Boundary Road
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boundary Road clear sky chart from 2005-11-06 to 2025-01-07.

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	17% 10% 3 8% 5% 5% 5% 4 5% 7% 32%	105178
	Night	17% 12% 4 9% 5% 5% 4 3 5% 6% 31%	103846
Transparency	Day	50% 7% 13% 21% 9% 1	108661
	Night	52% 6% 12% 19% 10% 1	108452
Seeing	Day	21% 11% 28% 23% 12% 4	14635
	Night	22% 9% 28% 29% 11% 1	23794

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	17% 16% 5% 12% 6% 5% 5% 3 5% 5% 21%	2565
	11th to 20th	Day	14% 13% 6% 12% 7% 7% 5% 3 5% 6% 22%	2520
	21st to end	Day	15% 18% 6% 12% 6% 7% 5% 4 5% 5% 18%	2850
February	1st to 10th	Day	14% 17% 5% 10% 7% 6% 3 4 5% 6% 24%	1710
	11th to 20th	Day	13% 16% 5% 11% 5% 5% 3 4 5% 6% 26%	1686
	21st to end	Day	19% 14% 3 8% 6% 5% 5% 5% 5% 5% 25%	1570
March	1st to 10th	Day	20% 12% 4 10% 6% 5% 5% 3 4 5% 26%	3116
	11th to 20th	Day	13% 16% 5% 9% 5% 6% 5% 3 4 6% 27%	3443
	21st to end	Day	14% 8% 3 8% 5% 4 4 4 5% 7% 39%	4056
April	1st to 10th	Day	21% 12% 3 7% 4 4 4 3 3 6% 33%	2280
	11th to 20th	Day	22% 8% 3 7% 5% 3 3 3 4 6% 36%	2254
	21st to end	Day	23% 10% 3 7% 5% 4 4 3 4 5% 32%	2405
May	1st to 10th	Day	20% 9% 3 7% 5% 5% 5% 4 6% 7% 30%	4426
	11th to 20th	Day	17% 9% 3 8% 6% 4 4 4 5% 7% 33%	4398
	21st to end	Day	15% 7% 2 7% 5% 5% 5% 4 6% 7% 37%	4837
June	1st to 10th	Day	22% 10% 2 7% 6% 4 4 4 6% 6% 29%	2660
	11th to 20th	Day	14% 6% 2 7% 4 6% 4 4 6% 8% 38%	2660
	21st to end	Day	20% 7% 2 7% 4 5% 5% 5% 7% 7% 31%	2660
July	1st to 10th	Day	11% 7% 2 8% 6% 5% 6% 5% 6% 10% 34%	4440
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 5% 7% 9% 38%	4440
	21st to end	Day	11% 6% 2 6% 4 4 5% 4 7% 9% 43%	4814
August	1st to 10th	Day	14% 7% 3 8% 5% 5% 5% 4 6% 8% 35%	4397
	11th to 20th	Day	15% 8% 2 5% 4 4 5% 4 6% 9% 37%	3959
	21st to end	Day	15% 6% 2 7% 4 4 5% 4 6% 9% 39%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 5% 5% 4 5% 8% 43%	2280
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 8% 48%	2266
	21st to end	Day	17% 6% 2 5% 4 5% 4 3 5% 8% 40%	2050
October	1st to 10th	Day	19% 9% 3 6% 5% 4 4 4 6% 7% 34%	1900
	11th to 20th	Day	21% 8% 4 8% 5% 5% 5% 4 5% 6% 29%	1900
	21st to end	Day	25% 10% 2 7% 5% 5% 5% 3 4 6% 28%	1919
November	1st to 10th	Day	16% 9% 3 7% 5% 5% 6% 4 7% 7% 32%	1761
	11th to 20th	Day	18% 12% 3 9% 5% 4 5% 4 5% 6% 29%	1630
	21st to end	Day	21% 15% 4 9% 5% 5% 5% 3 5% 5% 22%	1600
December	1st to 10th	Day	20% 21% 5% 11% 5% 5% 5% 5% 5% 6% 14%	2600
	11th to 20th	Day	18% 16% 5% 11% 5% 4 5% 3 5% 5% 24%	2986
	21st to end	Day	23% 15% 5% 12% 4 5% 5% 3 4 4 21%	3960
January	1st to 10th	Night	17% 18% 6% 12% 5% 5% 4 3 4 5% 21%	4514
	11th to 20th	Night	13% 14% 5% 14% 7% 6% 5% 4 5% 5% 22%	4440
	21st to end	Night	17% 20% 7% 10% 5% 5% 4 2 4 5% 21%	4788
February	1st to 10th	Night	14% 15% 5% 11% 6% 6% 6% 4 4 6% 23%	2470
