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 2026-06-16.

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%	137770
	Night	16% 11% 4 9% 6% 5% 5% 4 6% 6% 28%	134475
Transparency	Day	51% 9% 13% 18% 8% 1	141225
	Night	54% 8% 12% 18% 8% 1	139110
Seeing	Day	20% 16% 30% 23% 8% 1	18413
	Night	21% 15% 30% 25% 8% 1	26169

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% 19% 5% 10% 6% 6% 6% 4 6% 5% 18%	4093
	11th to 20th	Day	11% 14% 6% 13% 10% 8% 6% 5% 6% 6% 16%	4213
	21st to end	Day	4 7% 4 10% 6% 7% 9% 7% 9% 9% 29%	5221
February	1st to 10th	Day	7% 10% 4 11% 7% 7% 7% 6% 7% 6% 28%	1890
	11th to 20th	Day	11% 14% 5% 12% 6% 6% 6% 5% 6% 7% 22%	1843
	21st to end	Day	8% 12% 4 10% 6% 6% 6% 5% 6% 7% 30%	1630
March	1st to 10th	Day	16% 11% 5% 12% 6% 4 7% 5% 6% 7% 23%	4799
	11th to 20th	Day	15% 15% 4 9% 7% 6% 5% 4 6% 7% 23%	5090
	21st to end	Day	19% 13% 4 10% 6% 5% 5% 3 4 5% 25%	5587
April	1st to 10th	Day	19% 11% 3 7% 7% 5% 5% 3 6% 6% 28%	2519
	11th to 20th	Day	17% 9% 3 7% 5% 5% 5% 4 6% 7% 32%	2702
	21st to end	Day	21% 9% 3 9% 5% 4 4 3 5% 6% 31%	2730
May	1st to 10th	Day	21% 10% 3 9% 5% 5% 5% 4 5% 6% 27%	7793
	11th to 20th	Day	19% 7% 2 7% 5% 5% 5% 4 6% 7% 33%	8256
	21st to end	Day	20% 9% 3 8% 5% 5% 5% 4 5% 7% 29%	9716
June	1st to 10th	Day	21% 9% 3 6% 4 5% 4 3 7% 7% 31%	3150
	11th to 20th	Day	18% 8% 2 6% 5% 4 5% 4 6% 8% 33%	3075
	21st to end	Day	21% 8% 3 7% 6% 5% 5% 5% 7% 7% 26%	3000
July	1st to 10th	Day	13% 6% 3 8% 5% 6% 6% 5% 7% 9% 33%	6820
	11th to 20th	Day	13% 6% 3 6% 5% 6% 5% 5% 7% 9% 35%	6820
	21st to end	Day	11% 5% 2 5% 4 4 5% 5% 6% 9% 44%	7169
August	1st to 10th	Day	14% 5% 2 6% 5% 5% 6% 5% 8% 9% 35%	4235
	11th to 20th	Day	15% 7% 3 7% 5% 6% 6% 5% 7% 8% 32%	4175
	21st to end	Day	13% 5% 2 7% 5% 4 5% 4 6% 8% 40%	4675
September	1st to 10th	Day	15% 6% 2 7% 5% 4 5% 3 5% 6% 41%	2438
	11th to 20th	Day	17% 6% 2 5% 4 4 5% 3 4 4 46%	2200
	21st to end	Day	17% 6% 3 7% 5% 4 5% 3 6% 7% 37%	2200
October	1st to 10th	Day	14% 6% 2 8% 5% 4 5% 4 6% 6% 40%	2020
	11th to 20th	Day	19% 9% 3 8% 5% 5% 5% 4 6% 6% 30%	2000
	21st to end	Day	23% 9% 3 8% 5% 4 5% 4 5% 7% 27%	2023
November	1st to 10th	Day	17% 10% 4 9% 7% 6% 6% 4 6% 6% 26%	1693
	11th to 20th	Day	21% 12% 4 10% 5% 5% 6% 5% 7% 5% 21%	1680
	21st to end	Day	19% 13% 5% 10% 7% 5% 6% 4 5% 7% 19%	1680
December	1st to 10th	Day	16% 10% 5% 10% 10% 7% 7% 4 5% 5% 22%	2394
	11th to 20th	Day	19% 9% 4 11% 7% 6% 6% 5% 6% 6% 22%	2688
	21st to end	Day	14% 15% 5% 10% 6% 4 5% 4 5% 5% 27%	3553
January	1st to 10th	Night	14% 16% 5% 13% 8% 7% 7% 4 6% 6% 13%	8831
	11th to 20th	Night	11% 15% 6% 12% 8% 7% 6% 5% 6% 6% 18%	8693
	21st to end	Night	4 8% 3 9% 7% 7% 10% 8% 8% 8% 28%	8958
February	1st to 10th	Night	8% 13% 5% 9% 7% 7% 5% 5% 6% 6% 28%	2730
	11th to 20th	Night	13% 16% 5% 9% 5% 6% 7% 5% 6% 6% 23%	2663
