Fred Bay Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fred Bay Observatory clear sky chart from 2005-11-06 to 2025-02-21.

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	14% 9% 3 8% 5% 4 4 4 5% 7% 35%	109409
	Night	15% 10% 4 8% 5% 4 4 3 5% 6% 36%	109158
Transparency	Day	46% 5% 11% 23% 14% 2	113093
	Night	47% 4 10% 22% 14% 2	113613
Seeing	Day	18% 10% 26% 23% 16% 7%	15642
	Night	18% 8% 26% 31% 15% 2	25364

By Month

Percentage Cloud Cover SE (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	15% 18% 8% 13% 5% 4 3 3 4 4 24%	3672
	11th to 20th	Day	14% 15% 6% 14% 7% 6% 5% 3 3 4 24%	4060
	21st to end	Day	22% 13% 7% 10% 6% 4 5% 3 4 5% 22%	4466
February	1st to 10th	Day	18% 14% 6% 10% 6% 4 4 3 4 5% 26%	1873
	11th to 20th	Day	17% 12% 4 12% 7% 5% 4 3 5% 5% 27%	1948
	21st to end	Day	19% 14% 5% 8% 5% 3 4 3 4 5% 29%	1570
March	1st to 10th	Day	22% 16% 5% 9% 4 4 4 3 4 4 25%	3174
	11th to 20th	Day	14% 8% 3 7% 5% 4 4 4 6% 7% 39%	3480
	21st to end	Day	18% 10% 3 7% 5% 4 4 4 5% 7% 33%	3815
April	1st to 10th	Day	14% 11% 4 7% 4 5% 5% 3 6% 8% 34%	2218
	11th to 20th	Day	16% 9% 3 8% 4 4 4 3 5% 6% 39%	2328
	21st to end	Day	18% 8% 3 8% 5% 4 5% 3 5% 7% 35%	2470
May	1st to 10th	Day	21% 10% 3 7% 5% 5% 4 3 5% 7% 31%	4082
	11th to 20th	Day	20% 8% 3 6% 4 4 4 3 5% 7% 36%	4081
	21st to end	Day	10% 7% 3 7% 5% 5% 6% 5% 7% 10% 34%	4476
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 9% 38%	2470
	11th to 20th	Day	9% 5% 2 7% 5% 4 5% 4 7% 10% 42%	2631
	21st to end	Day	10% 6% 3 6% 6% 5% 4 5% 7% 10% 39%	2660
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 4 6% 10% 37%	4440
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 4 7% 10% 45%	4230
	21st to end	Day	11% 7% 3 7% 6% 6% 7% 5% 7% 9% 34%	4466
August	1st to 10th	Day	9% 6% 2 6% 4 4 5% 4 8% 10% 42%	4104
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 8% 10% 41%	4044
	21st to end	Day	6% 4 2 5% 4 4 4 3 5% 9% 54%	4276
September	1st to 10th	Day	12% 5% 1 4 3 5% 4 4 5% 7% 48%	2147
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 6% 10% 48%	2090
	21st to end	Day	15% 7% 2 6% 4 3 3 2 5% 6% 45%	2090
October	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 3 6% 7% 48%	2090
	11th to 20th	Day	14% 8% 2 6% 4 3 4 3 5% 6% 46%	1910
	21st to end	Day	21% 8% 3 6% 4 4 3 2 5% 7% 38%	1964
November	1st to 10th	Day	10% 6% 2 6% 4 4 4 3 4 7% 51%	1762
