Og's Bog
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Og's Bog 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	15% 11% 4 9% 5% 5% 5% 4 5% 7% 31%	127376
	Night	14% 11% 4 9% 6% 5% 5% 4 5% 6% 32%	122285
Transparency	Day	51% 5% 11% 20% 11% 1	130970
	Night	51% 5% 12% 21% 11% 1	126781
Seeing	Day	21% 16% 31% 19% 9% 3	16019
	Night	20% 14% 31% 24% 10% 1	24986

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% 11% 5% 16% 9% 6% 5% 4 5% 5% 19%	3597
	11th to 20th	Day	11% 18% 7% 18% 10% 8% 5% 3 4 4 12%	3916
	21st to end	Day	16% 18% 7% 16% 6% 5% 5% 3 4 5% 15%	4466
February	1st to 10th	Day	14% 17% 5% 14% 8% 6% 6% 4 5% 4 18%	1800
	11th to 20th	Day	12% 16% 7% 14% 9% 5% 5% 4 4 5% 19%	1753
	21st to end	Day	16% 18% 4 11% 6% 4 5% 3 4 5% 24%	1540
March	1st to 10th	Day	17% 12% 6% 10% 7% 4 4 2 4 5% 30%	4932
	11th to 20th	Day	11% 15% 5% 9% 5% 5% 4 3 5% 7% 30%	4980
	21st to end	Day	17% 15% 4 8% 5% 4 3 3 3 5% 32%	5465
April	1st to 10th	Day	16% 17% 4 8% 5% 4 5% 3 5% 5% 29%	2350
	11th to 20th	Day	19% 12% 3 6% 3 4 3 4 5% 7% 34%	2474
	21st to end	Day	20% 10% 4 8% 4 4 4 3 5% 7% 33%	2600
May	1st to 10th	Day	18% 9% 3 7% 4 4 4 3 5% 7% 35%	5888
	11th to 20th	Day	16% 10% 3 8% 5% 4 5% 4 5% 6% 34%	6263
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 7% 38%	6949
June	1st to 10th	Day	16% 7% 2 6% 5% 5% 5% 4 7% 8% 36%	2800
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 4 7% 10% 39%	2866
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 4 6% 8% 38%	3000
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 5% 6% 9% 38%	6663
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 6% 9% 45%	6360
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 6% 9% 46%	6455
August	1st to 10th	Day	14% 7% 3 6% 4 5% 6% 4 7% 7% 36%	4234
	11th to 20th	Day	12% 8% 3 7% 5% 5% 5% 5% 7% 9% 34%	4174
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 6% 9% 42%	4520
September	1st to 10th	Day	12% 7% 3 7% 5% 5% 4 4 6% 7% 40%	2400
	11th to 20th	Day	11% 6% 2 6% 4 5% 4 4 5% 8% 44%	2264
	21st to end	Day	15% 7% 2 9% 6% 6% 5% 4 6% 7% 35%	2170
October	1st to 10th	Day	18% 9% 3 8% 5% 4 4 3 4 6% 35%	2000
	11th to 20th	Day	19% 10% 4 10% 6% 5% 4 4 5% 6% 28%	1980
	21st to end	Day	25% 16% 3 9% 4 4 4 4 5% 6% 20%	2090
November	1st to 10th	Day	13% 14% 3 9% 6% 5% 5% 4 5% 8% 29%	1681
	11th to 20th	Day	13% 16% 4 10% 7% 6% 5% 4 6% 7% 23%	1600
	21st to end	Day	16% 19% 6% 13% 6% 5% 5% 4 5% 5% 18%	1600
December	1st to 10th	Day	19% 21% 6% 12% 6% 6% 5% 3 5% 5% 13%	2600
	11th to 20th	Day	18% 22% 5% 12% 5% 4 5% 3 4 5% 18%	2986
	21st to end	Day	24% 18% 7% 17% 5% 4 4 3 4 4 10%	3960
January	1st to 10th	Night	13% 13% 6% 13% 10% 6% 5% 4 5% 5% 20%	6349
	11th to 20th	Night	11% 15% 8% 17% 12% 7% 5% 4 5% 5% 11%	5951
	21st to end	Night	14% 15% 7% 12% 8% 6% 5% 3 5% 6% 19%	6490
February	1st to 10th	Night	13% 16% 6% 11% 7% 5% 5% 4 5% 6% 23%	2600
