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 2024-11-05.

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%	101313
	Night	16% 8% 3 6% 5% 4 4 4 6% 7% 37%	97719
Transparency	Day	45% 6% 13% 21% 13% 2	104838
	Night	44% 6% 14% 22% 13% 2	102283
Seeing	Day	24% 18% 29% 19% 8% 3	14892
	Night	23% 16% 28% 24% 8% 1	23870

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	12% 10% 4 7% 6% 6% 6% 5% 6% 6% 33%	2517
	11th to 20th	Day	15% 9% 5% 11% 6% 6% 5% 4 6% 6% 26%	2520
	21st to end	Day	16% 15% 7% 10% 4 4 4 4 5% 5% 26%	2776
February	1st to 10th	Day	13% 10% 4 8% 7% 6% 5% 6% 7% 7% 28%	1737
	11th to 20th	Day	9% 9% 3 10% 7% 5% 6% 4 7% 7% 34%	1848
	21st to end	Day	18% 12% 3 8% 5% 4 5% 4 5% 5% 31%	1570
March	1st to 10th	Day	20% 15% 4 7% 5% 4 4 3 4 6% 28%	3116
	11th to 20th	Day	12% 10% 3 6% 5% 5% 6% 5% 7% 8% 33%	3250
	21st to end	Day	18% 11% 3 7% 4 3 3 3 5% 7% 37%	4166
April	1st to 10th	Day	17% 11% 3 7% 5% 4 5% 3 5% 6% 34%	2280
	11th to 20th	Day	15% 9% 2 6% 5% 4 6% 4 5% 7% 35%	2254
	21st to end	Day	18% 8% 3 8% 5% 5% 4 4 6% 6% 32%	2413
May	1st to 10th	Day	24% 11% 3 7% 5% 4 4 4 5% 6% 28%	4106
	11th to 20th	Day	24% 8% 3 6% 4 4 5% 3 5% 7% 31%	4365
	21st to end	Day	14% 5% 1 5% 4 5% 5% 4 7% 9% 41%	4837
June	1st to 10th	Day	17% 7% 2 5% 6% 5% 6% 4 8% 8% 31%	2660
	11th to 20th	Day	17% 6% 2 6% 6% 5% 5% 4 6% 9% 35%	2660
	21st to end	Day	17% 7% 2 7% 6% 4 6% 5% 7% 8% 31%	2660
July	1st to 10th	Day	11% 5% 2 7% 5% 6% 6% 6% 7% 10% 35%	4440
	11th to 20th	Day	11% 5% 2 6% 5% 5% 6% 5% 8% 11% 36%	4440
	21st to end	Day	17% 7% 3 7% 5% 5% 4 4 6% 7% 34%	4814
August	1st to 10th	Day	15% 6% 3 7% 5% 5% 5% 4 7% 8% 35%	4331
	11th to 20th	Day	14% 6% 2 7% 5% 5% 6% 6% 8% 9% 32%	3826
	21st to end	Day	10% 4 1 5% 5% 5% 6% 5% 8% 10% 40%	4180
September	1st to 10th	Day	13% 6% 2 4 4 5% 6% 4 5% 7% 43%	2280
	11th to 20th	Day	14% 5% 2 5% 3 4 4 3 5% 7% 47%	2153
	21st to end	Day	17% 6% 2 5% 4 3 4 4 5% 8% 40%	2090
October	1st to 10th	Day	22% 6% 2 6% 5% 5% 4 4 4 5% 37%	2045
	11th to 20th	Day	15% 7% 2 5% 4 5% 4 4 7% 6% 41%	1862
	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%	1708
	11th to 20th	Day	16% 8% 3 6% 4 5% 5% 4 5% 6% 39%	1710
	21st to end	Day	17% 8% 3 7% 3 4 5% 4 5% 6% 36%	1620
December	1st to 10th	Day	16% 12% 4 9% 6% 5% 5% 4 5% 7% 27%	1520
	11th to 20th	Day	20% 10% 3 6% 5% 3 4 3 4 5% 36%	1906
	21st to end	Day	17% 13% 3 8% 5% 5% 5% 4 6% 8% 27%	2772
January	1st to 10th	Night	15% 8% 4 7% 4 4 4 4 5% 6% 39%	4429
	11th to 20th	Night	12% 10% 5% 10% 7% 6% 5% 4 7% 7% 26%	4440
	21st to end	Night	16% 13% 4 10% 5% 5% 4 4 5% 7% 25%	4879
February	1st to 10th	Night	12% 9% 3 8% 5% 6% 6% 4 6% 8% 34%	2442
	11th to 20th	Night	8% 10% 3 7% 6% 6% 6% 5% 7% 9% 35%	2218
	21st to end	Night	14% 10% 2 8% 5% 4 4 3 5% 8% 35%	1884
March	1st to 10th	Night	21% 12% 2 7% 5% 4 4 4 5% 7% 30%	3724