	11th to 20th	Day	14% 11% 5% 9% 4 4 4 3 5% 6% 35%	1800
	21st to end	Day	19% 11% 3 9% 5% 4 3 3 4 6% 34%	1800
December	1st to 10th	Day	18% 10% 4 8% 5% 4 4 3 4 5% 36%	2930
	11th to 20th	Day	20% 15% 5% 10% 5% 4 4 3 4 4 25%	3373
	21st to end	Day	21% 20% 6% 10% 5% 4 4 3 4 5% 18%	4419
January	1st to 10th	Night	14% 18% 7% 14% 7% 5% 4 3 4 5% 18%	6261
	11th to 20th	Night	13% 17% 7% 13% 8% 5% 5% 3 4 5% 20%	5900
	21st to end	Night	21% 14% 5% 9% 5% 5% 3 3 3 4 29%	6490
February	1st to 10th	Night	21% 13% 4 8% 5% 4 5% 3 4 5% 28%	2525
	11th to 20th	Night	18% 12% 3 9% 6% 4 5% 3 4 6% 30%	2338
	21st to end	Night	21% 14% 6% 8% 4 3 3 3 3 5% 30%	1885
March	1st to 10th	Night	21% 12% 3 7% 4 4 3 3 5% 5% 33%	3679
	11th to 20th	Night	12% 7% 3 6% 5% 4 5% 3 6% 7% 43%	3480
	21st to end	Night	18% 8% 3 7% 5% 4 4 3 4 5% 39%	3817
April	1st to 10th	Night	15% 9% 2 6% 4 4 4 4 5% 6% 39%	1978
	11th to 20th	Night	14% 8% 3 6% 4 4 4 4 6% 8% 40%	1800
	21st to end	Night	16% 8% 3 6% 4 4 4 4 5% 7% 38%	1710
May	1st to 10th	Night	23% 11% 4 9% 4 4 4 4 5% 6% 27%	2813
	11th to 20th	Night	19% 6% 2 7% 4 3 4 3 6% 8% 38%	2812
	21st to end	Night	9% 5% 2 5% 4 4 5% 4 8% 8% 45%	3079
June	1st to 10th	Night	8% 5% 2 5% 4 4 4 3 5% 9% 49%	1710
	11th to 20th	Night	8% 4 2 5% 4 5% 4 4 8% 11% 46%	1543
	21st to end	Night	6% 5% 1 4 5% 5% 7% 5% 7% 9% 46%	1520
July	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 5% 8% 50%	2530
	11th to 20th	Night	7% 4 2 4 4 4 5% 4 6% 9% 51%	2716
	21st to end	Night	11% 7% 2 6% 5% 5% 5% 4 7% 9% 38%	3087
August	1st to 10th	Night	8% 7% 3 7% 5% 5% 5% 4 7% 8% 43%	2825
	11th to 20th	Night	7% 8% 3 8% 4 4 4 3 6% 8% 45%	2785
	21st to end	Night	4 3 1 4 4 4 4 4 6% 9% 57%	3376
September	1st to 10th	Night	12% 5% 2 4 4 4 3 3 5% 7% 51%	2012
	11th to 20th	Night	9% 5% 2 4 3 3 4 3 6% 7% 55%	2090
	21st to end	Night	14% 6% 2 5% 4 3 3 3 5% 5% 50%	2090
October	1st to 10th	Night	8% 5% 2 6% 3 4 3 3 5% 6% 54%	2090
	11th to 20th	Night	14% 8% 2 6% 4 3 4 3 4 6% 45%	2270
	21st to end	Night	19% 7% 2 7% 4 4 3 3 4 7% 40%	2631
November	1st to 10th	Night	13% 8% 2 5% 4 3 4 3 4 7% 46%	2536
	11th to 20th	Night	17% 9% 2 8% 4 4 4 3 6% 6% 39%	2600
	21st to end	Night	19% 13% 4 7% 5% 4 4 3 5% 6% 30%	2600
December	1st to 10th	Night	17% 11% 4 9% 4 3 4 3 4 6% 36%	4250
	11th to 20th	Night	19% 16% 6% 10% 4 5% 4 2 5% 5% 25%	4822
	21st to end	Night	22% 19% 6% 10% 6% 5% 4 2 4 5% 17%	6508