	21st to end	Night	11% 14% 4 8% 6% 5% 6% 5% 6% 6% 30%	2171
March	1st to 10th	Night	16% 14% 4 10% 6% 5% 7% 4 4 5% 26%	5261
	11th to 20th	Night	19% 15% 5% 12% 6% 6% 5% 4 4 5% 19%	5089
	21st to end	Night	25% 15% 3 11% 5% 4 3 2 3 4 23%	5587
April	1st to 10th	Night	21% 14% 3 9% 5% 4 4 3 5% 5% 27%	2096
	11th to 20th	Night	20% 9% 3 8% 4 5% 4 3 4 6% 33%	1868
	21st to end	Night	24% 11% 3 6% 4 3 3 3 4 5% 32%	1890
May	1st to 10th	Night	23% 10% 3 9% 5% 4 4 3 4 5% 29%	5113
	11th to 20th	Night	22% 7% 3 6% 5% 5% 4 4 5% 6% 32%	4670
	21st to end	Night	18% 11% 4 9% 5% 4 4 3 5% 5% 32%	4497
June	1st to 10th	Night	22% 9% 3 6% 4 5% 4 4 4 6% 33%	1470
	11th to 20th	Night	20% 6% 2 6% 5% 4 4 4 6% 8% 37%	1435
	21st to end	Night	26% 8% 2 7% 5% 5% 5% 3 4 6% 30%	1400
July	1st to 10th	Night	14% 8% 3 9% 6% 5% 5% 4 7% 7% 32%	3160
	11th to 20th	Night	14% 6% 2 6% 4 5% 6% 6% 8% 8% 35%	3160
	21st to end	Night	10% 5% 3 7% 5% 5% 4 4 6% 7% 45%	3648
August	1st to 10th	Night	14% 7% 3 9% 5% 5% 5% 4 6% 6% 36%	2915
	11th to 20th	Night	14% 7% 2 8% 5% 5% 6% 4 6% 8% 34%	2872
	21st to end	Night	14% 6% 2 6% 4 4 3 3 5% 7% 46%	3223
September	1st to 10th	Night	17% 6% 1 5% 4 4 5% 4 5% 8% 43%	1961
	11th to 20th	Night	16% 6% 2 6% 3 4 3 3 5% 6% 44%	2196
	21st to end	Night	17% 4 1 6% 5% 4 4 3 4 6% 46%	2200
October	1st to 10th	Night	17% 7% 2 7% 4 4 4 3 5% 6% 40%	2380
	11th to 20th	Night	21% 6% 2 6% 5% 4 5% 4 5% 6% 34%	2400
	21st to end	Night	21% 9% 2 7% 5% 5% 5% 4 6% 7% 29%	2815
November	1st to 10th	Night	19% 9% 3 8% 6% 5% 6% 4 5% 6% 29%	2823
	11th to 20th	Night	20% 11% 4 10% 5% 6% 5% 3 5% 6% 24%	2940
	21st to end	Night	20% 13% 4 9% 7% 5% 5% 4 6% 6% 20%	2940
December	1st to 10th	Night	13% 12% 4 12% 8% 5% 5% 4 6% 7% 23%	5012
	11th to 20th	Night	20% 10% 4 12% 6% 5% 5% 4 6% 7% 21%	5741
	21st to end	Night	17% 13% 5% 10% 6% 5% 4 4 5% 6% 25%	7667

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	65% 8% 12% 12% 4	4094
	11th to 20th	Day	63% 11% 12% 8% 6%	4239
	21st to end	Day	45% 8% 11% 21% 14%	5221
February	1st to 10th	Day	51% 10% 14% 16% 8% 2	1890
	11th to 20th	Day	59% 10% 10% 15% 6%	1851
	21st to end	Day	50% 12% 8% 20% 9% 1	1630
March	1st to 10th	Day	59% 7% 10% 12% 11% 1	4803
	11th to 20th	Day	59% 10% 11% 13% 6% 1	5090
	21st to end	Day	57% 9% 6% 17% 10% 1	5587
April	1st to 10th	Day	55% 8% 9% 15% 11% 2	2519
	11th to 20th	Day	51% 9% 10% 19% 10% 1	2702
	21st to end	Day	54% 8% 9% 17% 10% 3	2730
May	1st to 10th	Day	56% 9% 10% 15% 10%	7805
	11th to 20th	Day	49% 8% 15% 19% 9%	8260
	21st to end	Day	52% 9% 10% 17% 13%	9717
June	1st to 10th	Day	52% 8% 12% 17% 10% 1	3150
	11th to 20th	Day	48% 9% 13% 18% 10% 2	3075
	21st to end	Day	54% 12% 12% 15% 7% 1	3000
July	1st to 10th	Day	45% 9% 16% 21% 10%	6745
	11th to 20th	Day	42% 9% 21% 24% 4	6739
	21st to end	Day	37% 8% 19% 28% 8%	6961
August	1st to 10th	Day	43% 10% 19% 20% 8%	4222