	11th to 20th	Night	12% 15% 6% 12% 7% 6% 5% 4 5% 5% 23%	2533
	21st to end	Night	12% 14% 4 11% 7% 5% 5% 4 6% 6% 27%	2086
March	1st to 10th	Night	15% 11% 5% 10% 6% 5% 5% 4 5% 6% 28%	4908
	11th to 20th	Night	9% 11% 4 7% 5% 5% 5% 5% 7% 8% 35%	4979
	21st to end	Night	18% 10% 4 7% 4 4 4 3 5% 5% 36%	5467
April	1st to 10th	Night	18% 12% 4 9% 5% 4 4 4 5% 6% 30%	2049
	11th to 20th	Night	16% 10% 3 5% 4 4 5% 4 5% 7% 37%	1892
	21st to end	Night	14% 11% 3 7% 5% 5% 4 4 5% 6% 36%	1800
May	1st to 10th	Night	19% 9% 2 6% 4 4 4 4 5% 6% 36%	4053
	11th to 20th	Night	13% 8% 2 7% 5% 4 4 3 5% 7% 41%	3626
	21st to end	Night	18% 6% 2 5% 4 3 3 3 5% 7% 45%	3926
June	1st to 10th	Night	13% 5% 3 5% 5% 5% 5% 4 6% 9% 41%	1600
	11th to 20th	Night	9% 4 3 7% 4 4 6% 5% 6% 10% 43%	1526
	21st to end	Night	12% 5% 2 6% 5% 3 4 4 5% 7% 46%	1400
July	1st to 10th	Night	7% 6% 3 6% 5% 5% 4 4 6% 8% 47%	3290
	11th to 20th	Night	11% 5% 1 5% 4 4 4 3 5% 7% 52%	3600
	21st to end	Night	8% 5% 1 5% 3 4 4 4 6% 7% 52%	4380
August	1st to 10th	Night	9% 6% 2 6% 4 4 4 3 5% 7% 49%	2915
	11th to 20th	Night	13% 6% 2 5% 3 4 4 4 5% 8% 48%	2875
	21st to end	Night	11% 6% 2 4 4 4 4 3 6% 8% 47%	3345
September	1st to 10th	Night	8% 5% 2 6% 5% 4 4 4 6% 7% 48%	2000
	11th to 20th	Night	10% 5% 2 4 4 3 3 3 5% 7% 55%	2133
	21st to end	Night	16% 7% 3 7% 5% 3 4 2 4 7% 41%	2230
October	1st to 10th	Night	17% 7% 2 6% 5% 5% 4 4 5% 7% 40%	2400
	11th to 20th	Night	17% 10% 3 8% 4 5% 4 4 5% 6% 34%	2377
	21st to end	Night	21% 11% 4 8% 5% 5% 5% 4 5% 6% 26%	2508
November	1st to 10th	Night	15% 11% 3 7% 5% 4 4 4 5% 6% 35%	2613
	11th to 20th	Night	16% 14% 4 7% 6% 5% 5% 5% 6% 7% 27%	2800
	21st to end	Night	17% 16% 5% 10% 5% 5% 5% 4 5% 6% 22%	2800
December	1st to 10th	Night	17% 17% 6% 13% 6% 5% 6% 5% 7% 6% 14%	4580
	11th to 20th	Night	14% 21% 7% 13% 6% 5% 4 3 5% 5% 18%	5208
	21st to end	Night	22% 18% 7% 15% 5% 5% 4 3 3 5% 12%	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	64% 5% 13% 13% 5%	3598
	11th to 20th	Day	74% 5% 9% 8% 3	3950
	21st to end	Day	71% 4 10% 9% 5%	4466
February	1st to 10th	Day	68% 5% 7% 15% 4 1	1800
	11th to 20th	Day	65% 6% 7% 14% 6% 1	1762
	21st to end	Day	63% 4 8% 16% 7% 1	1540
March	1st to 10th	Day	57% 3 13% 14% 12% 1	4935
	11th to 20th	Day	53% 5% 13% 20% 7% 2	4980
	21st to end	Day	55% 4 8% 18% 13% 2	5465
April	1st to 10th	Day	57% 4 9% 19% 10% 1	2350
	11th to 20th	Day	50% 4 8% 24% 14% 1	2474
	21st to end	Day	52% 4 10% 20% 12% 2	2600
May	1st to 10th	Day	49% 4 12% 19% 12% 5%	5897
	11th to 20th	Day	51% 4 10% 17% 17% 1	6267
	21st to end	Day	46% 4 11% 22% 15% 2	6949
June	1st to 10th	Day	44% 6% 12% 22% 16% 1	2800
	11th to 20th	Day	39% 5% 14% 25% 16% 1	2866
	21st to end	Day	42% 5% 14% 25% 13% 1	3000
July	1st to 10th	Day	40% 7% 14% 23% 15% 1	6593
	11th to 20th	Day	33% 6% 12% 32% 16% 1	6285
	21st to end	Day	34% 7% 12% 31% 15% 1	6270
