Dorcas Bay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dorcas Bay clear sky chart from 2005-11-06 to 2025-09-19.

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	14% 11% 4 9% 5% 4 4 3 5% 6% 34%	126302
	Night	15% 13% 5% 11% 6% 5% 4 3 4 5% 27%	122854
Transparency	Day	51% 5% 12% 21% 11% 1	129832
	Night	59% 4 10% 16% 9% 1	127411
Seeing	Day	19% 6% 22% 30% 18% 5%	16413
	Night	23% 9% 24% 27% 15% 3	25586

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	13% 22% 11% 15% 11% 7% 3 3 3 3 9%	3181
	11th to 20th	Day	12% 21% 10% 27% 11% 5% 4 2 2 2 4	3589
	21st to end	Day	15% 19% 10% 17% 7% 6% 5% 3 4 4 10%	4377
February	1st to 10th	Day	14% 19% 7% 16% 10% 4 5% 3 3 5% 14%	1800
	11th to 20th	Day	12% 17% 6% 12% 8% 8% 5% 3 5% 5% 19%	1753
	21st to end	Day	13% 17% 6% 12% 6% 5% 5% 4 5% 5% 22%	1621
March	1st to 10th	Day	16% 11% 4 10% 5% 4 4 4 5% 6% 31%	4739
	11th to 20th	Day	11% 15% 6% 11% 5% 5% 5% 4 5% 6% 27%	4980
	21st to end	Day	18% 12% 5% 9% 5% 4 4 3 4 5% 31%	5465
April	1st to 10th	Day	18% 12% 4 7% 5% 4 4 3 5% 5% 33%	2327
	11th to 20th	Day	18% 8% 2 6% 4 4 3 3 5% 6% 40%	2572
	21st to end	Day	18% 9% 4 6% 3 4 3 3 6% 7% 37%	2600
May	1st to 10th	Day	16% 7% 3 6% 3 4 4 4 5% 7% 42%	5888
	11th to 20th	Day	12% 8% 2 7% 5% 4 4 3 5% 6% 44%	6106
	21st to end	Day	17% 8% 2 6% 3 4 4 3 4 7% 41%	7031
June	1st to 10th	Day	13% 7% 2 7% 4 4 5% 4 6% 7% 42%	3000
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 7% 9% 43%	3000
	21st to end	Day	14% 6% 3 6% 5% 4 5% 3 6% 8% 41%	3000
July	1st to 10th	Day	12% 7% 2 7% 4 5% 4 3 5% 8% 43%	6819
	11th to 20th	Day	9% 5% 2 5% 3 3 3 3 5% 7% 54%	6691
	21st to end	Day	7% 4 2 5% 3 4 3 4 5% 8% 54%	6565
August	1st to 10th	Day	11% 5% 2 5% 4 4 5% 4 6% 9% 45%	4234
	11th to 20th	Day	15% 6% 2 5% 3 3 3 4 6% 8% 45%	4174
	21st to end	Day	8% 6% 2 6% 4 4 5% 4 6% 8% 47%	4675
September	1st to 10th	Day	13% 7% 2 6% 5% 5% 4 4 5% 7% 43%	2361
	11th to 20th	Day	12% 6% 2 5% 3 4 4 4 6% 8% 48%	2178
	21st to end	Day	18% 8% 2 6% 4 4 4 3 4 8% 37%	2090
October	1st to 10th	Day	17% 10% 3 8% 4 4 4 3 5% 8% 33%	1942
	11th to 20th	Day	26% 11% 4 8% 5% 3 5% 4 5% 6% 23%	1900
	21st to end	Day	26% 17% 4 10% 4 4 3 4 4 6% 19%	1978
November	1st to 10th	Day	18% 18% 4 9% 5% 4 5% 3 5% 7% 22%	1727
	11th to 20th	Day	18% 18% 5% 12% 6% 5% 4 4 4 5% 18%	1600
	21st to end	Day	23% 23% 5% 12% 7% 4 4 3 4 4 10%	1600
December	1st to 10th	Day	16% 31% 9% 14% 7% 4 3 3 3 4 8%	2600
	11th to 20th	Day	21% 23% 8% 19% 6% 4 3 2 3 3 8%	2663
	21st to end	Day	26% 24% 9% 16% 4 6% 3 2 3 3 4	3476
January	1st to 10th	Night	13% 20% 10% 20% 12% 8% 3 2 2 2 7%	6780
	11th to 20th	Night	13% 19% 9% 23% 10% 6% 4 3 3 3 8%	6316
	21st to end	Night	12% 17% 9% 18% 8% 7% 5% 3 4 4 13%	6567
February	1st to 10th	Night	11% 20% 7% 18% 8% 6% 4 3 4 4 15%	2600
	11th to 20th	Night	13% 17% 6% 14% 7% 5% 5% 3 5% 5% 20%	2533
