Dalhousie
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dalhousie 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	16% 9% 3 8% 6% 5% 6% 4 6% 7% 30%	108208
	Night	16% 11% 4 9% 6% 5% 5% 4 6% 6% 29%	110501
Transparency	Day	50% 10% 14% 18% 8% 1	111712
	Night	53% 9% 13% 18% 7% 1	115162
Seeing	Day	20% 16% 30% 24% 8% 1	16584
	Night	21% 15% 30% 25% 8% 1	24501

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	16% 19% 5% 11% 7% 5% 4 3 4 5% 19%	3157
	11th to 20th	Day	8% 12% 7% 13% 12% 9% 7% 5% 5% 6% 16%	3237
	21st to end	Day	4 5% 3 9% 5% 7% 9% 7% 9% 10% 32%	4029
February	1st to 10th	Day	8% 10% 4 10% 7% 7% 7% 6% 7% 7% 27%	1800
	11th to 20th	Day	11% 14% 5% 12% 7% 6% 6% 5% 6% 7% 22%	1753
	21st to end	Day	8% 12% 4 9% 6% 6% 6% 5% 7% 7% 29%	1479
March	1st to 10th	Day	14% 11% 5% 8% 5% 5% 8% 6% 6% 6% 26%	3265
	11th to 20th	Day	16% 18% 4 9% 6% 4 4 4 5% 6% 23%	3480
	21st to end	Day	22% 12% 4 10% 6% 5% 4 3 4 5% 25%	3816
April	1st to 10th	Day	19% 11% 3 7% 7% 5% 5% 3 6% 6% 28%	2279
	11th to 20th	Day	16% 9% 3 7% 5% 5% 6% 4 6% 7% 33%	2442
	21st to end	Day	21% 9% 3 9% 5% 4 4 3 5% 6% 31%	2470
May	1st to 10th	Day	13% 11% 4 12% 7% 6% 6% 4 6% 5% 26%	4192
	11th to 20th	Day	17% 8% 3 8% 5% 5% 5% 4 6% 7% 32%	4420
	21st to end	Day	18% 8% 3 8% 5% 5% 6% 5% 6% 7% 28%	5185
June	1st to 10th	Day	22% 9% 3 7% 4 5% 4 3 7% 7% 30%	2850
	11th to 20th	Day	18% 7% 2 6% 5% 4 5% 4 6% 8% 35%	2850
	21st to end	Day	21% 8% 3 7% 6% 5% 5% 5% 7% 7% 25%	2850
July	1st to 10th	Day	14% 6% 3 8% 5% 6% 5% 4 7% 9% 32%	4760
	11th to 20th	Day	13% 6% 2 5% 5% 6% 6% 5% 8% 9% 35%	4760
	21st to end	Day	12% 5% 2 5% 5% 4 5% 5% 7% 9% 42%	4987
August	1st to 10th	Day	15% 5% 2 6% 5% 6% 6% 5% 8% 9% 33%	4105
	11th to 20th	Day	15% 7% 3 7% 5% 6% 6% 5% 7% 8% 31%	4045
	21st to end	Day	14% 4 2 7% 5% 4 5% 4 6% 8% 41%	4532
September	1st to 10th	Day	16% 6% 2 7% 5% 5% 5% 3 5% 6% 40%	2316
	11th to 20th	Day	18% 6% 2 5% 4 4 4 3 4 4 45%	2090
	21st to end	Day	18% 7% 3 7% 5% 5% 4 3 6% 7% 37%	2090
October	1st to 10th	Day	15% 6% 2 8% 5% 4 5% 4 6% 6% 39%	1919
	11th to 20th	Day	20% 9% 3 8% 5% 5% 5% 4 5% 6% 30%	1900
	21st to end	Day	23% 9% 3 8% 5% 4 5% 4 5% 6% 27%	1922
November	1st to 10th	Day	17% 9% 4 9% 7% 6% 6% 4 5% 6% 26%	1611
	11th to 20th	Day	21% 11% 4 9% 5% 5% 6% 5% 7% 5% 22%	1600
	21st to end	Day	19% 13% 5% 10% 7% 5% 6% 4 5% 7% 19%	1600
December	1st to 10th	Day	16% 10% 5% 10% 10% 7% 7% 4 5% 5% 22%	2323
	11th to 20th	Day	19% 9% 4 10% 7% 6% 6% 5% 6% 6% 22%	2618
	21st to end	Day	14% 15% 5% 10% 6% 4 5% 4 5% 5% 27%	3476
January	1st to 10th	Night	15% 17% 6% 13% 9% 7% 5% 4 6% 6% 13%	6808
	11th to 20th	Night	12% 14% 6% 12% 8% 7% 6% 5% 6% 6% 18%	6661
	21st to end	Night	4 7% 3 7% 6% 7% 10% 6% 8% 8% 33%	6925
February	1st to 10th	Night	8% 14% 5% 9% 7% 7% 5% 5% 7% 7% 27%	2600
	11th to 20th	Night	13% 15% 4 10% 6% 6% 7% 5% 6% 6% 22%	2533