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% 3 9% 17% 7%	3673
	11th to 20th	Day	65% 3 8% 12% 10% 1	4060
	21st to end	Day	66% 3 10% 13% 7% 2	4466
February	1st to 10th	Day	61% 4 10% 15% 9% 1	1873
	11th to 20th	Day	59% 4 10% 19% 8% 1	1958
	21st to end	Day	58% 5% 9% 20% 7% 2	1570
March	1st to 10th	Day	63% 4 7% 18% 7% 1	3174
	11th to 20th	Day	43% 6% 10% 26% 15% 1	3480
	21st to end	Day	49% 5% 12% 23% 10% 1	3815
April	1st to 10th	Day	48% 6% 10% 24% 11% 1	2218
	11th to 20th	Day	46% 4 11% 22% 14% 3	2328
	21st to end	Day	49% 5% 11% 22% 12% 2	2470
May	1st to 10th	Day	54% 5% 11% 23% 7% 1	4120
	11th to 20th	Day	48% 6% 13% 23% 10%	4085
	21st to end	Day	40% 9% 14% 20% 14% 2	4489
June	1st to 10th	Day	40% 7% 11% 24% 15% 2	2470
	11th to 20th	Day	37% 5% 16% 24% 17% 2	2631
	21st to end	Day	38% 8% 15% 21% 15% 3	2660
July	1st to 10th	Day	42% 5% 12% 22% 17% 2	4371
	11th to 20th	Day	32% 7% 10% 32% 19% 1	4157
	21st to end	Day	44% 6% 14% 18% 13% 3	4324
August	1st to 10th	Day	35% 7% 16% 30% 11% 1	4091
	11th to 20th	Day	36% 6% 11% 31% 16% 1	4083
	21st to end	Day	28% 5% 14% 30% 20% 3	4275
September	1st to 10th	Day	35% 5% 10% 25% 20% 5%	2147
	11th to 20th	Day	33% 5% 11% 26% 22% 3	2090
	21st to end	Day	40% 4 10% 23% 22% 2	2090
October	1st to 10th	Day	24% 4 9% 30% 31% 2	3480
	11th to 20th	Day	31% 3 9% 31% 25% 1	3150
	21st to end	Day	37% 2 11% 24% 23% 2	3214
November	1st to 10th	Day	35% 4 10% 29% 20% 3	1762
	11th to 20th	Day	50% 3 10% 19% 15% 3	1800
	21st to end	Day	53% 3 11% 21% 12% 1	1800
December	1st to 10th	Day	52% 3 10% 26% 9%	2930
	11th to 20th	Day	63% 4 8% 18% 6%	3370
	21st to end	Day	70% 3 10% 11% 5%	4419
January	1st to 10th	Night	69% 4 7% 13% 7%	6261
	11th to 20th	Night	67% 3 8% 12% 8% 2	5900
	21st to end	Night	61% 2 8% 19% 8% 1	6490
February	1st to 10th	Night	59% 4 9% 19% 8% 1	2525
	11th to 20th	Night	55% 4 9% 19% 12% 1	2339
	21st to end	Night	58% 4 10% 16% 10% 2	1885
March	1st to 10th	Night	52% 5% 7% 24% 11% 1	3679
	11th to 20th	Night	39% 6% 13% 27% 13% 2	3480
	21st to end	Night	46% 5% 8% 28% 11% 1	3817
April	1st to 10th	Night	44% 6% 11% 25% 12% 2	1978
	11th to 20th	Night	41% 5% 11% 19% 19% 5%	1800
	21st to end	Night	44% 6% 11% 23% 13% 3	1710
May	1st to 10th	Night	58% 6% 8% 19% 7% 2	2840
	11th to 20th	Night	43% 7% 13% 27% 9% 1	2813
	21st to end	Night	31% 8% 15% 28% 12% 5%	3087
June	1st to 10th	Night	33% 6% 11% 28% 20% 2	1710
	11th to 20th	Night	30% 7% 17% 22% 19% 4	1543
	21st to end	Night	30% 9% 17% 22% 17% 5%	1520
July	1st to 10th	Night	34% 6% 11% 25% 18% 6%	2490
	11th to 20th	Night	28% 5% 13% 32% 19% 2	2666
	21st to end	Night	41% 5% 12% 23% 15% 4	2988
August	1st to 10th	Night	37% 7% 13% 30% 12% 1	2814
	11th to 20th	Night	37% 6% 10% 28% 19% 1	2812
	21st to end	Night	24% 5% 16% 30% 21% 3	3376
September	1st to 10th	Night	32% 5% 12% 25% 19% 6%	2012
	11th to 20th	Night	28% 5% 9% 27% 27% 3	2090
	21st to end	Night	37% 4 9% 22% 26% 2	2090
October	1st to 10th	Night	23% 3 7% 29% 36% 3	3480
	11th to 20th	Night	29% 3 9% 27% 30% 2	3769
	21st to end	Night	34% 3 8% 24% 26% 5%	4331
November	1st to 10th	Night	39% 4 11% 22% 21% 4	2536
	11th to 20th	Night	46% 4 8% 22% 17% 3	2600
	21st to end	Night	55% 3 11% 20% 10% 1	2600
December	1st to 10th	Night	51% 3 11% 25% 9%	4250
	11th to 20th	Night	62% 4 10% 17% 7%	4824
	21st to end	Night	72% 3 8% 14% 3	6508

