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 2026-05-02.

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	16% 10% 3 8% 5% 5% 5% 4 5% 7% 31%	131911
	Night	16% 12% 4 9% 5% 5% 4 3 5% 6% 29%	130547
Transparency	Day	51% 6% 12% 21% 9% 1	135376
	Night	54% 6% 11% 18% 10% 1	135169
Seeing	Day	21% 11% 29% 23% 12% 4	16292
	Night	22% 9% 29% 29% 11% 1	25488

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% 17% 6% 14% 6% 5% 5% 3 5% 5% 20%	4661
	11th to 20th	Day	13% 17% 8% 13% 7% 6% 5% 3 4 5% 19%	4650
	21st to end	Day	9% 14% 6% 14% 9% 8% 5% 4 5% 5% 20%	5286
February	1st to 10th	Day	13% 16% 5% 11% 7% 5% 3 4 5% 6% 24%	1890
	11th to 20th	Day	13% 16% 5% 11% 6% 5% 3 4 6% 6% 26%	1869
	21st to end	Day	18% 15% 4 9% 7% 5% 5% 5% 5% 5% 24%	1730
March	1st to 10th	Day	18% 12% 4 12% 6% 5% 5% 3 5% 6% 24%	4576
	11th to 20th	Day	12% 13% 4 9% 5% 6% 5% 4 6% 7% 29%	5038
	21st to end	Day	18% 10% 3 8% 5% 4 4 3 4 5% 35%	5932
April	1st to 10th	Day	21% 13% 3 7% 4 4 4 3 3 6% 33%	2520
	11th to 20th	Day	22% 9% 3 7% 5% 3 3 3 4 6% 35%	2494
	21st to end	Day	22% 10% 3 7% 5% 4 4 3 4 5% 32%	2659
May	1st to 10th	Day	23% 11% 4 8% 5% 5% 5% 3 5% 6% 25%	6360
	11th to 20th	Day	16% 10% 4 9% 6% 5% 5% 4 5% 7% 30%	6318
	21st to end	Day	21% 8% 3 7% 5% 5% 4 4 5% 6% 33%	6949
June	1st to 10th	Day	21% 10% 2 7% 6% 4 4 4 5% 6% 30%	2800
	11th to 20th	Day	13% 6% 2 7% 4 6% 4 4 6% 8% 39%	2800
	21st to end	Day	19% 7% 2 7% 4 5% 5% 5% 7% 7% 32%	2800
July	1st to 10th	Day	12% 7% 2 8% 6% 5% 6% 5% 6% 9% 34%	6360
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 4 6% 9% 42%	6360
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 7% 10% 44%	6926
August	1st to 10th	Day	14% 7% 3 8% 5% 5% 5% 4 6% 8% 37%	4536
	11th to 20th	Day	15% 8% 2 5% 4 4 5% 4 6% 9% 38%	4086
	21st to end	Day	14% 6% 2 7% 4 4 5% 4 6% 9% 39%	4312
September	1st to 10th	Day	13% 6% 2 6% 4 5% 5% 4 5% 8% 43%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 7% 49%	2385
	21st to end	Day	17% 6% 3 5% 4 5% 4 3 5% 7% 40%	2158
October	1st to 10th	Day	18% 8% 3 6% 5% 4 4 3 5% 7% 37%	2000
	11th to 20th	Day	20% 8% 4 8% 5% 5% 5% 4 5% 7% 30%	2000
	21st to end	Day	25% 10% 3 7% 5% 5% 5% 3 4 6% 28%	2020
November	1st to 10th	Day	16% 9% 3 7% 5% 5% 6% 4 7% 7% 31%	1851
	11th to 20th	Day	18% 12% 3 9% 5% 5% 5% 4 5% 6% 28%	1711
	21st to end	Day	21% 16% 4 9% 5% 5% 5% 3 5% 5% 22%	1680
December	1st to 10th	Day	19% 21% 5% 11% 5% 5% 5% 4 5% 5% 14%	2680
	11th to 20th	Day	18% 16% 5% 11% 6% 4 5% 3 5% 5% 23%	3066
	21st to end	Day	22% 15% 5% 12% 5% 5% 5% 3 4 4 20%	4048
January	1st to 10th	Night	13% 18% 8% 16% 6% 4 4 3 4 4 20%	8234
	11th to 20th	Night	12% 17% 7% 16% 8% 6% 5% 3 5% 4 18%	8235
	21st to end	Night	12% 16% 6% 11% 7% 6% 5% 4 5% 6% 22%	8877
February	1st to 10th	Night	13% 15% 5% 11% 6% 6% 5% 4 5% 6% 24%	2730
	11th to 20th	Night	13% 13% 5% 11% 7% 6% 4 4 5% 5% 28%	2637
