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 2025-01-22.

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% 3 8% 5% 4 4 4 5% 7% 31%	105153
	Night	19% 13% 4 9% 5% 4 4 3 4 5% 29%	104211
Transparency	Day	51% 8% 12% 19% 9% 1	108652
	Night	56% 7% 11% 16% 8% 1	108801
Seeing	Day	25% 20% 29% 18% 7% 2	14920
	Night	24% 17% 28% 23% 8% 1	24011

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% 16% 7% 12% 6% 5% 4 4 5% 4 22%	2597
	11th to 20th	Day	13% 14% 7% 17% 9% 5% 5% 3 3 4 21%	2600
	21st to end	Day	17% 18% 6% 10% 6% 5% 5% 4 5% 5% 20%	2780
February	1st to 10th	Day	14% 15% 4 11% 8% 5% 5% 4 6% 5% 24%	1685
	11th to 20th	Day	13% 15% 6% 13% 6% 5% 4 3 4 6% 25%	1826
	21st to end	Day	16% 14% 5% 11% 7% 4 3 4 5% 6% 26%	1570
March	1st to 10th	Day	23% 16% 5% 10% 5% 5% 4 3 4 4 22%	3116
	11th to 20th	Day	15% 22% 5% 9% 5% 4 4 3 5% 4 25%	3268
	21st to end	Day	18% 12% 4 11% 4 4 4 3 4 4 33%	4166
April	1st to 10th	Day	23% 15% 3 6% 3 4 3 3 4 6% 31%	2280
	11th to 20th	Day	19% 11% 2 7% 5% 4 3 3 4 6% 35%	2254
	21st to end	Day	23% 11% 3 7% 5% 4 4 3 5% 5% 30%	2429
May	1st to 10th	Day	26% 10% 3 8% 4 5% 4 3 4 5% 27%	4197
	11th to 20th	Day	17% 9% 3 8% 6% 5% 5% 4 6% 8% 30%	4398
	21st to end	Day	13% 7% 3 6% 5% 5% 5% 4 6% 9% 37%	4837
June	1st to 10th	Day	20% 8% 3 8% 5% 5% 5% 4 5% 7% 29%	2660
	11th to 20th	Day	17% 7% 2 6% 5% 5% 4 4 6% 8% 34%	2660
	21st to end	Day	20% 9% 3 7% 5% 5% 5% 5% 6% 7% 29%	2660
July	1st to 10th	Day	13% 6% 2 6% 5% 4 5% 4 7% 10% 37%	4440
	11th to 20th	Day	14% 6% 2 7% 5% 5% 6% 5% 7% 11% 33%	4440
	21st to end	Day	15% 6% 2 6% 5% 4 5% 5% 7% 8% 37%	4814
August	1st to 10th	Day	17% 7% 2 6% 4 5% 5% 4 6% 9% 34%	4409
	11th to 20th	Day	15% 8% 3 8% 5% 5% 5% 4 7% 8% 33%	3866
	21st to end	Day	15% 5% 2 6% 4 4 5% 4 6% 9% 41%	4180
September	1st to 10th	Day	15% 5% 1 5% 4 4 4 3 6% 8% 45%	2280
	11th to 20th	Day	15% 6% 2 5% 2 4 3 3 5% 7% 47%	2170
	21st to end	Day	17% 6% 2 6% 4 3 4 4 5% 8% 39%	2090
October	1st to 10th	Day	23% 9% 3 8% 4 4 4 3 4 5% 32%	2014
	11th to 20th	Day	18% 10% 3 8% 4 4 3 3 6% 6% 35%	1854
	21st to end	Day	26% 12% 3 6% 4 4 4 3 4 7% 27%	1881
November	1st to 10th	Day	16% 12% 3 8% 5% 3 5% 3 5% 7% 34%	1761
	11th to 20th	Day	18% 15% 3 8% 5% 5% 4 3 5% 5% 29%	1800
	21st to end	Day	18% 14% 4 9% 4 5% 4 4 5% 6% 26%	1625
December	1st to 10th	Day	19% 18% 6% 12% 6% 4 4 4 5% 6% 16%	2600
	11th to 20th	Day	23% 17% 5% 9% 5% 4 3 3 4 5% 24%	2986
	21st to end	Day	22% 18% 5% 9% 4 5% 5% 3 4 4 19%	3960
January	1st to 10th	Night	15% 18% 7% 13% 5% 5% 4 3 3 4 23%	4569
	11th to 20th	Night	13% 19% 6% 18% 9% 5% 4 3 4 4 16%	4580
	21st to end	Night	21% 17% 6% 14% 6% 4 4 3 3 3 19%	4899
February	1st to 10th	Night	15% 16% 5% 13% 8% 6% 4 4 4 5% 21%	2494
	11th to 20th	Night	13% 16% 5% 13% 8% 5% 4 4 5% 5% 23%	2239
	21st to end	Night	15% 15% 5% 10% 6% 4 3 3 5% 4 29%	1884
