Borderline Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Borderline Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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% 8% 3 7% 5% 5% 5% 4 6% 7% 34%	126518
	Night	16% 8% 3 7% 5% 4 4 4 6% 7% 37%	123185
Transparency	Day	46% 6% 12% 21% 13% 2	130025
	Night	44% 6% 13% 21% 14% 2	127765
Seeing	Day	25% 18% 28% 18% 8% 3	16121
	Night	23% 16% 28% 24% 8% 1	25059

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	10% 8% 4 12% 7% 6% 6% 5% 6% 7% 29%	3597
	11th to 20th	Day	12% 9% 5% 10% 5% 6% 5% 5% 6% 6% 31%	3570
	21st to end	Day	13% 12% 6% 9% 4 4 4 4 5% 5% 34%	3978
February	1st to 10th	Day	12% 10% 3 9% 7% 6% 5% 5% 7% 7% 28%	1829
	11th to 20th	Day	9% 8% 3 10% 7% 5% 6% 4 7% 7% 33%	1948
	21st to end	Day	17% 12% 3 8% 5% 4 5% 4 5% 5% 31%	1650
March	1st to 10th	Day	17% 12% 4 7% 5% 4 4 4 5% 6% 31%	4476
	11th to 20th	Day	15% 9% 3 7% 5% 5% 5% 4 6% 7% 36%	4652
	21st to end	Day	21% 11% 2 6% 3 3 3 3 5% 7% 35%	5970
April	1st to 10th	Day	17% 10% 3 7% 5% 4 5% 3 5% 6% 34%	2400
	11th to 20th	Day	15% 9% 2 6% 5% 5% 6% 4 5% 7% 34%	2374
	21st to end	Day	18% 8% 2 8% 5% 5% 4 4 6% 6% 33%	2540
May	1st to 10th	Day	32% 9% 3 6% 4 4 4 3 5% 6% 23%	5886
	11th to 20th	Day	21% 8% 3 9% 5% 5% 5% 3 5% 6% 29%	6271
	21st to end	Day	20% 7% 2 6% 4 5% 5% 4 7% 8% 31%	6949
June	1st to 10th	Day	18% 7% 2 6% 6% 5% 6% 4 8% 8% 31%	2800
	11th to 20th	Day	17% 6% 2 6% 6% 5% 5% 4 6% 9% 35%	2800
	21st to end	Day	17% 7% 2 7% 6% 4 5% 5% 7% 8% 32%	2800
July	1st to 10th	Day	12% 6% 3 7% 5% 6% 6% 6% 7% 10% 34%	6360
	11th to 20th	Day	10% 5% 2 6% 5% 5% 6% 5% 8% 11% 37%	6360
	21st to end	Day	16% 6% 2 6% 4 4 4 4 6% 8% 42%	6926
August	1st to 10th	Day	15% 6% 3 7% 5% 5% 5% 4 7% 7% 35%	4468
	11th to 20th	Day	14% 6% 2 7% 5% 5% 6% 5% 8% 9% 33%	3949
	21st to end	Day	10% 4 1 5% 5% 5% 6% 5% 8% 10% 40%	4312
September	1st to 10th	Day	13% 6% 2 4 4 5% 6% 4 6% 7% 44%	2400
	11th to 20th	Day	13% 5% 2 5% 3 4 4 3 5% 7% 47%	2266
	21st to end	Day	17% 6% 2 5% 4 3 4 4 5% 8% 40%	2200
October	1st to 10th	Day	21% 6% 2 6% 4 5% 4 4 4 5% 39%	2152
	11th to 20th	Day	15% 7% 2 5% 4 5% 4 4 6% 6% 41%	1942
	21st to end	Day	20% 9% 2 7% 4 5% 5% 4 6% 7% 32%	1881
November	1st to 10th	Day	12% 7% 2 5% 5% 4 4 3 4 7% 45%	1761
	11th to 20th	Day	16% 8% 3 5% 4 5% 5% 4 5% 6% 41%	1800
	21st to end	Day	18% 9% 3 7% 3 4 5% 4 5% 6% 36%	1705
December	1st to 10th	Day	14% 11% 3 10% 5% 5% 5% 4 5% 7% 31%	2600
	11th to 20th	Day	19% 11% 4 6% 5% 4 4 3 5% 6% 34%	2986
	21st to end	Day	15% 11% 3 8% 5% 5% 5% 4 6% 7% 32%	3960
January	1st to 10th	Night	12% 7% 4 12% 6% 5% 4 4 5% 6% 36%	6349
	11th to 20th	Night	10% 10% 4 10% 7% 6% 6% 4 7% 8% 27%	6315
	21st to end	Night	12% 10% 3 9% 5% 5% 4 4 6% 7% 34%	6977
February	1st to 10th	Night	12% 8% 3 8% 6% 6% 6% 4 6% 8% 33%	2570
	11th to 20th	Night	9% 9% 3 7% 7% 6% 6% 5% 7% 8% 34%	2338