	21st to end	Night	15% 14% 5% 8% 7% 4 5% 3 4 6% 29%	2076
March	1st to 10th	Night	21% 14% 4 8% 5% 5% 4 3 4 4 28%	5484
	11th to 20th	Night	13% 13% 4 8% 6% 5% 5% 3 6% 8% 29%	5139
	21st to end	Night	18% 12% 4 8% 5% 4 4 3 5% 5% 33%	5228
April	1st to 10th	Night	22% 10% 2 7% 5% 5% 4 4 5% 5% 32%	2100
	11th to 20th	Night	19% 10% 3 8% 5% 4 4 4 4 5% 34%	2075
	21st to end	Night	21% 9% 3 7% 5% 4 3 3 5% 5% 36%	1975
May	1st to 10th	Night	22% 10% 3 9% 5% 6% 5% 4 6% 7% 23%	3602
	11th to 20th	Night	16% 8% 3 6% 4 4 5% 4 5% 7% 36%	3575
	21st to end	Night	16% 10% 3 9% 5% 5% 5% 5% 6% 7% 31%	3926
June	1st to 10th	Night	16% 7% 2 7% 5% 6% 6% 5% 6% 8% 32%	1600
	11th to 20th	Night	14% 7% 2 6% 4 4 4 5% 6% 7% 41%	1600
	21st to end	Night	18% 7% 3 7% 5% 5% 4 4 6% 8% 35%	1600
July	1st to 10th	Night	14% 7% 2 8% 6% 5% 6% 5% 7% 8% 33%	3600
	11th to 20th	Night	11% 6% 2 7% 5% 4 5% 4 6% 8% 42%	3600
	21st to end	Night	9% 6% 3 6% 4 4 4 4 7% 7% 45%	3926
August	1st to 10th	Night	12% 7% 3 8% 4 4 5% 4 6% 9% 39%	2610
	11th to 20th	Night	15% 6% 2 7% 4 3 5% 4 6% 8% 41%	2928
	21st to end	Night	11% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	3586
September	1st to 10th	Night	10% 6% 2 7% 5% 4 5% 3 5% 6% 47%	2000
	11th to 20th	Night	12% 5% 2 6% 3 3 3 3 6% 7% 50%	2015
	21st to end	Night	16% 6% 2 6% 4 4 4 3 4 6% 44%	2239
October	1st to 10th	Night	18% 7% 2 6% 5% 5% 4 3 5% 6% 39%	2400
	11th to 20th	Night	18% 9% 3 7% 6% 5% 4 4 5% 6% 33%	2400
	21st to end	Night	22% 10% 3 8% 4 5% 4 3 5% 7% 29%	2820
November	1st to 10th	Night	20% 11% 3 7% 4 4 4 3 4 6% 33%	2667
	11th to 20th	Night	18% 10% 3 9% 5% 5% 5% 4 4 6% 30%	2905
	21st to end	Night	20% 14% 5% 10% 6% 4 4 3 5% 6% 24%	2940
December	1st to 10th	Night	18% 17% 5% 11% 7% 6% 5% 3 5% 6% 16%	4720
	11th to 20th	Night	23% 14% 5% 9% 5% 4 4 3 3 4 25%	5348
	21st to end	Night	26% 17% 4 9% 5% 3 4 3 5% 4 20%	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	64% 6% 11% 14% 4	4662
	11th to 20th	Day	66% 5% 9% 11% 9% 1	4680
	21st to end	Day	64% 6% 11% 10% 8% 1	5286
February	1st to 10th	Day	60% 5% 10% 19% 5% 1	1890
	11th to 20th	Day	57% 5% 13% 17% 7% 1	1878
	21st to end	Day	61% 5% 10% 13% 9% 1	1730
March	1st to 10th	Day	59% 4 11% 13% 10% 3	4579
	11th to 20th	Day	52% 6% 17% 15% 9% 1	5038
	21st to end	Day	50% 4 7% 20% 16% 2	5932
April	1st to 10th	Day	54% 4 8% 18% 13% 2	2520
	11th to 20th	Day	51% 4 8% 21% 14% 2	2494
	21st to end	Day	54% 4 9% 16% 14% 3	2659
May	1st to 10th	Day	58% 6% 8% 18% 8% 2	6371
	11th to 20th	Day	52% 7% 12% 18% 9% 1	6322
	21st to end	Day	50% 6% 12% 20% 11% 1	6949
June	1st to 10th	Day	53% 6% 11% 18% 11% 1	2800
	11th to 20th	Day	41% 8% 13% 24% 12% 2	2800
	21st to end	Day	48% 8% 13% 21% 10% 1	2800
July	1st to 10th	Day	45% 7% 14% 26% 9%	6290
	11th to 20th	Day	38% 6% 14% 29% 13%	6285
	21st to end	Day	33% 7% 14% 36% 9%	6728
August	1st to 10th	Day	45% 6% 16% 23% 9%	4522