	11th to 20th	Night	12% 9% 3 5% 5% 5% 5% 4 7% 7% 36%	3694
	21st to end	Night	18% 9% 3 7% 5% 4 4 4 5% 7% 34%	3466
April	1st to 10th	Night	16% 8% 3 6% 5% 4 5% 4 5% 8% 35%	1900
	11th to 20th	Night	17% 8% 2 5% 5% 4 4 4 5% 7% 38%	1875
	21st to end	Night	20% 8% 3 6% 3 4 4 3 6% 7% 37%	1766
May	1st to 10th	Night	24% 10% 3 6% 4 4 4 3 5% 6% 29%	2834
	11th to 20th	Night	17% 10% 3 8% 6% 5% 5% 4 6% 6% 29%	2544
	21st to end	Night	15% 6% 2 6% 4 4 4 4 7% 9% 39%	2738
June	1st to 10th	Night	18% 7% 2 6% 4 4 4 3 5% 8% 39%	1520
	11th to 20th	Night	18% 6% 1 5% 4 4 4 5% 5% 9% 41%	1520
	21st to end	Night	16% 6% 2 5% 3 4 4 4 6% 8% 42%	1520
July	1st to 10th	Night	11% 5% 2 6% 6% 5% 5% 5% 7% 9% 41%	2520
	11th to 20th	Night	15% 5% 2 5% 4 4 5% 4 6% 9% 41%	2520
	21st to end	Night	17% 6% 2 6% 4 4 4 3 5% 7% 42%	2739
August	1st to 10th	Night	22% 5% 1 6% 5% 4 5% 3 5% 7% 37%	2565
	11th to 20th	Night	15% 6% 3 6% 3 4 4 4 6% 7% 41%	2997
	21st to end	Night	10% 3 1 3 3 3 4 4 6% 9% 55%	3476
September	1st to 10th	Night	13% 5% 2 4 4 3 4 5% 6% 8% 46%	1900
	11th to 20th	Night	13% 4 2 4 4 4 3 3 4 6% 51%	2025
	21st to end	Night	17% 6% 1 6% 4 4 4 3 4 5% 45%	2090
October	1st to 10th	Night	21% 6% 2 5% 4 4 4 3 5% 6% 40%	2133
	11th to 20th	Night	14% 7% 2 5% 4 3 4 4 6% 6% 43%	2316
	21st to end	Night	21% 8% 2 6% 4 4 4 3 5% 7% 36%	2717
November	1st to 10th	Night	14% 7% 2 5% 5% 4 4 3 4 6% 46%	2470
	11th to 20th	Night	17% 6% 2 5% 5% 5% 4 4 5% 8% 38%	2470
	21st to end	Night	13% 8% 3 6% 4 4 4 4 6% 9% 40%	2556
December	1st to 10th	Night	16% 9% 3 7% 5% 5% 4 4 7% 7% 32%	2660
	11th to 20th	Night	18% 8% 3 6% 4 4 4 4 7% 8% 34%	3288
	21st to end	Night	20% 10% 4 9% 5% 4 5% 4 5% 6% 28%	4884

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	44% 5% 15% 26% 9% 1	2518
	11th to 20th	Day	53% 6% 13% 19% 7% 1	2520
	21st to end	Day	58% 5% 9% 19% 8% 2	2776
February	1st to 10th	Day	51% 6% 14% 18% 10% 1	1737
	11th to 20th	Day	47% 6% 11% 20% 14% 2	1857
	21st to end	Day	53% 5% 10% 18% 13% 2	1570
March	1st to 10th	Day	57% 4 13% 12% 11% 2	3119
	11th to 20th	Day	42% 7% 19% 18% 12% 2	3250
	21st to end	Day	48% 3 7% 18% 20% 4	4166
April	1st to 10th	Day	50% 4 10% 20% 13% 3	2280
	11th to 20th	Day	45% 7% 13% 19% 15% 2	2254
	21st to end	Day	50% 6% 10% 18% 14% 3	2413
May	1st to 10th	Day	55% 7% 10% 16% 11% 1	4120
	11th to 20th	Day	51% 7% 15% 17% 9% 1	4369
	21st to end	Day	36% 7% 16% 26% 14% 1	4837
June	1st to 10th	Day	47% 8% 13% 20% 11% 1	2660
	11th to 20th	Day	44% 7% 14% 20% 13% 1	2660
	21st to end	Day	48% 8% 13% 20% 11%	2660
July	1st to 10th	Day	41% 8% 13% 22% 15% 1	4370
	11th to 20th	Day	40% 7% 16% 26% 11% 1	4365
	21st to end	Day	46% 5% 14% 20% 14% 1	4661
August	1st to 10th	Day	45% 8% 16% 20% 10% 1	4317
	11th to 20th	Day	43% 6% 17% 25% 7% 1	3865
	21st to end	Day	36% 6% 15% 29% 13% 1	4180
September	1st to 10th	Day	37% 5% 13% 28% 16% 1	2280