March	1st to 10th	Night	24% 14% 3 8% 5% 4 5% 3 3 5% 26%	3724
	11th to 20th	Night	15% 20% 4 10% 5% 4 3 2 4 5% 28%	3687
	21st to end	Night	17% 15% 4 10% 5% 3 3 2 3 3 33%	3466
April	1st to 10th	Night	22% 14% 4 7% 5% 4 4 3 4 5% 30%	1900
	11th to 20th	Night	24% 11% 4 7% 4 4 4 3 4 6% 30%	1875
	21st to end	Night	22% 11% 4 7% 5% 3 3 3 4 5% 32%	1749
May	1st to 10th	Night	27% 10% 3 7% 4 4 4 4 5% 7% 25%	2770
	11th to 20th	Night	21% 10% 4 9% 5% 5% 4 3 5% 5% 30%	2495
	21st to end	Night	16% 7% 2 6% 4 4 4 4 7% 8% 37%	2738
June	1st to 10th	Night	21% 8% 2 8% 5% 4 5% 3 6% 7% 32%	1520
	11th to 20th	Night	20% 6% 2 5% 4 4 4 4 5% 8% 38%	1520
	21st to end	Night	20% 7% 3 7% 5% 4 4 4 6% 7% 33%	1520
July	1st to 10th	Night	15% 6% 2 7% 6% 5% 5% 5% 7% 7% 36%	2520
	11th to 20th	Night	18% 6% 2 5% 5% 4 4 4 6% 8% 37%	2520
	21st to end	Night	14% 8% 4 7% 5% 4 5% 4 6% 7% 37%	2739
August	1st to 10th	Night	21% 8% 2 6% 5% 4 4 3 4 6% 36%	2506
	11th to 20th	Night	16% 7% 3 7% 5% 5% 4 3 6% 8% 36%	2928
	21st to end	Night	13% 6% 2 5% 4 4 4 4 6% 8% 42%	3476
September	1st to 10th	Night	16% 7% 2 5% 3 3 4 3 5% 7% 45%	1900
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 6% 51%	2009
	21st to end	Night	16% 6% 2 5% 5% 4 4 4 5% 7% 42%	2090
October	1st to 10th	Night	24% 9% 3 7% 4 4 4 3 5% 5% 32%	2162
	11th to 20th	Night	21% 9% 4 7% 5% 4 5% 4 4 5% 32%	2324
	21st to end	Night	26% 12% 3 7% 5% 4 4 3 3 5% 27%	2717
November	1st to 10th	Night	19% 12% 4 8% 5% 4 4 3 5% 4 32%	2537
	11th to 20th	Night	20% 13% 4 8% 5% 4 3 3 4 6% 31%	2600
	21st to end	Night	19% 16% 5% 9% 5% 4 4 3 6% 6% 25%	2770
December	1st to 10th	Night	19% 24% 9% 13% 6% 4 4 2 3 4 12%	4580
	11th to 20th	Night	26% 16% 4 8% 5% 3 3 3 3 4 24%	5208
	21st to end	Night	23% 17% 5% 10% 6% 6% 4 3 3 4 19%	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	62% 7% 13% 15% 3	2598
	11th to 20th	Day	68% 6% 9% 11% 5% 1	2600
	21st to end	Day	66% 7% 12% 9% 5% 1	2780
February	1st to 10th	Day	62% 6% 10% 15% 5% 2	1685
	11th to 20th	Day	61% 8% 9% 15% 7% 1	1835
	21st to end	Day	59% 8% 10% 12% 11% 2	1570
March	1st to 10th	Day	65% 7% 9% 11% 7% 1	3119
	11th to 20th	Day	59% 9% 12% 11% 8% 1	3268
	21st to end	Day	52% 8% 7% 14% 17% 2	4166
April	1st to 10th	Day	56% 5% 9% 19% 9% 2	2280
	11th to 20th	Day	51% 7% 10% 17% 14% 1	2254
	21st to end	Day	56% 6% 9% 15% 11% 2	2429
May	1st to 10th	Day	58% 8% 9% 18% 7% 1	4211
	11th to 20th	Day	49% 10% 14% 17% 9% 1	4402
	21st to end	Day	40% 10% 13% 25% 11% 1	4837
June	1st to 10th	Day	53% 8% 14% 16% 9% 1	2660
	11th to 20th	Day	47% 8% 12% 19% 12% 2	2660
	21st to end	Day	53% 8% 11% 18% 8% 1	2660
July	1st to 10th	Day	40% 8% 15% 25% 10%	4370
	11th to 20th	Day	41% 10% 17% 26% 6%	4365
	21st to end	Day	43% 10% 14% 25% 8%	4661
August	1st to 10th	Day	47% 8% 16% 20% 8%	4395
	11th to 20th	Day	47% 8% 18% 22% 5%	3905