	21st to end	Night	11% 14% 4 9% 6% 6% 6% 5% 6% 6% 29%	1972
March	1st to 10th	Night	12% 13% 4 9% 7% 5% 8% 4 4 6% 28%	3575
	11th to 20th	Night	20% 18% 5% 10% 5% 5% 4 4 4 5% 19%	3479
	21st to end	Night	26% 14% 3 11% 5% 4 3 2 3 5% 24%	3816
April	1st to 10th	Night	22% 14% 3 8% 5% 4 4 3 5% 5% 28%	1896
	11th to 20th	Night	19% 9% 3 7% 4 5% 4 3 5% 6% 34%	1688
	21st to end	Night	25% 11% 3 6% 4 3 3 3 4 5% 33%	1710
May	1st to 10th	Night	18% 10% 3 10% 6% 5% 5% 4 5% 6% 29%	2751
	11th to 20th	Night	18% 8% 3 7% 6% 6% 5% 4 6% 8% 30%	2506
	21st to end	Night	16% 9% 3 9% 6% 5% 5% 4 6% 7% 29%	2407
June	1st to 10th	Night	23% 9% 3 6% 4 5% 4 4 4 6% 33%	1330
	11th to 20th	Night	19% 6% 2 5% 5% 4 4 4 6% 8% 37%	1330
	21st to end	Night	26% 8% 2 7% 5% 5% 5% 3 4 5% 30%	1330
July	1st to 10th	Night	13% 7% 3 8% 6% 5% 4 4 6% 7% 38%	2210
	11th to 20th	Night	16% 7% 3 7% 4 4 5% 6% 7% 7% 36%	2210
	21st to end	Night	11% 5% 3 6% 4 4 4 4 6% 8% 45%	2538
August	1st to 10th	Night	15% 7% 3 9% 6% 5% 5% 4 6% 6% 34%	2825
	11th to 20th	Night	15% 8% 2 8% 6% 5% 6% 4 6% 8% 33%	2782
	21st to end	Night	15% 6% 2 6% 4 4 3 3 5% 7% 46%	3124
September	1st to 10th	Night	17% 6% 1 5% 4 4 5% 4 5% 8% 42%	1863
	11th to 20th	Night	17% 6% 2 6% 3 4 3 3 5% 6% 44%	2086
	21st to end	Night	17% 4 1 6% 5% 4 4 3 4 6% 45%	2090
October	1st to 10th	Night	18% 7% 2 7% 4 4 4 3 5% 6% 39%	2261
	11th to 20th	Night	21% 6% 2 6% 5% 4 5% 4 5% 6% 34%	2280
	21st to end	Night	21% 9% 2 7% 5% 5% 5% 4 6% 7% 29%	2674
November	1st to 10th	Night	19% 9% 3 7% 6% 5% 6% 5% 5% 6% 30%	2685
	11th to 20th	Night	20% 11% 4 10% 5% 6% 5% 3 6% 6% 25%	2800
	21st to end	Night	21% 13% 4 9% 7% 5% 4 4 6% 6% 21%	2800
December	1st to 10th	Night	13% 12% 5% 12% 8% 5% 5% 4 6% 7% 23%	4863
	11th to 20th	Night	20% 10% 4 11% 5% 5% 5% 4 6% 7% 21%	5591
	21st to end	Night	17% 13% 5% 10% 6% 5% 4 4 5% 6% 25%	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	67% 9% 11% 10% 3	3158
	11th to 20th	Day	62% 13% 13% 9% 4	3263
	21st to end	Day	41% 10% 12% 22% 16%	4029
February	1st to 10th	Day	51% 11% 13% 16% 8% 2	1800
	11th to 20th	Day	60% 10% 9% 14% 6% 1	1761
	21st to end	Day	49% 12% 8% 20% 9% 1	1479
March	1st to 10th	Day	54% 10% 13% 14% 8% 1	3269
	11th to 20th	Day	61% 11% 8% 14% 5% 1	3480
	21st to end	Day	56% 12% 8% 15% 9% 1	3816
April	1st to 10th	Day	55% 8% 10% 15% 10% 2	2279
	11th to 20th	Day	49% 9% 10% 18% 11% 2	2442
	21st to end	Day	54% 8% 8% 17% 10% 3	2470
May	1st to 10th	Day	54% 13% 10% 16% 7% 1	4206
	11th to 20th	Day	49% 10% 17% 16% 8%	4424
	21st to end	Day	47% 14% 11% 18% 9%	5185
June	1st to 10th	Day	53% 9% 12% 16% 10% 1	2850
	11th to 20th	Day	47% 10% 14% 18% 10% 2	2850
	21st to end	Day	54% 12% 12% 15% 6%	2850
July	1st to 10th	Day	46% 9% 15% 20% 9%	4685
	11th to 20th	Day	42% 10% 19% 24% 5%	4679
	21st to end	Day	38% 10% 21% 22% 9%	4827
August	1st to 10th	Day	44% 10% 19% 19% 8%	4092