August	1st to 10th	Day	44% 6% 14% 23% 12% 1	4221
	11th to 20th	Day	43% 6% 17% 24% 10%	4213
	21st to end	Day	37% 7% 13% 32% 10% 1	4520
September	1st to 10th	Day	42% 6% 12% 28% 10% 2	2400
	11th to 20th	Day	37% 5% 13% 32% 14%	2264
	21st to end	Day	48% 5% 12% 26% 9%	2170
October	1st to 10th	Day	42% 6% 13% 27% 12% 1	3260
	11th to 20th	Day	43% 5% 14% 24% 14%	3220
	21st to end	Day	58% 6% 13% 18% 5%	3428
November	1st to 10th	Day	53% 6% 12% 20% 9% 1	1681
	11th to 20th	Day	60% 4 13% 14% 8% 1	1600
	21st to end	Day	68% 5% 11% 13% 2	1600
December	1st to 10th	Day	73% 6% 11% 8% 3	2600
	11th to 20th	Day	68% 8% 9% 9% 7%	2986
	21st to end	Day	77% 5% 6% 9% 4	3960
January	1st to 10th	Night	63% 6% 11% 16% 4	6349
	11th to 20th	Night	74% 5% 9% 9% 3	5992
	21st to end	Night	65% 4 11% 11% 8% 1	6490
February	1st to 10th	Night	60% 6% 9% 19% 6%	2600
	11th to 20th	Night	61% 5% 9% 17% 8%	2535
	21st to end	Night	56% 4 12% 20% 8% 1	2086
March	1st to 10th	Night	56% 5% 10% 18% 10% 2	4918
	11th to 20th	Night	45% 5% 17% 22% 10% 2	4980
	21st to end	Night	50% 4 8% 20% 15% 2	5467
April	1st to 10th	Night	55% 4 8% 19% 12% 1	2049
	11th to 20th	Night	47% 4 9% 22% 16% 2	1892
	21st to end	Night	48% 5% 10% 20% 16% 2	1800
May	1st to 10th	Night	49% 4 11% 16% 14% 6%	4060
	11th to 20th	Night	44% 4 13% 23% 14% 2	3627
	21st to end	Night	41% 4 13% 26% 14% 2	3927
June	1st to 10th	Night	39% 6% 10% 26% 17% 2	1600
	11th to 20th	Night	37% 5% 13% 25% 19% 2	1526
	21st to end	Night	37% 5% 13% 28% 16% 1	1400
July	1st to 10th	Night	34% 7% 14% 25% 18% 2	3253
	11th to 20th	Night	31% 6% 11% 32% 19% 1	3552
	21st to end	Night	29% 7% 11% 33% 18% 1	4251
August	1st to 10th	Night	35% 4 14% 30% 16% 2	2904
	11th to 20th	Night	35% 5% 15% 32% 12% 1	2902
	21st to end	Night	33% 6% 14% 34% 12% 1	3345
September	1st to 10th	Night	33% 6% 16% 31% 13% 1	2000
	11th to 20th	Night	29% 4 11% 36% 18% 1	2133
	21st to end	Night	44% 4 12% 28% 11% 1	2230
October	1st to 10th	Night	34% 5% 10% 29% 22% 1	3920
	11th to 20th	Night	42% 4 14% 25% 14%	3876
	21st to end	Night	45% 8% 15% 24% 9%	4120
November	1st to 10th	Night	48% 5% 12% 24% 10% 1	2613
	11th to 20th	Night	55% 5% 13% 18% 9% 1	2800
	21st to end	Night	62% 5% 15% 14% 3 1	2800
December	1st to 10th	Night	66% 8% 11% 11% 3	4580
	11th to 20th	Night	67% 6% 9% 12% 6%	5208
	21st to end	Night	74% 5% 9% 8% 4	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	23% 23% 34% 16% 4 1	200
	11th to 20th	Day	25% 28% 29% 14% 3 1	277
	21st to end	Day	29% 25% 32% 12% 2	330
February	1st to 10th	Day	33% 28% 26% 10% 3	300
	11th to 20th	Day	28% 28% 38% 5% 1	300
	21st to end	Day	38% 28% 29% 6%	252
March	1st to 10th	Day	25% 21% 35% 15% 4	500
	11th to 20th	Day	25% 21% 35% 15% 4	500
	21st to end	Day	32% 20% 30% 13% 4 1	550
April	1st to 10th	Day	27% 23% 31% 13% 5%	500
	11th to 20th	Day	34% 19% 25% 13% 7% 2	492
	21st to end	Day	27% 20% 33% 15% 5% 2	500