	21st to end	Day	38% 9% 19% 23% 11% 1	4180
September	1st to 10th	Day	37% 7% 14% 29% 12%	2280
	11th to 20th	Day	37% 6% 12% 29% 16% 1	2170
	21st to end	Day	42% 8% 15% 23% 10% 2	2090
October	1st to 10th	Day	48% 7% 9% 25% 11% 1	3352
	11th to 20th	Day	46% 5% 15% 21% 12%	3057
	21st to end	Day	45% 7% 12% 27% 8% 1	3081
November	1st to 10th	Day	51% 7% 14% 19% 9%	1761
	11th to 20th	Day	56% 6% 12% 13% 11% 2	1800
	21st to end	Day	58% 9% 15% 15% 4	1625
December	1st to 10th	Day	68% 9% 11% 9% 2	2600
	11th to 20th	Day	63% 6% 9% 15% 6%	2986
	21st to end	Day	65% 6% 8% 13% 7% 1	3960
January	1st to 10th	Night	63% 7% 15% 12% 3	4569
	11th to 20th	Night	70% 9% 10% 8% 3	4580
	21st to end	Night	71% 5% 9% 9% 5% 1	4899
February	1st to 10th	Night	65% 6% 10% 11% 6% 1	2494
	11th to 20th	Night	62% 6% 9% 14% 8% 1	2241
	21st to end	Night	58% 6% 11% 16% 7% 2	1884
March	1st to 10th	Night	61% 7% 9% 15% 8% 1	3734
	11th to 20th	Night	57% 8% 12% 15% 8% 1	3688
	21st to end	Night	53% 8% 7% 14% 15% 3	3466
April	1st to 10th	Night	57% 5% 8% 17% 11% 2	1900
	11th to 20th	Night	55% 6% 9% 17% 11% 1	1875
	21st to end	Night	53% 7% 8% 17% 11% 3	1749
May	1st to 10th	Night	56% 9% 9% 16% 8% 1	2776
	11th to 20th	Night	55% 7% 11% 17% 9% 1	2496
	21st to end	Night	41% 10% 14% 19% 14% 2	2739
June	1st to 10th	Night	49% 9% 13% 17% 11% 1	1520
	11th to 20th	Night	45% 8% 14% 20% 11% 2	1520
	21st to end	Night	49% 8% 13% 21% 8% 1	1520
July	1st to 10th	Night	45% 9% 16% 18% 12% 1	2480
	11th to 20th	Night	43% 10% 14% 24% 9%	2472
	21st to end	Night	45% 8% 12% 26% 9%	2651
August	1st to 10th	Night	50% 7% 12% 21% 9%	2496
	11th to 20th	Night	46% 7% 17% 22% 8%	2955
	21st to end	Night	36% 9% 18% 25% 11% 1	3476
September	1st to 10th	Night	38% 8% 15% 27% 11% 1	1900
	11th to 20th	Night	35% 5% 15% 30% 14% 1	2009
	21st to end	Night	41% 7% 14% 23% 13% 2	2090
October	1st to 10th	Night	54% 7% 10% 20% 7% 1	3604
	11th to 20th	Night	49% 5% 12% 20% 14% 1	3860
	21st to end	Night	51% 6% 13% 23% 6%	4467
November	1st to 10th	Night	55% 7% 12% 17% 8% 2	2537
	11th to 20th	Night	54% 7% 12% 16% 10% 2	2600
	21st to end	Night	59% 7% 15% 14% 5%	2770
December	1st to 10th	Night	77% 6% 8% 8% 1	4580
	11th to 20th	Night	64% 7% 8% 11% 9%	5208
	21st to end	Night	69% 5% 7% 13% 5% 2	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	26% 31% 27% 14% 2	200
	11th to 20th	Day	27% 28% 32% 11% 1 1	200
	21st to end	Day	23% 30% 34% 11% 3	200
February	1st to 10th	Day	31% 39% 24% 5% 2	258
	11th to 20th	Day	27% 32% 29% 10% 2	434
	21st to end	Day	32% 24% 32% 10% 2	382
March	1st to 10th	Day	28% 21% 34% 15% 2	460
	11th to 20th	Day	28% 30% 26% 13% 2 1	460
	21st to end	Day	32% 24% 26% 11% 5% 2	506
April	1st to 10th	Day	36% 20% 25% 14% 5% 1	460
	11th to 20th	Day	35% 23% 24% 12% 6% 1	452
	21st to end	Day	31% 24% 25% 13% 5% 2	460