	11th to 20th	Day	36% 6% 10% 25% 21% 2	2153
	21st to end	Day	41% 6% 12% 23% 16% 3	2090
October	1st to 10th	Day	44% 5% 9% 21% 19% 2	3396
	11th to 20th	Day	36% 6% 10% 26% 18% 4	3078
	21st to end	Day	36% 5% 11% 29% 14% 4	3081
November	1st to 10th	Day	39% 5% 15% 23% 15% 2	1708
	11th to 20th	Day	44% 5% 12% 20% 16% 2	1710
	21st to end	Day	45% 5% 17% 21% 9% 2	1620
December	1st to 10th	Day	54% 6% 15% 14% 10% 1	1520
	11th to 20th	Day	50% 4 13% 19% 13% 1	1906
	21st to end	Day	53% 7% 14% 16% 10%	2772
January	1st to 10th	Night	43% 7% 13% 24% 11% 2	4429
	11th to 20th	Night	52% 6% 16% 15% 8% 2	4440
	21st to end	Night	56% 6% 11% 16% 9% 1	4879
February	1st to 10th	Night	47% 7% 15% 20% 10% 2	2442
	11th to 20th	Night	44% 7% 13% 21% 14% 2	2220
	21st to end	Night	46% 4 13% 23% 10% 4	1884
March	1st to 10th	Night	52% 6% 11% 20% 10% 1	3734
	11th to 20th	Night	42% 8% 14% 24% 11% 2	3695
	21st to end	Night	47% 4 11% 13% 21% 5%	3466
April	1st to 10th	Night	45% 6% 12% 22% 14% 1	1900
	11th to 20th	Night	44% 7% 11% 22% 15% 1	1875
	21st to end	Night	46% 6% 11% 19% 15% 3	1766
May	1st to 10th	Night	53% 6% 11% 19% 9% 1	2840
	11th to 20th	Night	52% 9% 14% 14% 8% 1	2545
	21st to end	Night	38% 8% 15% 22% 14% 2	2739
June	1st to 10th	Night	43% 6% 14% 25% 11% 1	1520
	11th to 20th	Night	40% 9% 17% 22% 12% 1	1520
	21st to end	Night	39% 9% 15% 23% 13% 1	1520
July	1st to 10th	Night	38% 7% 16% 22% 15% 1	2480
	11th to 20th	Night	39% 7% 15% 25% 14%	2472
	21st to end	Night	41% 6% 12% 25% 14% 1	2651
August	1st to 10th	Night	47% 7% 14% 22% 9%	2555
	11th to 20th	Night	41% 6% 19% 26% 9%	3024
	21st to end	Night	25% 5% 16% 38% 15%	3476
September	1st to 10th	Night	34% 7% 13% 27% 18% 1	1900
	11th to 20th	Night	33% 4 12% 27% 22% 2	2025
	21st to end	Night	40% 5% 10% 26% 16% 2	2090
October	1st to 10th	Night	42% 5% 13% 24% 15% 1	3562
	11th to 20th	Night	31% 5% 12% 25% 21% 6%	3839
	21st to end	Night	36% 4 14% 32% 13% 1	4467
November	1st to 10th	Night	39% 5% 13% 23% 17% 3	2470
	11th to 20th	Night	43% 5% 13% 22% 15% 3	2470
	21st to end	Night	40% 6% 19% 24% 11% 1	2556
December	1st to 10th	Night	48% 7% 15% 20% 9% 1	2660
	11th to 20th	Night	47% 6% 17% 16% 13% 2	3288
	21st to end	Night	55% 5% 12% 20% 8% 1	4884

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	34% 19% 29% 14% 3	180
	11th to 20th	Day	30% 24% 32% 8% 4 1	180
	21st to end	Day	23% 28% 30% 14% 5% 1	200
February	1st to 10th	Day	33% 31% 25% 10% 1	297
	11th to 20th	Day	32% 27% 30% 9% 2	456
	21st to end	Day	31% 25% 28% 13% 3	382
March	1st to 10th	Day	30% 22% 31% 14% 2	460
	11th to 20th	Day	33% 25% 27% 11% 3 1	460
	21st to end	Day	30% 25% 28% 10% 5% 2	506
April	1st to 10th	Day	32% 23% 27% 10% 6% 2	460
	11th to 20th	Day	35% 20% 26% 10% 8% 1	452
	21st to end	Day	30% 21% 29% 14% 4 2	460
May	1st to 10th	Day	32% 20% 28% 13% 6% 2	463