	11th to 20th	Day	41% 7% 16% 29% 7% 1	4125
	21st to end	Day	40% 6% 16% 25% 11% 1	4312
September	1st to 10th	Day	37% 7% 15% 29% 12%	2400
	11th to 20th	Day	34% 7% 13% 33% 13% 1	2385
	21st to end	Day	43% 7% 15% 25% 11%	2158
October	1st to 10th	Day	46% 5% 11% 29% 9%	3260
	11th to 20th	Day	45% 6% 15% 25% 8% 1	3240
	21st to end	Day	48% 6% 17% 22% 7%	3245
November	1st to 10th	Day	49% 8% 13% 21% 7% 1	1851
	11th to 20th	Day	56% 6% 13% 16% 9% 1	1711
	21st to end	Day	64% 7% 13% 11% 6%	1680
December	1st to 10th	Day	70% 7% 12% 10% 1	2680
	11th to 20th	Day	62% 6% 10% 15% 7%	3066
	21st to end	Day	67% 4 9% 15% 4	4048
January	1st to 10th	Night	67% 4 11% 12% 5%	8234
	11th to 20th	Night	68% 6% 9% 11% 6%	8280
	21st to end	Night	62% 4 9% 13% 11% 1	8877
February	1st to 10th	Night	61% 6% 10% 16% 6% 1	2730
	11th to 20th	Night	57% 5% 11% 19% 7%	2639
	21st to end	Night	57% 4 9% 18% 10% 1	2076
March	1st to 10th	Night	59% 4 7% 15% 12% 3	5494
	11th to 20th	Night	52% 6% 16% 15% 8% 3	5140
	21st to end	Night	52% 4 7% 22% 13% 2	5228
April	1st to 10th	Night	53% 5% 9% 17% 13% 2	2100
	11th to 20th	Night	51% 6% 8% 20% 15% 1	2075
	21st to end	Night	49% 5% 8% 17% 16% 3	1975
May	1st to 10th	Night	57% 7% 9% 16% 8% 3	3608
	11th to 20th	Night	45% 8% 12% 19% 14% 1	3576
	21st to end	Night	50% 8% 11% 17% 14% 1	3927
June	1st to 10th	Night	47% 9% 12% 20% 11% 1	1600
	11th to 20th	Night	40% 7% 11% 22% 17% 2	1600
	21st to end	Night	46% 9% 13% 18% 14% 1	1600
July	1st to 10th	Night	45% 7% 17% 19% 11% 1	3560
	11th to 20th	Night	37% 7% 14% 31% 11%	3552
	21st to end	Night	36% 8% 12% 29% 15%	3809
August	1st to 10th	Night	41% 8% 15% 25% 10%	2600
	11th to 20th	Night	40% 7% 15% 31% 8%	2955
	21st to end	Night	40% 7% 15% 24% 12% 2	3586
September	1st to 10th	Night	37% 8% 11% 29% 15% 1	2000
	11th to 20th	Night	33% 7% 13% 35% 12% 1	2015
	21st to end	Night	41% 6% 12% 28% 13%	2239
October	1st to 10th	Night	41% 5% 14% 28% 11% 1	3920
	11th to 20th	Night	43% 6% 16% 26% 10%	3899
	21st to end	Night	48% 6% 16% 20% 9%	4545
November	1st to 10th	Night	52% 5% 14% 20% 8% 1	2667
	11th to 20th	Night	54% 6% 12% 17% 10% 1	2905
	21st to end	Night	61% 6% 13% 15% 5%	2940
December	1st to 10th	Night	68% 6% 12% 12% 3	4720
	11th to 20th	Night	63% 4 8% 14% 10% 1	5348
	21st to end	Night	68% 4 7% 12% 9%	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	20% 16% 33% 24% 6% 1	220
	11th to 20th	Day	25% 13% 36% 18% 6% 1	220
	21st to end	Day	17% 11% 38% 27% 6% 1	272
February	1st to 10th	Day	26% 15% 37% 16% 5%	330
	11th to 20th	Day	22% 17% 40% 19% 2	356
	21st to end	Day	26% 12% 37% 22% 3	446
March	1st to 10th	Day	23% 14% 34% 23% 6%	540
	11th to 20th	Day	24% 15% 33% 22% 5% 1	540
	21st to end	Day	27% 14% 31% 18% 8% 2	594
April	1st to 10th	Day	28% 16% 32% 16% 7% 1	540
	11th to 20th	Day	30% 17% 24% 16% 11% 2	532
	21st to end	Day	28% 15% 30% 16% 9% 1	540