	21st to end	Night	14% 15% 5% 11% 6% 6% 5% 3 5% 5% 23%	2005
March	1st to 10th	Night	15% 10% 4 10% 6% 5% 4 3 5% 5% 33%	5099
	11th to 20th	Night	11% 12% 4 8% 5% 4 6% 4 6% 7% 32%	4979
	21st to end	Night	20% 11% 5% 10% 5% 4 4 3 4 4 31%	5467
April	1st to 10th	Night	20% 12% 3 6% 4 4 4 3 5% 5% 33%	2087
	11th to 20th	Night	18% 8% 3 7% 4 4 3 4 6% 7% 37%	1777
	21st to end	Night	20% 9% 3 8% 5% 5% 4 3 5% 6% 33%	1800
May	1st to 10th	Night	14% 7% 4 9% 5% 5% 5% 4 5% 6% 36%	4053
	11th to 20th	Night	11% 7% 3 7% 5% 4 4 3 5% 8% 43%	3787
	21st to end	Night	18% 8% 3 7% 4 4 4 3 4 6% 38%	3845
June	1st to 10th	Night	12% 7% 3 7% 6% 4 5% 3 6% 7% 40%	1400
	11th to 20th	Night	9% 7% 2 5% 5% 5% 5% 4 6% 9% 43%	1400
	21st to end	Night	12% 7% 2 7% 4 5% 5% 5% 6% 8% 40%	1400
July	1st to 10th	Night	11% 6% 2 6% 4 4 4 3 6% 8% 45%	3160
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 8% 49%	3286
	21st to end	Night	9% 7% 2 5% 4 3 4 3 5% 9% 50%	4289
August	1st to 10th	Night	10% 7% 2 6% 4 4 4 4 6% 8% 45%	2915
	11th to 20th	Night	12% 7% 2 6% 4 4 5% 4 6% 8% 42%	2875
	21st to end	Night	9% 6% 2 6% 4 5% 5% 3 6% 7% 47%	3223
September	1st to 10th	Night	12% 4 2 5% 4 4 5% 3 6% 7% 47%	2015
	11th to 20th	Night	12% 5% 2 4 5% 4 5% 3 6% 6% 49%	2179
	21st to end	Night	19% 7% 3 6% 4 4 4 3 5% 7% 38%	2090
October	1st to 10th	Night	20% 9% 3 7% 6% 5% 4 3 4 6% 33%	2237
	11th to 20th	Night	24% 12% 4 9% 5% 5% 4 3 4 6% 24%	2280
	21st to end	Night	25% 14% 5% 9% 5% 5% 4 3 5% 5% 19%	2617
November	1st to 10th	Night	21% 15% 4 8% 5% 5% 4 3 4 6% 25%	2571
	11th to 20th	Night	17% 21% 6% 11% 6% 5% 5% 4 4 5% 17%	2800
	21st to end	Night	23% 23% 5% 11% 7% 4 4 3 4 4 11%	2800
December	1st to 10th	Night	19% 24% 7% 16% 7% 5% 4 3 3 4 8%	4580
	11th to 20th	Night	16% 22% 10% 18% 8% 5% 4 3 3 3 8%	5540
	21st to end	Night	23% 22% 8% 16% 7% 6% 3 3 3 2 6%	7502

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	83% 4 5% 4 2	3182
	11th to 20th	Day	91% 3 3 3 1	3620
	21st to end	Day	78% 5% 8% 7% 2	4377
February	1st to 10th	Day	75% 3 9% 11% 2	1800
	11th to 20th	Day	68% 4 11% 12% 5%	1762
	21st to end	Day	64% 4 11% 16% 5%	1621
March	1st to 10th	Day	54% 3 17% 13% 12% 1	4742
	11th to 20th	Day	56% 4 13% 19% 7% 1	4980
	21st to end	Day	52% 5% 14% 18% 8% 1	5465
April	1st to 10th	Day	51% 4 10% 22% 11% 1	2327
	11th to 20th	Day	45% 4 7% 26% 16% 2	2572
	21st to end	Day	46% 4 13% 21% 13% 3	2600
May	1st to 10th	Day	42% 5% 11% 21% 17% 4	5897
	11th to 20th	Day	43% 5% 12% 19% 20% 2	6110
	21st to end	Day	45% 4 11% 19% 17% 4	7031
June	1st to 10th	Day	40% 6% 10% 27% 16% 1	3000
	11th to 20th	Day	38% 5% 10% 28% 17% 1	3000
	21st to end	Day	42% 6% 13% 25% 13% 2	3000
July	1st to 10th	Day	41% 8% 14% 23% 13% 1	6746
	11th to 20th	Day	31% 5% 13% 37% 13% 1	6610
	21st to end	Day	28% 6% 13% 34% 18% 1	6378
August	1st to 10th	Day	33% 6% 15% 31% 14% 1	4221