	11th to 20th	Night	13% 13% 5% 10% 7% 6% 4 4 5% 5% 28%	2380
	21st to end	Night	16% 14% 4 8% 7% 3 5% 3 4 5% 30%	1884
March	1st to 10th	Night	19% 13% 4 8% 5% 5% 4 3 4 5% 31%	3724
	11th to 20th	Night	17% 16% 5% 10% 6% 4 4 2 5% 5% 26%	3514
	21st to end	Night	15% 11% 3 7% 5% 3 4 3 5% 5% 39%	3576
April	1st to 10th	Night	22% 11% 2 7% 5% 5% 4 4 4 5% 33%	1900
	11th to 20th	Night	19% 10% 3 8% 5% 4 4 4 4 5% 35%	1875
	21st to end	Night	21% 9% 3 7% 5% 4 4 3 5% 5% 34%	1787
May	1st to 10th	Night	20% 10% 3 8% 5% 5% 4 4 6% 7% 28%	2514
	11th to 20th	Night	20% 9% 3 6% 4 4 5% 4 5% 7% 34%	2495
	21st to end	Night	14% 7% 2 7% 4 5% 5% 5% 6% 8% 38%	2738
June	1st to 10th	Night	16% 7% 2 7% 5% 6% 6% 5% 6% 8% 31%	1520
	11th to 20th	Night	14% 7% 3 6% 4 4 4 5% 6% 7% 40%	1520
	21st to end	Night	18% 8% 3 7% 5% 4 4 4 6% 8% 35%	1520
July	1st to 10th	Night	13% 8% 3 9% 6% 5% 6% 5% 6% 7% 34%	2520
	11th to 20th	Night	12% 7% 2 7% 5% 4 5% 4 6% 8% 40%	2520
	21st to end	Night	10% 7% 3 6% 5% 5% 5% 4 7% 7% 42%	2739
August	1st to 10th	Night	12% 7% 3 8% 4 4 5% 4 6% 9% 38%	2529
	11th to 20th	Night	15% 6% 2 6% 4 3 5% 4 6% 8% 40%	2836
	21st to end	Night	11% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	3476
September	1st to 10th	Night	11% 6% 2 7% 5% 4 5% 3 5% 6% 46%	1900
	11th to 20th	Night	13% 5% 2 6% 3 3 3 3 6% 7% 50%	1914
	21st to end	Night	16% 6% 2 6% 5% 4 4 3 4 6% 44%	2127
October	1st to 10th	Night	19% 7% 2 6% 5% 5% 4 3 5% 6% 37%	2280
	11th to 20th	Night	19% 10% 3 7% 6% 5% 4 4 5% 5% 33%	2280
	21st to end	Night	22% 10% 3 8% 5% 5% 4 3 5% 7% 28%	2679
November	1st to 10th	Night	20% 11% 3 7% 4 4 4 3 4 6% 34%	2537
	11th to 20th	Night	18% 10% 3 9% 5% 5% 5% 4 4 6% 31%	2766
	21st to end	Night	20% 14% 4 10% 6% 4 4 3 5% 6% 24%	2800
December	1st to 10th	Night	19% 17% 5% 11% 7% 6% 5% 3 5% 6% 16%	4580
	11th to 20th	Night	24% 14% 5% 9% 6% 4 4 3 3 4 25%	5208
	21st to end	Night	27% 17% 3 9% 5% 3 4 3 5% 4 20%	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	63% 9% 12% 10% 5% 1	2566
	11th to 20th	Day	61% 6% 12% 13% 7% 1	2520
	21st to end	Day	67% 8% 12% 9% 5%	2850
February	1st to 10th	Day	61% 5% 10% 19% 4 1	1710
	11th to 20th	Day	57% 5% 13% 16% 8% 1	1695
	21st to end	Day	59% 6% 10% 14% 10% 1	1570
March	1st to 10th	Day	59% 5% 10% 17% 8% 1	3119
	11th to 20th	Day	56% 6% 13% 15% 9% 2	3443
	21st to end	Day	42% 6% 9% 24% 16% 3	4056
April	1st to 10th	Day	54% 4 8% 19% 13% 3	2280
	11th to 20th	Day	50% 4 8% 21% 14% 2	2254
	21st to end	Day	55% 5% 9% 14% 14% 3	2405
May	1st to 10th	Day	50% 8% 10% 21% 9% 2	4440
	11th to 20th	Day	49% 9% 13% 19% 9% 1	4402
	21st to end	Day	43% 7% 13% 22% 14% 1	4837
June	1st to 10th	Day	53% 6% 11% 18% 11% 1	2660
	11th to 20th	Day	42% 8% 13% 23% 12% 2	2660
	21st to end	Day	48% 9% 13% 20% 9% 1	2660
July	1st to 10th	Day	43% 7% 13% 25% 11%	4370
	11th to 20th	Day	39% 7% 16% 30% 8%	4365
	21st to end	Day	36% 8% 13% 32% 10%	4661
