14% 14% 5%	2909
December	1st to 10th	Night	76% 6% 8% 8% 1	4720
	11th to 20th	Night	64% 7% 8% 11% 9%	5348
	21st to end	Night	69% 5% 7% 13% 5% 2	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	26% 29% 28% 15% 2	220
	11th to 20th	Day	30% 28% 30% 10% 1 1	220
	21st to end	Day	26% 30% 31% 10% 2	242
February	1st to 10th	Day	31% 34% 27% 7% 1	316
	11th to 20th	Day	28% 30% 30% 10% 2	512
	21st to end	Day	31% 26% 32% 9% 2	446
March	1st to 10th	Day	30% 21% 34% 13% 2	540
	11th to 20th	Day	31% 30% 26% 11% 2 1	540
	21st to end	Day	35% 25% 25% 10% 4 1	594
April	1st to 10th	Day	39% 20% 24% 12% 4 1	540
	11th to 20th	Day	38% 21% 24% 10% 5% 1	532
	21st to end	Day	31% 24% 25% 13% 5% 2	540
May	1st to 10th	Day	34% 21% 25% 13% 7% 1	543
	11th to 20th	Day	26% 16% 34% 19% 4 1	551
	21st to end	Day	21% 19% 33% 21% 6% 1	573
June	1st to 10th	Day	24% 16% 36% 17% 6%	530
	11th to 20th	Day	21% 18% 34% 19% 8% 1	530
	21st to end	Day	25% 17% 32% 20% 6% 1	530
July	1st to 10th	Day	17% 13% 35% 24% 10% 1	525
	11th to 20th	Day	17% 12% 34% 26% 10% 1	524
	21st to end	Day	17% 12% 40% 23% 7% 1	576
August	1st to 10th	Day	15% 11% 30% 30% 12% 2	526
	11th to 20th	Day	19% 11% 28% 23% 14% 5%	519
	21st to end	Day	15% 15% 30% 25% 11% 4	583
September	1st to 10th	Day	17% 12% 28% 26% 10% 6%	530
	11th to 20th	Day	16% 13% 23% 23% 19% 7%	530
	21st to end	Day	18% 16% 30% 19% 12% 5%	530
October	1st to 10th	Day	27% 15% 25% 18% 12% 4	530
	11th to 20th	Day	27% 18% 28% 20% 7%	502
	21st to end	Day	33% 21% 28% 16% 2	462
November	1st to 10th	Day	31% 22% 26% 16% 5%	425
	11th to 20th	Day	29% 25% 24% 17% 4 1	430
	21st to end	Day	34% 24% 27% 14% 1	246
December	1st to 10th	Day	30% 25% 28% 14% 2	220
	11th to 20th	Day	36% 27% 29% 7% 1	220
	21st to end	Day	32% 24% 33% 8% 2	242
January	1st to 10th	Night	24% 25% 34% 15% 2	840
	11th to 20th	Night	28% 24% 29% 16% 3	840
	21st to end	Night	19% 23% 34% 20% 3	924
February	1st to 10th	Night	23% 22% 36% 18% 1	840
	11th to 20th	Night	22% 25% 35% 16% 2	823
	21st to end	Night	25% 22% 33% 18% 2	692
March	1st to 10th	Night	27% 21% 31% 19% 2	840
	11th to 20th	Night	26% 24% 28% 17% 4	840
	21st to end	Night	28% 21% 27% 18% 5%	924
April	1st to 10th	Night	31% 19% 27% 19% 4	840
	11th to 20th	Night	31% 21% 25% 17% 6%	831
	21st to end	Night	28% 18% 28% 21% 5%	673
May	1st to 10th	Night	30% 15% 28% 20% 7%	588
	11th to 20th	Night	22% 11% 36% 24% 6% 1	406
	21st to end	Night	20% 10% 32% 30% 8% 1	433
June	1st to 10th	Night	23% 9% 31% 27% 10%	400
	11th to 20th	Night	23% 8% 27% 31% 11% 1	400
	21st to end	Night	21% 6% 26% 37% 8% 1	400
July	1st to 10th	Night	15% 4 27% 38% 15% 1	390
	11th to 20th	Night	16% 5% 23% 38% 15% 2	388
	21st to end	Night	13% 6% 24% 44% 13%	438
August	1st to 10th	Night	16% 5% 19% 42% 18% 1	398
	11th to 20th	Night	18% 5% 21% 34% 19% 2	676
	21st to end	Night	16% 8% 22% 35% 18% 1	880
September	1st to 10th	Night	19% 8% 26% 31% 14% 2	800
	11th to 20th	Night	17% 8% 22% 30% 20% 3	800
	21st to end	Night	20% 11% 26% 25% 16% 2	800
October	1st to 10th	Night	29% 11% 21% 23% 15% 1	800
	11th to 20th	Night	26% 16% 24% 24% 9% 1	800
	21st to end	Night	33% 16% 28% 19% 4	880
November	1st to 10th	Night	28% 17% 30% 17% 6%	820
	11th to 20th	Night	26% 21% 29% 16% 6% 1	840
	21st to end	Night	27% 24% 28% 16% 4	840
December	1st to 10th	Night	32% 24% 27% 15% 3	840
	11th to 20th	Night	27% 22% 28% 20% 3	840
	21st to end	Night	33% 20% 30% 15% 2	924

Page generated at 2026-05-14 01:58:58. Version ac07

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