	11th to 20th	Day	37% 4 17% 28% 12%	4213
	21st to end	Day	34% 5% 16% 33% 11% 1	4674
September	1st to 10th	Day	39% 6% 14% 29% 11% 1	2361
	11th to 20th	Day	35% 4 16% 32% 12% 1	2178
	21st to end	Day	46% 5% 15% 25% 9%	2090
October	1st to 10th	Day	41% 4 12% 27% 16%	3228
	11th to 20th	Day	53% 4 12% 22% 9%	3140
	21st to end	Day	60% 4 13% 18% 5%	3241
November	1st to 10th	Day	63% 4 11% 16% 5%	1727
	11th to 20th	Day	69% 5% 11% 10% 5%	1600
	21st to end	Day	79% 4 8% 6% 2	1600
December	1st to 10th	Day	85% 6% 5% 4 1	2600
	11th to 20th	Day	84% 2 9% 4	2663
	21st to end	Day	87% 4 5% 3	3476
January	1st to 10th	Night	87% 3 5% 4 1	6780
	11th to 20th	Night	85% 5% 5% 4 2	6360
	21st to end	Night	78% 3 9% 6% 3	6567
February	1st to 10th	Night	73% 4 9% 11% 3	2600
	11th to 20th	Night	69% 4 9% 14% 5%	2535
	21st to end	Night	62% 3 13% 15% 6%	2005
March	1st to 10th	Night	52% 4 16% 15% 11% 1	5109
	11th to 20th	Night	51% 4 16% 20% 8% 1	4980
	21st to end	Night	58% 4 8% 16% 12% 1	5467
April	1st to 10th	Night	53% 4 8% 22% 12% 1	2087
	11th to 20th	Night	46% 4 7% 23% 18% 2	1777
	21st to end	Night	52% 4 8% 17% 16% 3	1800
May	1st to 10th	Night	49% 5% 8% 17% 17% 5%	4060
	11th to 20th	Night	41% 4 9% 24% 19% 2	3788
	21st to end	Night	48% 4 10% 21% 13% 4	3846
June	1st to 10th	Night	42% 4 9% 24% 19% 3	1400
	11th to 20th	Night	37% 5% 12% 25% 20% 1	1400
	21st to end	Night	41% 5% 13% 24% 15% 2	1400
July	1st to 10th	Night	37% 6% 12% 26% 15% 4	3125
	11th to 20th	Night	33% 4 11% 32% 18% 1	3244
	21st to end	Night	32% 5% 11% 30% 21% 1	4162
August	1st to 10th	Night	36% 5% 13% 30% 15% 1	2904
	11th to 20th	Night	39% 6% 17% 23% 14% 1	2902
	21st to end	Night	35% 5% 14% 33% 12% 1	3223
September	1st to 10th	Night	35% 5% 15% 29% 14% 2	2015
	11th to 20th	Night	36% 4 15% 31% 13% 1	2179
	21st to end	Night	48% 3 14% 24% 10%	2090
October	1st to 10th	Night	40% 5% 10% 29% 15%	3731
	11th to 20th	Night	53% 4 13% 22% 9%	3779
	21st to end	Night	58% 5% 14% 19% 4	4304
November	1st to 10th	Night	62% 4 10% 19% 5%	2571
	11th to 20th	Night	72% 4 11% 9% 4 1	2800
	21st to end	Night	79% 4 10% 6% 1	2800
December	1st to 10th	Night	83% 6% 7% 3 1	4580
	11th to 20th	Night	85% 3 7% 4 1	5539
	21st to end	Night	85% 4 7% 3 1	7502

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	27% 12% 26% 25% 7% 2	205
	11th to 20th	Day	22% 8% 36% 23% 10% 1	300
	21st to end	Day	28% 7% 26% 27% 11% 1	330
February	1st to 10th	Day	24% 16% 30% 22% 7% 1	300
	11th to 20th	Day	18% 14% 35% 27% 5%	300
	21st to end	Day	23% 16% 33% 22% 6%	249
March	1st to 10th	Day	21% 6% 34% 30% 8%	406
	11th to 20th	Day	20% 11% 27% 32% 9% 2	500
	21st to end	Day	24% 6% 27% 32% 10% 1	550
April	1st to 10th	Day	23% 7% 28% 29% 10% 3	500
	11th to 20th	Day	26% 6% 27% 23% 13% 5%	492
	21st to end	Day	23% 6% 25% 32% 11% 3	500
May	1st to 10th	Day	21% 3 22% 37% 13% 3	503