May	1st to 10th	Day	25% 12% 28% 23% 10% 2	507
	11th to 20th	Day	15% 14% 32% 24% 11% 4	527
	21st to end	Day	19% 11% 28% 24% 14% 3	573
June	1st to 10th	Day	23% 10% 27% 26% 10% 3	530
	11th to 20th	Day	15% 11% 29% 26% 15% 4	530
	21st to end	Day	22% 10% 27% 24% 12% 4	530
July	1st to 10th	Day	14% 5% 26% 33% 17% 6%	525
	11th to 20th	Day	11% 7% 29% 30% 17% 5%	524
	21st to end	Day	13% 6% 29% 30% 18% 4	576
August	1st to 10th	Day	13% 6% 21% 35% 20% 5%	526
	11th to 20th	Day	16% 5% 22% 24% 24% 9%	519
	21st to end	Day	13% 8% 26% 25% 18% 9%	583
September	1st to 10th	Day	15% 9% 26% 22% 16% 12%	530
	11th to 20th	Day	11% 5% 20% 23% 25% 15%	530
	21st to end	Day	19% 9% 26% 23% 16% 6%	530
October	1st to 10th	Day	21% 10% 25% 22% 16% 6%	530
	11th to 20th	Day	22% 13% 33% 21% 9% 2	530
	21st to end	Day	30% 12% 26% 24% 8%	484
November	1st to 10th	Day	24% 17% 28% 20% 10% 1	425
	11th to 20th	Day	25% 12% 36% 17% 6% 3	251
	21st to end	Day	30% 19% 30% 14% 7% 1	220
December	1st to 10th	Day	28% 13% 32% 20% 6%	220
	11th to 20th	Day	29% 19% 33% 15% 3 1	220
	21st to end	Day	31% 14% 31% 16% 7% 1	242
January	1st to 10th	Night	24% 11% 35% 25% 5%	840
	11th to 20th	Night	26% 13% 30% 25% 5% 1	840
	21st to end	Night	20% 8% 34% 30% 8% 1	924
February	1st to 10th	Night	20% 10% 35% 30% 5%	840
	11th to 20th	Night	20% 12% 37% 26% 5%	823
	21st to end	Night	23% 12% 31% 30% 4	692
March	1st to 10th	Night	23% 11% 34% 26% 5%	840
	11th to 20th	Night	24% 13% 31% 27% 5%	840
	21st to end	Night	24% 12% 29% 27% 8% 1	924
April	1st to 10th	Night	27% 10% 30% 28% 5%	840
	11th to 20th	Night	23% 13% 32% 24% 9%	831
	21st to end	Night	25% 10% 28% 28% 9% 1	715
May	1st to 10th	Night	25% 7% 28% 29% 9% 2	402
	11th to 20th	Night	20% 8% 29% 32% 9% 1	394
	21st to end	Night	14% 6% 34% 32% 14%	433
June	1st to 10th	Night	16% 4 29% 40% 10% 1	400
	11th to 20th	Night	16% 5% 29% 36% 13% 1	400
	21st to end	Night	18% 5% 29% 33% 15% 1	400
July	1st to 10th	Night	9% 5% 26% 43% 17%	390
	11th to 20th	Night	12% 5% 20% 46% 17% 1	388
	21st to end	Night	11% 4 23% 46% 16% 1	438
August	1st to 10th	Night	12% 2 23% 40% 22% 1	409
	11th to 20th	Night	14% 3 15% 37% 28% 2	621
	21st to end	Night	14% 5% 23% 36% 21% 2	880
September	1st to 10th	Night	14% 5% 24% 37% 20% 1	800
	11th to 20th	Night	16% 4 18% 33% 26% 3	800
	21st to end	Night	18% 7% 24% 28% 22% 2	800
October	1st to 10th	Night	22% 5% 26% 30% 16% 2	800
	11th to 20th	Night	23% 8% 31% 29% 9% 1	800
	21st to end	Night	27% 10% 28% 28% 8%	880
November	1st to 10th	Night	26% 11% 26% 28% 9% 1	820
	11th to 20th	Night	24% 12% 34% 21% 9%	840
	21st to end	Night	26% 13% 30% 24% 7%	840
December	1st to 10th	Night	29% 12% 29% 25% 5% 1	840
	11th to 20th	Night	28% 12% 32% 22% 5% 1	840
	21st to end	Night	33% 10% 29% 23% 5%	924

Page generated at 2026-04-29 02:21:41. Version ac07

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