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 2024-09-14.

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% 5% 4 6% 7% 30%	101211
	Night	16% 10% 3 8% 6% 5% 5% 4 6% 6% 29%	96780
Transparency	Day	50% 11% 14% 17% 8% 1	100952
	Night	53% 10% 13% 17% 8% 1	96660
Seeing	Day	20% 16% 30% 24% 8% 1	16146
	Night	21% 15% 30% 25% 8% 1	23788

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% 12% 4 9% 7% 6% 5% 4 6% 6% 26%	2207
	11th to 20th	Day	10% 12% 4 9% 14% 10% 8% 6% 6% 6% 16%	2274
	21st to end	Day	6% 7% 4 10% 6% 6% 6% 5% 8% 9% 34%	2813
February	1st to 10th	Day	8% 11% 4 10% 7% 7% 6% 6% 7% 7% 28%	1710
	11th to 20th	Day	11% 14% 5% 12% 6% 6% 6% 5% 6% 7% 21%	1663
	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 44%	2015
	21st to end	Day	18% 6% 3 7% 5% 4 4 3 6% 7% 37%	1980
October	1st to 10th	Day	14% 7% 2 8% 5% 4 5% 4 6% 6% 39%	1818
	11th to 20th	Day	20% 9% 3 8% 5% 5% 5% 4 5% 7% 30%	1800
	21st to end	Day	23% 10% 3 8% 5% 4 5% 4 5% 7% 26%	1821
November	1st to 10th	Day	16% 10% 4 9% 6% 6% 6% 4 5% 6% 27%	1529
	11th to 20th	Day	19% 11% 3 9% 6% 5% 6% 5% 8% 6% 22%	1520
	21st to end	Day	18% 13% 4 10% 7% 5% 6% 4 5% 7% 20%	1520
December	1st to 10th	Day	18% 11% 6% 9% 7% 8% 6% 5% 6% 5% 19%	1359
	11th to 20th	Day	18% 11% 5% 9% 7% 6% 7% 5% 7% 7% 19%	1668
	21st to end	Day	17% 13% 4 11% 7% 4 5% 4 5% 4 25%	2431
January	1st to 10th	Night	9% 11% 5% 13% 11% 9% 7% 5% 7% 8% 16%	4748
	11th to 20th	Night	14% 12% 4 11% 9% 8% 7% 6% 6% 6% 17%	4692
	21st to end	Night	6% 10% 4 7% 7% 7% 6% 5% 7% 8% 33%	4841
February	1st to 10th	Night	9% 14% 5% 9% 7% 7% 5% 5% 6% 7% 27%	2470
	11th to 20th	Night	13% 15% 4 9% 6% 6% 7% 5% 6% 6% 22%	2403
	21st to end	Night	11% 14% 4 9% 6% 6% 6% 5% 6% 6% 29%	1971
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	18% 6% 2 6% 3 3 3 3 5% 6% 43%	2019
	21st to end	Night	17% 4 1 5% 5% 4 4 3 4 6% 47%	1980
October	1st to 10th	Night	17% 7% 2 7% 4 5% 4 3 5% 6% 39%	2142
	11th to 20th	Night	21% 6% 2 7% 6% 4 5% 4 5% 6% 33%	2160
	21st to end	Night	22% 9% 2 7% 5% 5% 5% 4 6% 7% 28%	2533
November	1st to 10th	Night	20% 8% 2 7% 6% 6% 6% 4 5% 6% 30%	2547
	11th to 20th	Night	19% 10% 4 10% 5% 6% 5% 3 6% 7% 25%	2660
	21st to end	Night	19% 13% 4 9% 8% 5% 5% 4 6% 6% 21%	2660
December	1st to 10th	Night	19% 12% 4 11% 7% 4 5% 4 5% 6% 21%	2817
	11th to 20th	Night	20% 12% 4 10% 6% 6% 6% 4 7% 6% 18%	3531
	21st to end	Night	16% 12% 4 10% 6% 5% 5% 4 6% 6% 25%	5236