	11th to 20th	Day	47% 11% 21% 16% 4	4084
	21st to end	Day	39% 9% 18% 21% 11%	4532
September	1st to 10th	Day	43% 9% 16% 23% 9%	2316
	11th to 20th	Day	43% 7% 15% 25% 12%	2090
	21st to end	Day	45% 10% 12% 24% 9%	2090
October	1st to 10th	Day	47% 7% 16% 25% 6%	3192
	11th to 20th	Day	53% 6% 13% 22% 6%	3140
	21st to end	Day	49% 8% 14% 19% 9%	3146
November	1st to 10th	Day	56% 10% 11% 17% 6%	1611
	11th to 20th	Day	59% 13% 11% 13% 5%	1600
	21st to end	Day	64% 9% 14% 10% 2	1600
December	1st to 10th	Day	64% 8% 12% 14% 2	2323
	11th to 20th	Day	59% 9% 11% 15% 5% 1	2618
	21st to end	Day	55% 11% 12% 14% 7% 1	3476
January	1st to 10th	Night	68% 12% 11% 9% 1	6808
	11th to 20th	Night	60% 11% 12% 13% 3	6710
	21st to end	Night	44% 8% 12% 22% 14% 1	6925
February	1st to 10th	Night	55% 8% 11% 16% 9% 2	2600
	11th to 20th	Night	60% 9% 10% 14% 6% 1	2536
	21st to end	Night	53% 9% 10% 17% 9% 2	1972
March	1st to 10th	Night	55% 8% 11% 18% 7% 1	3584
	11th to 20th	Night	67% 8% 6% 13% 5% 1	3480
	21st to end	Night	60% 9% 7% 14% 9% 1	3816
April	1st to 10th	Night	58% 7% 9% 13% 11% 2	1896
	11th to 20th	Night	50% 6% 11% 18% 14% 2	1688
	21st to end	Night	53% 5% 10% 17% 14% 2	1710
May	1st to 10th	Night	52% 10% 13% 13% 11% 1	2757
	11th to 20th	Night	46% 11% 15% 20% 7%	2507
	21st to end	Night	46% 14% 14% 17% 9% 1	2408
June	1st to 10th	Night	52% 8% 12% 17% 11% 1	1330
	11th to 20th	Night	44% 8% 13% 19% 14% 2	1330
	21st to end	Night	56% 9% 10% 16% 9% 1	1330
July	1st to 10th	Night	43% 8% 18% 21% 10%	2175
	11th to 20th	Night	45% 9% 16% 24% 6%	2168
	21st to end	Night	36% 10% 22% 23% 9%	2458
August	1st to 10th	Night	49% 9% 16% 21% 6%	2813
	11th to 20th	Night	46% 10% 22% 17% 5%	2809
	21st to end	Night	38% 8% 17% 23% 14%	3124
September	1st to 10th	Night	40% 8% 15% 27% 9% 1	1863
	11th to 20th	Night	40% 8% 16% 25% 11% 1	2086
	21st to end	Night	39% 9% 13% 26% 12% 1	2090
October	1st to 10th	Night	49% 6% 13% 25% 7% 1	3768
	11th to 20th	Night	46% 6% 12% 30% 5%	3779
	21st to end	Night	46% 9% 16% 20% 8%	4400
November	1st to 10th	Night	53% 9% 14% 16% 7%	2685
	11th to 20th	Night	60% 9% 12% 15% 5%	2800
	21st to end	Night	64% 10% 13% 11% 3 1	2800
December	1st to 10th	Night	58% 8% 14% 17% 2	4863
	11th to 20th	Night	59% 9% 10% 16% 5%	5592
	21st to end	Night	58% 8% 12% 16% 4 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	26% 23% 29% 19% 3 1	200
	11th to 20th	Day	21% 29% 28% 14% 9% 2	200
	21st to end	Day	13% 22% 36% 20% 8% 1	250
February	1st to 10th	Day	16% 22% 35% 19% 7% 1	400
	11th to 20th	Day	17% 29% 34% 17% 4	396
	21st to end	Day	14% 25% 35% 21% 4 1	339
March	1st to 10th	Day	20% 18% 35% 23% 4	460
	11th to 20th	Day	18% 23% 28% 21% 8% 1	460
	21st to end	Day	21% 19% 35% 16% 7% 2	506
April	1st to 10th	Day	24% 23% 31% 18% 3 1	460
	11th to 20th	Day	18% 17% 37% 22% 6% 1	452
	21st to end	Day	23% 18% 32% 20% 5% 2	460