	11th to 20th	Day	24% 17% 33% 18% 6% 2	487
	21st to end	Day	19% 16% 31% 22% 10% 3	529
June	1st to 10th	Day	19% 17% 32% 22% 9% 1	490
	11th to 20th	Day	18% 14% 33% 22% 12% 1	490
	21st to end	Day	21% 17% 32% 17% 10% 3	490
July	1st to 10th	Day	15% 11% 31% 26% 14% 2	485
	11th to 20th	Day	15% 7% 33% 30% 12% 2	484
	21st to end	Day	14% 11% 38% 23% 10% 2	532
August	1st to 10th	Day	12% 9% 28% 29% 16% 5%	486
	11th to 20th	Day	16% 9% 28% 28% 11% 7%	479
	21st to end	Day	14% 12% 27% 28% 12% 7%	539
September	1st to 10th	Day	21% 10% 22% 24% 16% 7%	490
	11th to 20th	Day	17% 11% 22% 23% 17% 10%	490
	21st to end	Day	21% 12% 29% 21% 12% 5%	490
October	1st to 10th	Day	27% 15% 24% 18% 11% 4	490
	11th to 20th	Day	21% 21% 26% 21% 9% 2	470
	21st to end	Day	32% 15% 26% 20% 6% 1	429
November	1st to 10th	Day	25% 20% 24% 21% 8% 1	378
	11th to 20th	Day	29% 25% 24% 16% 5% 1	370
	21st to end	Day	32% 18% 27% 19% 4 1	280
December	1st to 10th	Day	29% 26% 25% 15% 4 1	180
	11th to 20th	Day	41% 18% 32% 7% 2	180
	21st to end	Day	34% 22% 27% 14% 4 1	198
January	1st to 10th	Night	25% 22% 33% 18% 3	760
	11th to 20th	Night	27% 19% 32% 19% 2 1	760
	21st to end	Night	21% 23% 33% 19% 3	836
February	1st to 10th	Night	23% 21% 37% 16% 2	760
	11th to 20th	Night	24% 25% 34% 16% 1	743
	21st to end	Night	29% 22% 31% 17% 2	628
March	1st to 10th	Night	25% 24% 31% 18% 2	760
	11th to 20th	Night	26% 21% 29% 20% 4	760
	21st to end	Night	26% 19% 29% 20% 6%	836
April	1st to 10th	Night	26% 21% 31% 18% 4	760
	11th to 20th	Night	24% 22% 28% 20% 7%	751
	21st to end	Night	29% 19% 26% 21% 5%	626
May	1st to 10th	Night	28% 13% 28% 22% 8% 1	570
	11th to 20th	Night	21% 15% 30% 29% 6%	410
	21st to end	Night	21% 8% 27% 30% 13% 1	411
June	1st to 10th	Night	21% 11% 29% 29% 11%	380
	11th to 20th	Night	18% 9% 29% 32% 12%	380
	21st to end	Night	17% 9% 27% 33% 13% 1	380
July	1st to 10th	Night	13% 8% 25% 39% 14%	370
	11th to 20th	Night	15% 5% 24% 40% 15% 2	368
	21st to end	Night	13% 6% 25% 38% 17%	416
August	1st to 10th	Night	13% 4 24% 37% 20% 2	411
	11th to 20th	Night	16% 6% 24% 32% 19% 3	686
	21st to end	Night	17% 6% 21% 35% 20% 2	836
September	1st to 10th	Night	15% 8% 25% 32% 19% 2	760
	11th to 20th	Night	16% 8% 22% 29% 20% 5%	760
	21st to end	Night	21% 9% 23% 29% 16% 2	760
October	1st to 10th	Night	27% 12% 20% 25% 13% 2	760
	11th to 20th	Night	21% 13% 30% 24% 11% 1	760
	21st to end	Night	28% 18% 27% 22% 5%	836
November	1st to 10th	Night	21% 19% 28% 23% 8% 1	760
	11th to 20th	Night	24% 18% 29% 20% 7% 1	760
	21st to end	Night	23% 20% 28% 23% 6% 1	760
December	1st to 10th	Night	25% 23% 32% 15% 5%	760
	11th to 20th	Night	29% 19% 29% 19% 4	760
	21st to end	Night	29% 24% 28% 17% 2	836

Page generated at 2024-11-03 02:11:26. Version ac07

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