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	56% 12% 13% 15% 4	2208
	11th to 20th	Day	58% 16% 11% 10% 5%	2274
	21st to end	Day	43% 12% 12% 21% 12%	2813
February	1st to 10th	Day	51% 11% 13% 16% 8% 1	1710
	11th to 20th	Day	59% 10% 10% 14% 6% 1	1671
	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	44% 7% 14% 24% 12%	2015
	21st to end	Day	45% 10% 12% 24% 9%	1980
October	1st to 10th	Day	46% 8% 15% 21% 9% 1	1818
	11th to 20th	Day	53% 8% 16% 18% 5%	1800
	21st to end	Day	58% 10% 11% 15% 7%	1821
November	1st to 10th	Day	56% 10% 11% 16% 6%	1529
	11th to 20th	Day	58% 13% 11% 14% 4	1520
	21st to end	Day	64% 10% 14% 10% 2	1520
December	1st to 10th	Day	64% 10% 11% 10% 4	1359
	11th to 20th	Day	60% 12% 12% 10% 5%	1668
	21st to end	Day	56% 13% 12% 9% 7% 2	2431
January	1st to 10th	Night	59% 15% 13% 11% 2	4748
	11th to 20th	Night	60% 13% 10% 13% 5%	4692
	21st to end	Night	46% 10% 12% 18% 13% 1	4841
February	1st to 10th	Night	55% 8% 11% 16% 9% 2	2470
	11th to 20th	Night	60% 10% 10% 13% 6% 1	2406
	21st to end	Night	53% 9% 10% 17% 9% 2	1971
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	41% 8% 15% 24% 12% 1	2019
	21st to end	Night	38% 9% 13% 27% 13% 1	1980
October	1st to 10th	Night	46% 7% 14% 22% 10% 1	2142
	11th to 20th	Night	50% 8% 14% 22% 6%	2160
	21st to end	Night	55% 9% 14% 16% 6% 1	2533
November	1st to 10th	Night	53% 9% 15% 15% 7%	2547
	11th to 20th	Night	59% 9% 13% 15% 5%	2660
	21st to end	Night	63% 10% 13% 10% 3 1	2660
December	1st to 10th	Night	62% 10% 12% 13% 3	2817
	11th to 20th	Night	62% 11% 10% 12% 3	3532
	21st to end	Night	58% 10% 14% 12% 4 2	5236

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% 22% 29% 20% 3 1	180
	11th to 20th	Day	21% 31% 28% 13% 6%	180
	21st to end	Day	13% 21% 37% 20% 8% 1	226
February	1st to 10th	Day	17% 19% 35% 19% 8% 1	370
	11th to 20th	Day	16% 28% 34% 17% 4	366
	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	22% 11% 29% 27% 10% 2	463
	21st to end	Day	18% 19% 28% 22% 11% 2	450
October	1st to 10th	Day	14% 17% 33% 24% 11% 2	369
	11th to 20th	Day	24% 21% 26% 20% 9% 1	360
	21st to end	Day	30% 23% 23% 16% 8%	396
November	1st to 10th	Day	19% 24% 30% 21% 5% 1	365
	11th to 20th	Day	26% 23% 28% 17% 5% 1	370
	21st to end	Day	24% 27% 28% 16% 4 1	370
December	1st to 10th	Day	23% 22% 32% 15% 6% 1	209
	11th to 20th	Day	23% 29% 32% 13% 2	180
	21st to end	Day	19% 30% 36% 13% 2	198
January	1st to 10th	Night	20% 20% 36% 19% 5%	850
	11th to 20th	Night	21% 23% 33% 20% 4	830
	21st to end	Night	14% 21% 36% 22% 6%	836
February	1st to 10th	Night	15% 22% 37% 20% 6%	760
	11th to 20th	Night	20% 19% 38% 17% 4	743
	21st to end	Night	17% 16% 39% 23% 5%	628
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% 33% 12% 1	732
	21st to end	Night	16% 13% 30% 32% 10%	720
October	1st to 10th	Night	20% 14% 30% 27% 9% 1	720
	11th to 20th	Night	22% 14% 32% 22% 9% 1	720
	21st to end	Night	27% 19% 28% 21% 5%	792
November	1st to 10th	Night	25% 19% 31% 20% 4 1	807
	11th to 20th	Night	26% 21% 28% 18% 5% 1	850
	21st to end	Night	25% 21% 28% 20% 5%	850
December	1st to 10th	Night	26% 17% 32% 18% 6%	850
	11th to 20th	Night	23% 27% 30% 14% 5%	850
	21st to end	Night	25% 24% 30% 18% 2 2	935

Page generated at 2024-09-11 11:00:06. Version ac07

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