	11th to 20th	Day	18% 3 17% 34% 22% 6%	527
	21st to end	Day	14% 3 19% 33% 24% 7%	603
June	1st to 10th	Day	15% 2 18% 39% 22% 4	730
	11th to 20th	Day	15% 2 18% 36% 25% 5%	730
	21st to end	Day	17% 3 15% 38% 22% 4	730
July	1st to 10th	Day	10% 2 11% 42% 31% 5%	721
	11th to 20th	Day	12% 2 15% 37% 27% 8%	718
	21st to end	Day	11% 2 15% 36% 30% 6%	639
August	1st to 10th	Day	16% 2 12% 30% 33% 8%	526
	11th to 20th	Day	11% 2 10% 33% 33% 11%	519
	21st to end	Day	14% 3 16% 30% 29% 8%	583
September	1st to 10th	Day	14% 4 19% 26% 28% 8%	530
	11th to 20th	Day	10% 3 15% 29% 28% 14%	522
	21st to end	Day	19% 4 18% 27% 23% 8%	490
October	1st to 10th	Day	25% 6% 22% 22% 18% 6%	490
	11th to 20th	Day	32% 8% 25% 24% 9% 2	490
	21st to end	Day	32% 10% 28% 22% 7% 1	427
November	1st to 10th	Day	25% 11% 33% 23% 8% 1	283
	11th to 20th	Day	31% 12% 35% 18% 6%	200
	21st to end	Day	33% 14% 33% 18% 3	200
December	1st to 10th	Day	31% 14% 30% 19% 7% 1	200
	11th to 20th	Day	32% 19% 26% 19% 5%	200
	21st to end	Day	35% 13% 34% 14% 3 1	220
January	1st to 10th	Night	26% 16% 28% 22% 7% 1	1000
	11th to 20th	Night	28% 13% 28% 23% 8% 1	1000
	21st to end	Night	22% 9% 32% 26% 9% 2	918
February	1st to 10th	Night	22% 15% 34% 23% 6% 1	800
	11th to 20th	Night	22% 10% 32% 27% 8% 1	783
	21st to end	Night	22% 10% 35% 27% 6% 1	660
March	1st to 10th	Night	19% 9% 32% 31% 8% 1	800
	11th to 20th	Night	19% 11% 31% 28% 10% 1	800
	21st to end	Night	21% 6% 28% 30% 13% 1	880
April	1st to 10th	Night	25% 8% 27% 30% 9% 1	770
	11th to 20th	Night	23% 6% 23% 31% 14% 3	593
	21st to end	Night	25% 5% 21% 31% 16% 3	600
May	1st to 10th	Night	17% 2 19% 38% 21% 3	600
	11th to 20th	Night	17% 3 14% 39% 24% 2	489
	21st to end	Night	16% 4 15% 36% 24% 5%	433
June	1st to 10th	Night	17% 4 16% 39% 21% 3	400
	11th to 20th	Night	13% 3 16% 39% 25% 4	400
	21st to end	Night	15% 2 15% 38% 27% 5%	400
July	1st to 10th	Night	8% 3 12% 41% 31% 5%	390
	11th to 20th	Night	9% 3 15% 34% 34% 5%	436
	21st to end	Night	9% 3 13% 39% 32% 5%	657
August	1st to 10th	Night	11% 3 12% 36% 32% 6%	597
	11th to 20th	Night	12% 3 11% 30% 34% 11%	594
	21st to end	Night	14% 3 17% 32% 29% 6%	660
September	1st to 10th	Night	14% 5% 16% 31% 24% 10%	770
	11th to 20th	Night	15% 3 16% 28% 26% 11%	793
	21st to end	Night	22% 5% 17% 23% 24% 8%	760
October	1st to 10th	Night	26% 5% 22% 25% 19% 3	760
	11th to 20th	Night	31% 8% 22% 26% 11% 2	760
	21st to end	Night	36% 8% 24% 23% 7% 1	836
November	1st to 10th	Night	32% 11% 24% 22% 9% 2	780
	11th to 20th	Night	31% 16% 25% 18% 9% 1	800
	21st to end	Night	39% 13% 26% 17% 5%	800
December	1st to 10th	Night	37% 14% 27% 14% 7%	800
	11th to 20th	Night	30% 20% 32% 14% 5%	967
	21st to end	Night	35% 13% 30% 17% 5%	1100

Page generated at 2025-09-16 01:49:14. Version ac07

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