	21st to end	Night	14% 10% 2 8% 6% 4 4 4 5% 8% 35%	1980
March	1st to 10th	Night	18% 9% 3 6% 5% 4 4 3 6% 7% 33%	5364
	11th to 20th	Night	14% 8% 3 6% 5% 5% 5% 4 6% 8% 38%	5292
	21st to end	Night	20% 10% 3 7% 4 4 4 3 5% 7% 33%	4962
April	1st to 10th	Night	16% 8% 3 6% 5% 5% 5% 4 5% 8% 34%	2000
	11th to 20th	Night	17% 9% 2 5% 5% 4 5% 4 5% 7% 37%	1975
	21st to end	Night	19% 8% 3 6% 3 4 4 4 6% 7% 37%	1859
May	1st to 10th	Night	32% 9% 2 6% 3 3 4 3 5% 7% 25%	4054
	11th to 20th	Night	16% 11% 3 8% 6% 5% 5% 4 6% 6% 29%	3652
	21st to end	Night	21% 7% 3 7% 4 4 4 4 6% 8% 31%	3926
June	1st to 10th	Night	18% 6% 2 6% 4 4 4 3 5% 8% 40%	1600
	11th to 20th	Night	17% 6% 1 5% 5% 4 4 4 5% 9% 40%	1600
	21st to end	Night	16% 6% 2 5% 3 4 4 4 6% 7% 42%	1600
July	1st to 10th	Night	12% 6% 2 6% 5% 4 4 4 7% 9% 42%	3600
	11th to 20th	Night	14% 6% 2 6% 5% 5% 5% 5% 6% 9% 37%	3600
	21st to end	Night	12% 5% 2 5% 3 4 3 3 5% 7% 51%	3926
August	1st to 10th	Night	22% 5% 1 6% 5% 4 5% 3 6% 7% 37%	2647
	11th to 20th	Night	15% 6% 3 6% 3 4 4 4 6% 7% 42%	3094
	21st to end	Night	10% 3 1 3 3 3 4 4 6% 9% 55%	3586
September	1st to 10th	Night	13% 5% 2 4 4 3 4 5% 6% 8% 47%	2000
	11th to 20th	Night	13% 4 2 4 4 4 3 3 4 6% 52%	2132
	21st to end	Night	17% 6% 2 6% 4 4 4 3 4 6% 45%	2200
October	1st to 10th	Night	20% 6% 2 5% 4 3 4 3 5% 6% 42%	2245
	11th to 20th	Night	15% 7% 2 5% 4 3 4 4 7% 7% 43%	2413
	21st to end	Night	21% 8% 2 6% 4 4 4 3 5% 7% 36%	2717
November	1st to 10th	Night	13% 7% 2 5% 5% 4 5% 3 5% 6% 46%	2537
	11th to 20th	Night	17% 6% 2 6% 5% 5% 4 4 5% 8% 39%	2600
	21st to end	Night	13% 9% 3 6% 4 3 4 4 6% 8% 40%	2691
December	1st to 10th	Night	14% 11% 4 9% 4 5% 4 4 6% 8% 30%	4580
	11th to 20th	Night	22% 9% 3 6% 3 4 4 3 6% 7% 34%	5208
	21st to end	Night	17% 10% 4 9% 5% 4 4 3 5% 5% 33%	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	48% 7% 11% 24% 11% 1	3598
	11th to 20th	Day	48% 6% 11% 17% 15% 3	3600
	21st to end	Day	49% 5% 7% 21% 16% 2	3978
February	1st to 10th	Day	51% 6% 14% 17% 10% 1	1829
	11th to 20th	Day	48% 6% 11% 19% 13% 2	1957
	21st to end	Day	52% 5% 10% 18% 13% 2	1650
March	1st to 10th	Day	52% 5% 11% 13% 11% 7%	4479
	11th to 20th	Day	44% 5% 18% 20% 9% 3	4652
	21st to end	Day	49% 3 7% 16% 20% 5%	5970
April	1st to 10th	Day	51% 4 10% 20% 13% 3	2400
	11th to 20th	Day	46% 6% 12% 19% 15% 2	2374
	21st to end	Day	49% 6% 10% 18% 14% 3	2540
May	1st to 10th	Day	61% 6% 10% 14% 8% 1	5897
	11th to 20th	Day	54% 6% 14% 16% 9% 1	6275
	21st to end	Day	48% 6% 14% 21% 10% 1	6949
June	1st to 10th	Day	47% 8% 13% 20% 12% 1	2800
	11th to 20th	Day	44% 7% 14% 20% 13% 1	2800
	21st to end	Day	47% 8% 13% 20% 12% 1	2800
July	1st to 10th	Day	45% 7% 13% 22% 13% 1	6290
	11th to 20th	Day	39% 7% 16% 25% 12% 1	6285
	21st to end	Day	40% 5% 13% 27% 14%	6728
August	1st to 10th	Day	45% 8% 16% 21% 11% 1	4454
	11th to 20th	Day	43% 6% 17% 26% 8% 1	3988