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	25% 18% 35% 16% 5%	300
	11th to 20th	Day	26% 20% 37% 14% 3	300
	21st to end	Day	29% 19% 33% 15% 3	330
February	1st to 10th	Day	28% 18% 36% 15% 3	300
	11th to 20th	Day	29% 23% 33% 13% 3	300
	21st to end	Day	28% 18% 44% 9% 1	225
March	1st to 10th	Day	24% 22% 35% 15% 3	298
	11th to 20th	Day	24% 14% 35% 18% 7% 1	460
	21st to end	Day	28% 15% 33% 19% 5%	506
April	1st to 10th	Day	20% 16% 31% 22% 9% 1	460
	11th to 20th	Day	22% 16% 26% 22% 10% 4	452
	21st to end	Day	21% 13% 35% 19% 8% 3	460
May	1st to 10th	Day	21% 13% 31% 22% 8% 5%	463
	11th to 20th	Day	21% 6% 27% 23% 16% 7%	487
	21st to end	Day	12% 5% 24% 28% 20% 11%	529
June	1st to 10th	Day	13% 4 21% 28% 25% 8%	490
	11th to 20th	Day	12% 4 19% 32% 26% 8%	651
	21st to end	Day	10% 5% 24% 30% 22% 9%	680
July	1st to 10th	Day	12% 4 17% 24% 24% 19%	671
	11th to 20th	Day	10% 3 21% 26% 26% 15%	551
	21st to end	Day	12% 2 14% 30% 27% 16%	532
August	1st to 10th	Day	12% 2 12% 27% 29% 19%	486
	11th to 20th	Day	12% 4 19% 28% 24% 13%	479
	21st to end	Day	10% 3 11% 28% 31% 17%	539
September	1st to 10th	Day	13% 2 14% 26% 32% 12%	490
	11th to 20th	Day	10% 3 14% 24% 35% 14%	490
	21st to end	Day	18% 8% 20% 24% 21% 9%	490
October	1st to 10th	Day	10% 5% 21% 35% 20% 8%	490
	11th to 20th	Day	16% 11% 30% 24% 14% 5%	490
	21st to end	Day	23% 11% 36% 22% 7% 1	413
November	1st to 10th	Day	14% 10% 36% 27% 10% 4	300
	11th to 20th	Day	23% 16% 30% 19% 10% 2	300
	21st to end	Day	27% 11% 37% 20% 5%	300
December	1st to 10th	Day	23% 19% 34% 17% 6% 1	300
	11th to 20th	Day	24% 15% 39% 18% 4	300
	21st to end	Day	30% 16% 33% 18% 3	330
January	1st to 10th	Night	22% 16% 35% 22% 5%	965
	11th to 20th	Night	27% 15% 34% 22% 3	800
	21st to end	Night	23% 12% 38% 24% 3	880
February	1st to 10th	Night	28% 10% 34% 26% 3	800
	11th to 20th	Night	25% 14% 36% 24% 2	783
	21st to end	Night	28% 12% 36% 22% 2	629
March	1st to 10th	Night	21% 12% 36% 25% 6%	760
	11th to 20th	Night	21% 13% 33% 27% 6% 1	760
	21st to end	Night	24% 9% 34% 26% 8%	836
April	1st to 10th	Night	19% 12% 33% 29% 6% 1	648
	11th to 20th	Night	17% 10% 33% 28% 11% 1	563
	21st to end	Night	17% 8% 36% 30% 8% 1	570
May	1st to 10th	Night	21% 8% 30% 28% 12% 1	570
	11th to 20th	Night	18% 4 23% 35% 17% 3	561
	21st to end	Night	10% 3 19% 38% 25% 5%	617
June	1st to 10th	Night	10% 2 18% 45% 23% 1	570
	11th to 20th	Night	9% 4 14% 44% 26% 2	570
	21st to end	Night	7% 2 18% 44% 26% 2	570
July	1st to 10th	Night	10% 2 11% 35% 32% 9%	555
	11th to 20th	Night	7% 3 11% 39% 35% 6%	552
	21st to end	Night	12% 2 9% 42% 32% 3	624
August	1st to 10th	Night	8% 2 10% 38% 36% 5%	567
	11th to 20th	Night	10% 1 11% 45% 29% 5%	564
	21st to end	Night	8% 1 11% 35% 39% 6%	627
September	1st to 10th	Night	14% 2 12% 32% 35% 5%	682
	11th to 20th	Night	10% 2 12% 36% 35% 5%	760
	21st to end	Night	17% 4 21% 33% 24% 2	760
October	1st to 10th	Night	12% 4 21% 36% 23% 4	760
	11th to 20th	Night	18% 8% 26% 33% 14% 1	760
	21st to end	Night	22% 7% 33% 28% 11%	836
November	1st to 10th	Night	18% 7% 29% 30% 15% 1	780
	11th to 20th	Night	19% 12% 32% 27% 9% 1	800
	21st to end	Night	25% 10% 28% 29% 7% 1	800
December	1st to 10th	Night	26% 10% 33% 25% 6%	800
	11th to 20th	Night	27% 8% 34% 25% 5% 1	800
	21st to end	Night	30% 13% 30% 25% 3	885

Page generated at 2025-02-18 03:26:33. Version ac07

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