August	1st to 10th	Day	46% 6% 16% 23% 9%	4383
	11th to 20th	Day	41% 7% 16% 28% 7%	3998
	21st to end	Day	40% 6% 17% 24% 12% 1	4180
September	1st to 10th	Day	38% 7% 15% 28% 12%	2280
	11th to 20th	Day	35% 7% 13% 32% 13% 1	2266
	21st to end	Day	42% 7% 15% 26% 10%	2050
October	1st to 10th	Day	47% 5% 11% 29% 8%	3160
	11th to 20th	Day	46% 6% 15% 25% 7%	3140
	21st to end	Day	48% 6% 17% 22% 7%	3144
November	1st to 10th	Day	48% 9% 14% 22% 7% 1	1761
	11th to 20th	Day	55% 6% 13% 16% 10% 1	1630
	21st to end	Day	63% 7% 13% 11% 6%	1600
December	1st to 10th	Day	70% 7% 12% 10% 1	2600
	11th to 20th	Day	62% 6% 10% 15% 7%	2986
	21st to end	Day	67% 4 9% 15% 4	3960
January	1st to 10th	Night	65% 6% 15% 10% 4	4514
	11th to 20th	Night	61% 9% 14% 11% 5%	4440
	21st to end	Night	66% 6% 11% 10% 6% 1	4788
February	1st to 10th	Night	61% 6% 11% 15% 6% 1	2470
	11th to 20th	Night	57% 5% 11% 19% 8%	2382
	21st to end	Night	57% 4 9% 18% 11% 1	1884
March	1st to 10th	Night	55% 5% 9% 19% 9% 2	3734
	11th to 20th	Night	59% 6% 12% 14% 8% 2	3515
	21st to end	Night	45% 4 8% 23% 17% 3	3576
April	1st to 10th	Night	53% 5% 10% 17% 14% 2	1900
	11th to 20th	Night	50% 6% 8% 20% 15% 1	1875
	21st to end	Night	50% 6% 9% 17% 15% 4	1787
May	1st to 10th	Night	52% 9% 10% 18% 9% 3	2520
	11th to 20th	Night	46% 10% 10% 19% 12% 2	2496
	21st to end	Night	41% 9% 12% 19% 18% 1	2739
June	1st to 10th	Night	47% 9% 13% 20% 10% 1	1520
	11th to 20th	Night	40% 8% 11% 22% 17% 2	1520
	21st to end	Night	46% 9% 13% 18% 14% 1	1520
July	1st to 10th	Night	45% 8% 14% 19% 13% 1	2480
	11th to 20th	Night	39% 7% 15% 32% 8%	2472
	21st to end	Night	39% 9% 14% 24% 14%	2651
August	1st to 10th	Night	42% 8% 15% 25% 10%	2519
	11th to 20th	Night	40% 7% 15% 31% 7%	2863
	21st to end	Night	40% 7% 15% 24% 12% 2	3476
September	1st to 10th	Night	38% 8% 11% 28% 15% 1	1900
	11th to 20th	Night	34% 7% 13% 34% 11% 1	1914
	21st to end	Night	40% 6% 13% 29% 12%	2127
October	1st to 10th	Night	41% 6% 14% 28% 11%	3800
	11th to 20th	Night	43% 6% 16% 26% 9%	3779
	21st to end	Night	48% 6% 17% 20% 9%	4404
November	1st to 10th	Night	51% 5% 15% 20% 9% 1	2537
	11th to 20th	Night	54% 6% 12% 17% 11% 1	2766
	21st to end	Night	61% 6% 13% 15% 5%	2800
December	1st to 10th	Night	68% 6% 12% 11% 2	4580
	11th to 20th	Night	63% 4 8% 14% 10% 1	5208
	21st to end	Night	68% 4 7% 12% 9%	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	21% 17% 30% 26% 6% 1	192
	11th to 20th	Day	26% 11% 35% 21% 7% 2	180
	21st to end	Day	20% 9% 35% 29% 6% 1	222
February	1st to 10th	Day	29% 16% 36% 15% 4	270
	11th to 20th	Day	23% 17% 40% 19% 2	293
	21st to end	Day	27% 12% 35% 23% 4	382
March	1st to 10th	Day	23% 11% 33% 25% 7%	460
	11th to 20th	Day	22% 15% 33% 23% 6% 1	460
	21st to end	Day	27% 13% 30% 19% 8% 2	506
April	1st to 10th	Day	25% 14% 34% 18% 8% 1	460
	11th to 20th	Day	28% 17% 25% 17% 12% 2	452
	21st to end	Day	28% 13% 30% 17% 10% 2	460