	21st to end	Day	35% 6% 15% 29% 13% 1	4312
September	1st to 10th	Day	37% 5% 13% 28% 16% 1	2400
	11th to 20th	Day	35% 6% 10% 26% 21% 2	2266
	21st to end	Day	41% 6% 12% 23% 16% 3	2200
October	1st to 10th	Day	43% 5% 9% 22% 20% 2	3503
	11th to 20th	Day	36% 6% 9% 26% 18% 4	3158
	21st to end	Day	36% 5% 11% 29% 14% 4	3081
November	1st to 10th	Day	38% 5% 15% 24% 16% 2	1761
	11th to 20th	Day	43% 5% 11% 20% 19% 3	1800
	21st to end	Day	46% 5% 16% 22% 9% 2	1705
December	1st to 10th	Day	50% 5% 13% 25% 7% 1	2600
	11th to 20th	Day	50% 4 10% 18% 16% 1	2986
	21st to end	Day	49% 6% 10% 16% 13% 5%	3960
January	1st to 10th	Night	44% 8% 11% 24% 12% 2	6349
	11th to 20th	Night	50% 6% 15% 15% 12% 2	6360
	21st to end	Night	46% 6% 11% 16% 17% 3	6977
February	1st to 10th	Night	47% 8% 14% 20% 10% 2	2570
	11th to 20th	Night	45% 7% 13% 20% 13% 2	2340
	21st to end	Night	46% 4 12% 23% 11% 3	1980
March	1st to 10th	Night	47% 5% 12% 17% 15% 3	5374
	11th to 20th	Night	43% 6% 18% 21% 10% 3	5293
	21st to end	Night	49% 4 8% 14% 22% 3	4962
April	1st to 10th	Night	46% 5% 12% 21% 14% 1	2000
	11th to 20th	Night	44% 7% 11% 21% 15% 1	1975
	21st to end	Night	45% 5% 11% 20% 16% 3	1859
May	1st to 10th	Night	59% 6% 12% 14% 8% 1	4060
	11th to 20th	Night	53% 8% 13% 16% 9% 2	3653
	21st to end	Night	50% 6% 14% 18% 11% 1	3927
June	1st to 10th	Night	44% 6% 13% 25% 11% 1	1600
	11th to 20th	Night	41% 9% 16% 22% 12% 1	1600
	21st to end	Night	38% 9% 15% 24% 13% 1	1600
July	1st to 10th	Night	39% 6% 16% 24% 15% 1	3560
	11th to 20th	Night	42% 7% 15% 20% 15% 1	3552
	21st to end	Night	33% 5% 14% 33% 15% 1	3809
August	1st to 10th	Night	46% 7% 15% 22% 9%	2637
	11th to 20th	Night	40% 6% 19% 26% 9%	3121
	21st to end	Night	25% 5% 16% 38% 15% 1	3586
September	1st to 10th	Night	34% 7% 13% 27% 18% 1	2000
	11th to 20th	Night	32% 4 12% 28% 22% 2	2132
	21st to end	Night	40% 5% 10% 26% 16% 2	2200
October	1st to 10th	Night	41% 5% 12% 24% 16% 2	3674
	11th to 20th	Night	31% 5% 12% 25% 21% 6%	3936
	21st to end	Night	36% 4 14% 32% 13% 1	4467
November	1st to 10th	Night	38% 6% 12% 24% 18% 3	2537
	11th to 20th	Night	42% 5% 12% 21% 16% 3	2600
	21st to end	Night	40% 6% 18% 24% 11% 1	2691
December	1st to 10th	Night	49% 6% 11% 26% 7%	4580
	11th to 20th	Night	49% 4 14% 13% 17% 2	5208
	21st to end	Night	52% 4 10% 16% 13% 5%	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	36% 21% 28% 13% 3	200
	11th to 20th	Day	31% 24% 32% 8% 4 1	200
	21st to end	Day	28% 26% 28% 13% 4	223
February	1st to 10th	Day	35% 31% 24% 9% 1	329
	11th to 20th	Day	34% 26% 29% 9% 2	496
	21st to end	Day	32% 25% 29% 12% 3	414
March	1st to 10th	Day	32% 23% 30% 13% 2	500
	11th to 20th	Day	32% 25% 27% 11% 3 1	500
	21st to end	Day	31% 25% 28% 9% 5% 2	550
April	1st to 10th	Day	35% 23% 25% 9% 6% 2	500
	11th to 20th	Day	37% 21% 25% 9% 7% 1	492
	21st to end	Day	30% 21% 29% 14% 4 2	500