	11th to 20th	Day	47% 10% 21% 17% 5%	4214
	21st to end	Day	40% 9% 18% 21% 11%	4675
September	1st to 10th	Day	42% 9% 16% 24% 9%	2438
	11th to 20th	Day	42% 6% 15% 25% 11%	2200
	21st to end	Day	44% 9% 12% 25% 9%	2200
October	1st to 10th	Day	46% 7% 16% 25% 6%	3293
	11th to 20th	Day	53% 6% 13% 22% 6%	3240
	21st to end	Day	49% 8% 14% 19% 9%	3247
November	1st to 10th	Day	57% 9% 12% 16% 6%	1693
	11th to 20th	Day	59% 12% 10% 13% 5%	1680
	21st to end	Day	65% 9% 13% 10% 3	1680
December	1st to 10th	Day	63% 8% 12% 14% 2	2394
	11th to 20th	Day	60% 9% 10% 15% 5% 1	2688
	21st to end	Day	55% 10% 12% 14% 7% 1	3553
January	1st to 10th	Night	67% 9% 11% 11% 2	8831
	11th to 20th	Night	61% 10% 11% 13% 5%	8742
	21st to end	Night	49% 7% 11% 20% 12% 1	8958
February	1st to 10th	Night	54% 7% 12% 16% 9% 2	2730
	11th to 20th	Night	60% 9% 10% 14% 6% 1	2666
	21st to end	Night	52% 9% 10% 18% 10% 2	2171
March	1st to 10th	Night	59% 6% 8% 15% 10% 1	5270
	11th to 20th	Night	64% 7% 9% 12% 7% 1	5090
	21st to end	Night	63% 7% 6% 13% 10% 1	5587
April	1st to 10th	Night	58% 7% 9% 13% 12% 2	2096
	11th to 20th	Night	51% 6% 10% 18% 13% 2	1868
	21st to end	Night	53% 5% 10% 17% 13% 2	1890
May	1st to 10th	Night	56% 7% 9% 15% 12% 1	5119
	11th to 20th	Night	50% 7% 11% 19% 11% 1	4671
	21st to end	Night	51% 9% 10% 15% 15% 1	4498
June	1st to 10th	Night	51% 7% 12% 17% 11% 1	1470
	11th to 20th	Night	45% 8% 13% 19% 14% 2	1435
	21st to end	Night	56% 9% 10% 15% 9% 1	1400
July	1st to 10th	Night	49% 8% 17% 20% 7%	3125
	11th to 20th	Night	43% 9% 18% 26% 4	3118
	21st to end	Night	38% 8% 20% 24% 10%	3542
August	1st to 10th	Night	48% 9% 15% 22% 6%	2903
	11th to 20th	Night	45% 10% 21% 17% 6%	2899
	21st to end	Night	38% 8% 17% 23% 14%	3223
September	1st to 10th	Night	40% 8% 15% 28% 9% 1	1961
	11th to 20th	Night	39% 8% 16% 25% 11% 1	2196
	21st to end	Night	39% 8% 13% 27% 13% 1	2200
October	1st to 10th	Night	48% 6% 13% 25% 7% 1	3887
	11th to 20th	Night	46% 6% 13% 29% 5%	3899
	21st to end	Night	47% 9% 16% 19% 8%	4541
November	1st to 10th	Night	54% 9% 14% 16% 7%	2823
	11th to 20th	Night	60% 8% 12% 15% 4	2940
	21st to end	Night	63% 10% 13% 11% 3 1	2940
December	1st to 10th	Night	58% 8% 14% 17% 2	5012
	11th to 20th	Night	59% 9% 10% 16% 5%	5742
	21st to end	Night	58% 8% 12% 16% 4 1	7667

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% 23% 29% 19% 3 1	220
	11th to 20th	Day	22% 27% 27% 15% 8% 1	220
	21st to end	Day	14% 22% 36% 20% 7% 1	274
February	1st to 10th	Day	15% 21% 33% 20% 8% 2	430
	11th to 20th	Day	16% 29% 34% 16% 5%	426
	21st to end	Day	15% 25% 34% 20% 5% 1	394
March	1st to 10th	Day	22% 19% 34% 21% 4	540
	11th to 20th	Day	20% 24% 28% 20% 7% 1	540
	21st to end	Day	22% 20% 34% 17% 6% 2	594
April	1st to 10th	Day	26% 24% 30% 17% 2 1	540
	11th to 20th	Day	20% 19% 35% 19% 5% 1	532
	21st to end	Day	23% 16% 31% 20% 6% 2	540