May	1st to 10th	Day	30% 21% 25% 14% 8% 1	463
	11th to 20th	Day	23% 16% 35% 19% 5% 1	487
	21st to end	Day	19% 19% 33% 22% 6% 1	529
June	1st to 10th	Day	23% 17% 36% 17% 7%	490
	11th to 20th	Day	19% 18% 34% 20% 8% 1	490
	21st to end	Day	26% 17% 31% 19% 7% 1	490
July	1st to 10th	Day	17% 13% 33% 25% 11% 1	485
	11th to 20th	Day	17% 11% 34% 26% 10% 1	484
	21st to end	Day	17% 12% 39% 24% 7% 2	532
August	1st to 10th	Day	16% 12% 30% 29% 11% 2	486
	11th to 20th	Day	20% 11% 28% 23% 13% 5%	479
	21st to end	Day	15% 15% 28% 26% 11% 4	539
September	1st to 10th	Day	17% 12% 29% 25% 11% 6%	490
	11th to 20th	Day	17% 14% 22% 22% 19% 7%	490
	21st to end	Day	18% 17% 29% 19% 13% 5%	490
October	1st to 10th	Day	28% 15% 25% 16% 12% 4	490
	11th to 20th	Day	26% 17% 28% 20% 8%	465
	21st to end	Day	34% 20% 28% 16% 3	429
November	1st to 10th	Day	28% 22% 27% 17% 6%	395
	11th to 20th	Day	30% 25% 24% 17% 4 1	400
	21st to end	Day	32% 23% 28% 15% 1	225
December	1st to 10th	Day	29% 26% 29% 15% 3	200
	11th to 20th	Day	35% 28% 29% 8% 1	200
	21st to end	Day	32% 25% 33% 8% 3	220
January	1st to 10th	Night	24% 25% 34% 15% 2	800
	11th to 20th	Night	27% 23% 30% 16% 3	800
	21st to end	Night	18% 23% 34% 22% 3	841
February	1st to 10th	Night	23% 23% 36% 18% 1	760
	11th to 20th	Night	22% 26% 34% 16% 2	743
	21st to end	Night	25% 22% 32% 19% 2	628
March	1st to 10th	Night	26% 20% 32% 21% 2	760
	11th to 20th	Night	25% 24% 28% 18% 4	760
	21st to end	Night	26% 22% 26% 19% 6%	836
April	1st to 10th	Night	30% 18% 27% 21% 4	760
	11th to 20th	Night	29% 21% 25% 18% 7% 1	751
	21st to end	Night	27% 18% 28% 21% 5%	609
May	1st to 10th	Night	30% 14% 28% 20% 8%	532
	11th to 20th	Night	22% 10% 36% 25% 6% 1	374
	21st to end	Night	20% 9% 31% 30% 9% 1	411
June	1st to 10th	Night	23% 9% 30% 27% 11%	380
	11th to 20th	Night	22% 7% 27% 32% 12% 1	380
	21st to end	Night	22% 5% 27% 37% 8% 1	380
July	1st to 10th	Night	15% 4 27% 37% 16% 1	370
	11th to 20th	Night	15% 5% 23% 39% 16% 2	368
	21st to end	Night	13% 6% 21% 46% 13%	416
August	1st to 10th	Night	17% 5% 18% 42% 17% 1	378
	11th to 20th	Night	18% 5% 22% 34% 19% 2	642
	21st to end	Night	17% 8% 22% 34% 18% 1	836
September	1st to 10th	Night	19% 8% 26% 31% 14% 2	760
	11th to 20th	Night	18% 8% 22% 29% 20% 4	760
	21st to end	Night	19% 11% 26% 25% 17% 2	760
October	1st to 10th	Night	30% 11% 21% 22% 15% 1	760
	11th to 20th	Night	26% 16% 24% 25% 9% 1	760
	21st to end	Night	33% 16% 28% 19% 5%	836
November	1st to 10th	Night	27% 18% 30% 18% 7%	780
	11th to 20th	Night	26% 21% 29% 17% 6% 1	800
	21st to end	Night	27% 25% 28% 16% 5% 1	800
December	1st to 10th	Night	31% 24% 27% 15% 3	800
	11th to 20th	Night	27% 22% 28% 20% 3	800
	21st to end	Night	33% 19% 31% 15% 2	880

Page generated at 2025-01-19 03:06:48. Version ac07

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