May	1st to 10th	Day	25% 10% 27% 24% 11% 3	463
	11th to 20th	Day	15% 14% 31% 24% 11% 5%	487
	21st to end	Day	17% 12% 29% 24% 14% 3	529
June	1st to 10th	Day	25% 9% 25% 28% 10% 4	490
	11th to 20th	Day	16% 10% 28% 26% 16% 4	490
	21st to end	Day	23% 10% 25% 24% 13% 4	490
July	1st to 10th	Day	14% 4 25% 34% 17% 7%	485
	11th to 20th	Day	12% 7% 28% 30% 18% 5%	484
	21st to end	Day	14% 5% 26% 32% 19% 4	532
August	1st to 10th	Day	14% 6% 22% 33% 20% 5%	486
	11th to 20th	Day	16% 4 21% 24% 25% 10%	479
	21st to end	Day	13% 8% 26% 25% 19% 9%	539
September	1st to 10th	Day	15% 8% 27% 22% 16% 12%	490
	11th to 20th	Day	12% 6% 21% 22% 25% 15%	490
	21st to end	Day	19% 10% 24% 23% 17% 7%	490
October	1st to 10th	Day	22% 10% 25% 21% 16% 6%	490
	11th to 20th	Day	23% 12% 32% 20% 10% 2	490
	21st to end	Day	31% 12% 24% 24% 8%	449
November	1st to 10th	Day	24% 17% 27% 21% 10% 1	395
	11th to 20th	Day	26% 12% 36% 17% 6% 3	230
	21st to end	Day	28% 18% 31% 15% 8% 2	200
December	1st to 10th	Day	29% 12% 30% 22% 7% 1	200
	11th to 20th	Day	30% 20% 30% 17% 3 1	200
	21st to end	Day	30% 14% 32% 15% 7% 1	220
January	1st to 10th	Night	24% 11% 33% 26% 6%	785
	11th to 20th	Night	25% 12% 30% 26% 5% 1	760
	21st to end	Night	20% 8% 33% 31% 8% 1	836
February	1st to 10th	Night	21% 10% 36% 29% 4	760
	11th to 20th	Night	20% 12% 36% 27% 5%	743
	21st to end	Night	23% 11% 31% 30% 4	628
March	1st to 10th	Night	23% 10% 33% 28% 6%	760
	11th to 20th	Night	24% 13% 31% 27% 6%	760
	21st to end	Night	22% 11% 28% 29% 9% 1	836
April	1st to 10th	Night	27% 10% 30% 28% 6%	760
	11th to 20th	Night	23% 13% 31% 24% 9% 1	751
	21st to end	Night	25% 10% 27% 27% 10% 1	647
May	1st to 10th	Night	24% 7% 28% 30% 10% 2	380
	11th to 20th	Night	21% 7% 29% 32% 9% 1	374
	21st to end	Night	14% 6% 35% 32% 14%	411
June	1st to 10th	Night	17% 4 29% 39% 11% 1	380
	11th to 20th	Night	16% 5% 27% 37% 13% 1	380
	21st to end	Night	18% 5% 27% 34% 16% 1	380
July	1st to 10th	Night	9% 5% 25% 43% 17%	370
	11th to 20th	Night	12% 4 20% 46% 18% 1	368
	21st to end	Night	11% 3 22% 46% 17% 1	416
August	1st to 10th	Night	13% 2 23% 40% 21% 1	388
	11th to 20th	Night	14% 3 16% 37% 28% 2	589
	21st to end	Night	14% 4 22% 36% 21% 2	836
September	1st to 10th	Night	14% 4 24% 37% 19% 1	760
	11th to 20th	Night	16% 5% 18% 33% 25% 3	760
	21st to end	Night	18% 6% 24% 28% 22% 2	760
October	1st to 10th	Night	23% 5% 26% 28% 16% 2	760
	11th to 20th	Night	23% 8% 30% 28% 9% 1	760
	21st to end	Night	27% 11% 27% 27% 8%	836
November	1st to 10th	Night	26% 11% 26% 27% 10% 1	780
	11th to 20th	Night	24% 12% 34% 21% 9%	800
	21st to end	Night	26% 13% 30% 24% 8%	800
December	1st to 10th	Night	29% 11% 29% 25% 5% 1	800
	11th to 20th	Night	29% 12% 31% 22% 6% 1	800
	21st to end	Night	32% 10% 29% 23% 5%	880

Page generated at 2025-01-04 03:23:59. Version ac07

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