May	1st to 10th	Day	26% 14% 30% 23% 6% 1	595
	11th to 20th	Day	23% 14% 28% 22% 11% 1	774
	21st to end	Day	24% 16% 30% 22% 7% 1	845
June	1st to 10th	Day	25% 11% 30% 24% 9% 1	780
	11th to 20th	Day	22% 13% 29% 24% 11% 1	755
	21st to end	Day	24% 11% 28% 28% 7% 1	730
July	1st to 10th	Day	16% 10% 34% 27% 11% 2	720
	11th to 20th	Day	13% 9% 31% 36% 10% 1	718
	21st to end	Day	15% 8% 30% 35% 11% 1	717
August	1st to 10th	Day	13% 9% 30% 32% 14% 2	526
	11th to 20th	Day	15% 8% 27% 33% 15% 1	519
	21st to end	Day	13% 11% 29% 30% 13% 4	583
September	1st to 10th	Day	17% 10% 33% 26% 12% 3	530
	11th to 20th	Day	21% 10% 29% 28% 9% 2	530
	21st to end	Day	17% 18% 27% 22% 12% 3	530
October	1st to 10th	Day	13% 16% 32% 26% 11% 2	431
	11th to 20th	Day	24% 20% 26% 20% 10% 1	420
	21st to end	Day	28% 22% 24% 18% 8%	462
November	1st to 10th	Day	22% 23% 30% 19% 4 1	425
	11th to 20th	Day	28% 22% 27% 16% 6% 1	430
	21st to end	Day	26% 25% 28% 16% 4	430
December	1st to 10th	Day	24% 22% 31% 16% 6% 1	251
	11th to 20th	Day	26% 27% 32% 12% 2	220
	21st to end	Day	18% 28% 36% 15% 3	242
January	1st to 10th	Night	21% 20% 35% 19% 5%	950
	11th to 20th	Night	20% 21% 32% 21% 5% 1	925
	21st to end	Night	14% 22% 36% 22% 6%	924
February	1st to 10th	Night	14% 21% 37% 20% 7%	840
	11th to 20th	Night	22% 18% 37% 17% 5%	823
	21st to end	Night	16% 15% 39% 24% 5%	692
March	1st to 10th	Night	21% 18% 34% 23% 4	840
	11th to 20th	Night	22% 19% 30% 21% 7%	840
	21st to end	Night	21% 15% 35% 23% 6% 1	924
April	1st to 10th	Night	27% 17% 31% 21% 5%	714
	11th to 20th	Night	24% 14% 31% 23% 8%	624
	21st to end	Night	29% 11% 27% 27% 7% 1	630
May	1st to 10th	Night	26% 12% 27% 27% 9%	630
	11th to 20th	Night	23% 10% 28% 29% 9% 1	622
	21st to end	Night	21% 10% 33% 27% 8%	461
June	1st to 10th	Night	23% 9% 25% 34% 10%	420
	11th to 20th	Night	22% 6% 22% 35% 15%	410
	21st to end	Night	28% 7% 24% 31% 9% 1	400
July	1st to 10th	Night	18% 7% 28% 33% 13% 1	390
	11th to 20th	Night	13% 5% 26% 43% 12% 1	388
	21st to end	Night	15% 7% 23% 40% 16% 1	448
August	1st to 10th	Night	14% 4 26% 40% 15% 1	598
	11th to 20th	Night	15% 5% 22% 41% 16%	592
	21st to end	Night	13% 6% 28% 32% 18% 2	660
September	1st to 10th	Night	16% 8% 32% 33% 11%	600
	11th to 20th	Night	19% 11% 23% 34% 12% 1	796
	21st to end	Night	16% 13% 29% 31% 11% 1	800
October	1st to 10th	Night	19% 14% 29% 27% 10% 1	800
	11th to 20th	Night	23% 13% 31% 23% 9% 1	800
	21st to end	Night	26% 19% 28% 22% 5%	880
November	1st to 10th	Night	26% 19% 31% 20% 4 1	903
	11th to 20th	Night	27% 21% 27% 19% 5% 1	950
	21st to end	Night	26% 20% 28% 20% 5%	950
December	1st to 10th	Night	25% 17% 33% 19% 5%	950
	11th to 20th	Night	24% 27% 31% 14% 4	950
	21st to end	Night	24% 23% 30% 18% 3 1	1045

Page generated at 2026-06-13 03:31:42. Version ac07

Copyright © 2026 Attilla Danko.