May	1st to 10th	Day	34% 19% 27% 12% 5% 2	503
	11th to 20th	Day	24% 19% 33% 17% 5% 2	527
	21st to end	Day	21% 15% 31% 21% 9% 3	573
June	1st to 10th	Day	21% 15% 32% 22% 8% 2	530
	11th to 20th	Day	20% 14% 33% 22% 11% 1	530
	21st to end	Day	21% 17% 32% 18% 9% 3	530
July	1st to 10th	Day	15% 12% 32% 25% 13% 2	525
	11th to 20th	Day	15% 8% 34% 30% 12% 2	524
	21st to end	Day	14% 12% 37% 24% 10% 2	576
August	1st to 10th	Day	12% 9% 29% 28% 17% 6%	526
	11th to 20th	Day	16% 9% 28% 29% 12% 7%	519
	21st to end	Day	15% 11% 28% 28% 12% 6%	583
September	1st to 10th	Day	20% 10% 24% 23% 16% 7%	530
	11th to 20th	Day	16% 10% 23% 24% 17% 10%	530
	21st to end	Day	21% 12% 29% 21% 12% 5%	530
October	1st to 10th	Day	26% 15% 23% 19% 12% 5%	530
	11th to 20th	Day	22% 21% 26% 20% 9% 2	502
	21st to end	Day	32% 15% 26% 20% 6% 1	429
November	1st to 10th	Day	25% 21% 25% 21% 8% 1	395
	11th to 20th	Day	29% 25% 25% 16% 5% 1	400
	21st to end	Day	35% 18% 25% 17% 4 1	305
December	1st to 10th	Day	30% 27% 26% 14% 4 1	200
	11th to 20th	Day	44% 17% 31% 6% 3	200
	21st to end	Day	32% 21% 27% 15% 4	220
January	1st to 10th	Night	26% 22% 32% 17% 3	800
	11th to 20th	Night	27% 19% 32% 19% 3 1	800
	21st to end	Night	21% 24% 34% 19% 3	880
February	1st to 10th	Night	24% 22% 36% 16% 2	800
	11th to 20th	Night	26% 24% 34% 16% 1	783
	21st to end	Night	28% 22% 31% 17% 2	660
March	1st to 10th	Night	26% 24% 31% 17% 2	800
	11th to 20th	Night	26% 21% 29% 21% 4	800
	21st to end	Night	26% 19% 29% 20% 5%	880
April	1st to 10th	Night	28% 20% 31% 17% 4	800
	11th to 20th	Night	25% 22% 28% 19% 6%	791
	21st to end	Night	29% 19% 26% 21% 5%	659
May	1st to 10th	Night	30% 12% 28% 22% 8% 1	600
	11th to 20th	Night	21% 15% 30% 28% 5%	432
	21st to end	Night	23% 7% 27% 30% 12% 1	433
June	1st to 10th	Night	21% 11% 29% 28% 12%	400
	11th to 20th	Night	18% 9% 30% 32% 12%	400
	21st to end	Night	16% 9% 28% 33% 13% 1	400
July	1st to 10th	Night	13% 7% 26% 39% 14%	390
	11th to 20th	Night	15% 5% 23% 40% 15% 2	388
	21st to end	Night	12% 5% 26% 39% 16%	438
August	1st to 10th	Night	14% 4 23% 36% 21% 2	433
	11th to 20th	Night	15% 6% 24% 32% 20% 3	723
	21st to end	Night	17% 6% 22% 34% 19% 1	880
September	1st to 10th	Night	15% 8% 24% 33% 18% 2	800
	11th to 20th	Night	15% 8% 22% 30% 20% 5%	800
	21st to end	Night	21% 9% 22% 30% 16% 2	800
October	1st to 10th	Night	26% 12% 20% 25% 14% 2	800
	11th to 20th	Night	22% 13% 30% 24% 11% 1	793
	21st to end	Night	28% 18% 27% 22% 5%	836
November	1st to 10th	Night	21% 19% 28% 23% 8% 1	780
	11th to 20th	Night	25% 18% 29% 20% 7% 1	800
	21st to end	Night	25% 20% 27% 23% 5% 1	800
December	1st to 10th	Night	25% 23% 31% 15% 5%	800
	11th to 20th	Night	30% 19% 28% 19% 4	800
	21st to end	Night	29% 23% 28% 17% 3	880

Page generated at 2025-10-16 01:50:24. Version ac07

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