May	1st to 10th	Day	25% 13% 30% 24% 7% 1	510
	11th to 20th	Day	23% 13% 27% 22% 13% 2	674
	21st to end	Day	22% 17% 31% 22% 6% 1	735
June	1st to 10th	Day	27% 11% 28% 25% 8% 1	680
	11th to 20th	Day	21% 13% 27% 25% 13% 1	680
	21st to end	Day	24% 10% 28% 29% 8% 1	680
July	1st to 10th	Day	16% 10% 34% 28% 10% 1	670
	11th to 20th	Day	13% 9% 31% 36% 10% 1	668
	21st to end	Day	15% 7% 29% 36% 12% 1	666
August	1st to 10th	Day	14% 10% 31% 30% 13% 2	486
	11th to 20th	Day	16% 8% 27% 33% 15% 1	479
	21st to end	Day	13% 11% 27% 30% 14% 4	539
September	1st to 10th	Day	17% 9% 34% 26% 11% 2	490
	11th to 20th	Day	21% 11% 29% 28% 9% 2	490
	21st to end	Day	18% 18% 26% 23% 12% 2	490
October	1st to 10th	Day	14% 16% 32% 25% 11% 2	400
	11th to 20th	Day	24% 21% 26% 19% 8% 1	390
	21st to end	Day	29% 23% 24% 17% 8%	429
November	1st to 10th	Day	21% 24% 30% 19% 5% 1	395
	11th to 20th	Day	29% 21% 27% 17% 5% 1	400
	21st to end	Day	27% 26% 28% 16% 4 1	400
December	1st to 10th	Day	25% 21% 32% 15% 6% 1	230
	11th to 20th	Day	26% 28% 32% 13% 3	200
	21st to end	Day	19% 27% 36% 15% 2	220
January	1st to 10th	Night	21% 21% 35% 18% 5%	900
	11th to 20th	Night	20% 22% 32% 20% 5% 1	877
	21st to end	Night	14% 22% 36% 22% 6%	880
February	1st to 10th	Night	15% 22% 37% 19% 6%	800
	11th to 20th	Night	22% 19% 38% 17% 4	783
	21st to end	Night	17% 16% 39% 23% 5%	629
March	1st to 10th	Night	20% 18% 34% 24% 4	760
	11th to 20th	Night	21% 19% 30% 21% 7% 1	760
	21st to end	Night	21% 15% 36% 22% 6% 1	836
April	1st to 10th	Night	26% 17% 30% 21% 5%	646
	11th to 20th	Night	23% 14% 30% 23% 9%	564
	21st to end	Night	29% 11% 27% 25% 6% 1	570
May	1st to 10th	Night	25% 11% 27% 27% 9%	570
	11th to 20th	Night	22% 9% 27% 31% 10% 1	562
	21st to end	Night	20% 11% 35% 27% 7%	417
June	1st to 10th	Night	24% 8% 26% 33% 9%	380
	11th to 20th	Night	21% 6% 22% 36% 15% 1	380
	21st to end	Night	28% 6% 23% 32% 9% 1	380
July	1st to 10th	Night	17% 7% 29% 33% 13% 1	370
	11th to 20th	Night	13% 5% 27% 42% 13% 1	368
	21st to end	Night	15% 7% 22% 39% 17% 1	425
August	1st to 10th	Night	15% 4 26% 40% 14% 1	568
	11th to 20th	Night	16% 5% 22% 41% 15%	562
	21st to end	Night	14% 6% 28% 32% 18% 2	627
September	1st to 10th	Night	17% 8% 32% 32% 10%	570
	11th to 20th	Night	20% 11% 23% 34% 12% 1	756
	21st to end	Night	16% 12% 29% 31% 11%	760
October	1st to 10th	Night	20% 13% 29% 26% 10% 1	760
	11th to 20th	Night	23% 14% 32% 22% 9% 1	760
	21st to end	Night	26% 20% 28% 22% 5%	836
November	1st to 10th	Night	25% 19% 32% 20% 4 1	855
	11th to 20th	Night	27% 21% 27% 19% 5% 1	900
	21st to end	Night	27% 20% 28% 20% 5%	900
December	1st to 10th	Night	25% 17% 33% 19% 6%	900
	11th to 20th	Night	24% 27% 30% 15% 5%	900
	21st to end	Night	25% 23% 30% 18% 3 2	990

Page generated at 2025-02-18 04:26:28. Version ac07

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