May	1st to 10th	Day	23% 15% 34% 17% 9% 2	503
	11th to 20th	Day	21% 15% 29% 21% 9% 4	527
	21st to end	Day	16% 16% 30% 24% 10% 4	573
June	1st to 10th	Day	18% 15% 34% 24% 7% 2	530
	11th to 20th	Day	16% 13% 35% 23% 11% 2	596
	21st to end	Day	18% 13% 36% 22% 8% 3	730
July	1st to 10th	Day	11% 8% 34% 29% 15% 4	659
	11th to 20th	Day	12% 8% 32% 27% 14% 7%	524
	21st to end	Day	11% 7% 37% 28% 11% 6%	576
August	1st to 10th	Day	10% 9% 26% 30% 15% 10%	526
	11th to 20th	Day	10% 8% 29% 27% 18% 8%	519
	21st to end	Day	15% 9% 29% 25% 15% 7%	583
September	1st to 10th	Day	15% 12% 26% 25% 14% 9%	530
	11th to 20th	Day	10% 7% 25% 23% 21% 13%	530
	21st to end	Day	17% 13% 29% 22% 16% 4	530
October	1st to 10th	Day	23% 15% 25% 21% 13% 5%	530
	11th to 20th	Day	22% 20% 33% 17% 6% 2	522
	21st to end	Day	34% 18% 29% 15% 4	539
November	1st to 10th	Day	25% 23% 30% 17% 4 1	271
	11th to 20th	Day	29% 21% 34% 13% 4 1	200
	21st to end	Day	29% 24% 37% 10% 1	200
December	1st to 10th	Day	30% 26% 29% 14% 2	200
	11th to 20th	Day	31% 29% 28% 11% 2 1	200
	21st to end	Day	26% 27% 35% 8% 3	220
January	1st to 10th	Night	24% 23% 35% 14% 5% 1	800
	11th to 20th	Night	26% 20% 32% 17% 4	800
	21st to end	Night	21% 16% 36% 21% 6%	880
February	1st to 10th	Night	21% 23% 37% 17% 2	800
	11th to 20th	Night	22% 22% 37% 15% 3	783
	21st to end	Night	21% 23% 35% 19% 3	660
March	1st to 10th	Night	22% 22% 33% 20% 3	800
	11th to 20th	Night	23% 21% 33% 19% 5% 1	800
	21st to end	Night	23% 17% 31% 21% 7%	880
April	1st to 10th	Night	26% 16% 35% 20% 3	800
	11th to 20th	Night	21% 17% 34% 19% 8% 1	708
	21st to end	Night	21% 14% 31% 26% 8%	600
May	1st to 10th	Night	19% 7% 30% 32% 11% 2	600
	11th to 20th	Night	15% 10% 31% 33% 10% 1	419
	21st to end	Night	11% 8% 34% 33% 13% 1	433
June	1st to 10th	Night	15% 7% 35% 32% 11%	400
	11th to 20th	Night	12% 10% 29% 36% 14%	400
	21st to end	Night	12% 6% 31% 35% 15% 1	400
July	1st to 10th	Night	7% 3 26% 43% 18% 2	390
	11th to 20th	Night	12% 5% 27% 35% 19% 2	388
	21st to end	Night	9% 5% 29% 37% 19% 2	635
August	1st to 10th	Night	10% 2 22% 39% 25% 2	597
	11th to 20th	Night	8% 6% 19% 40% 24% 3	594
	21st to end	Night	12% 5% 28% 33% 22% 2	730
September	1st to 10th	Night	11% 8% 28% 30% 20% 4	800
	11th to 20th	Night	13% 5% 24% 30% 24% 6%	800
	21st to end	Night	19% 9% 22% 29% 17% 4	800
October	1st to 10th	Night	19% 9% 29% 24% 16% 2	800
	11th to 20th	Night	24% 14% 32% 21% 8% 1	793
	21st to end	Night	28% 17% 30% 21% 5%	836
November	1st to 10th	Night	24% 16% 30% 22% 8% 1	780
	11th to 20th	Night	23% 21% 28% 21% 6% 1	800
	21st to end	Night	27% 17% 32% 21% 4	800
December	1st to 10th	Night	29% 19% 35% 14% 4 1	800
	11th to 20th	Night	23% 23% 34% 17% 4	800
	21st to end	Night	31% 17% 32% 18% 3	880

Page generated at 2025-10-16 01:50:29. Version ac07

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