Gander
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gander clear sky chart from 2005-11-06 to 2026-05-17.

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	20% 14% 4 9% 5% 5% 4 3 5% 6% 23%	131942
	Night	19% 14% 5% 10% 5% 5% 4 3 5% 6% 24%	131165
Transparency	Day	62% 3 10% 17% 7% 1	135382
	Night	62% 4 11% 16% 8% 1	135837
Seeing	Day	32% 13% 29% 18% 7% 2	20210
	Night	31% 13% 31% 20% 5%	27010

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	20% 21% 8% 11% 5% 4 4 3 3 4 18%	4122
	11th to 20th	Day	19% 18% 7% 14% 5% 4 4 3 4 4 18%	4607
	21st to end	Day	7% 13% 5% 12% 9% 7% 6% 5% 6% 6% 24%	5157
February	1st to 10th	Day	17% 14% 5% 12% 8% 6% 5% 3 5% 5% 22%	1701
	11th to 20th	Day	19% 20% 7% 12% 6% 4 5% 3 5% 3 16%	1894
	21st to end	Day	13% 15% 5% 13% 7% 5% 6% 4 3 5% 23%	1730
March	1st to 10th	Day	15% 15% 6% 14% 8% 6% 5% 3 4 5% 19%	4574
	11th to 20th	Day	20% 17% 7% 12% 6% 5% 4 3 4 4 19%	4991
	21st to end	Day	23% 21% 8% 12% 5% 4 3 2 2 3 16%	5759
April	1st to 10th	Day	16% 20% 6% 13% 6% 5% 4 3 4 5% 17%	2520
	11th to 20th	Day	22% 19% 5% 8% 4 4 4 3 4 5% 22%	2701
	21st to end	Day	23% 22% 5% 8% 5% 4 4 3 3 4 20%	2730
May	1st to 10th	Day	26% 22% 5% 8% 4 4 3 3 4 4 16%	6149
	11th to 20th	Day	22% 17% 4 9% 5% 4 4 4 4 5% 22%	6636
	21st to end	Day	25% 19% 3 7% 4 3 3 3 4 6% 23%	7428
June	1st to 10th	Day	32% 16% 4 6% 5% 4 4 3 4 5% 18%	3000
	11th to 20th	Day	28% 12% 3 7% 5% 4 4 4 5% 6% 22%	3000
	21st to end	Day	24% 10% 3 8% 5% 4 5% 4 6% 6% 24%	3000
July	1st to 10th	Day	20% 10% 4 9% 5% 5% 5% 4 6% 7% 25%	6800
	11th to 20th	Day	12% 7% 2 8% 6% 6% 6% 5% 7% 10% 29%	6757
	21st to end	Day	20% 9% 3 7% 4 4 5% 4 6% 8% 29%	6927
August	1st to 10th	Day	20% 8% 3 9% 7% 6% 6% 4 6% 8% 24%	4515
	11th to 20th	Day	19% 7% 2 6% 4 4 4 3 5% 6% 39%	4176
	21st to end	Day	21% 7% 3 7% 4 4 5% 4 5% 8% 32%	4675
September	1st to 10th	Day	21% 9% 2 6% 4 5% 5% 3 7% 7% 33%	2401
	11th to 20th	Day	26% 9% 4 8% 5% 5% 4 3 4 6% 27%	2200
	21st to end	Day	19% 11% 3 8% 5% 4 4 3 5% 7% 31%	2200
October	1st to 10th	Day	22% 12% 3 7% 5% 4 4 3 5% 6% 30%	2140
	11th to 20th	Day	24% 14% 4 8% 5% 4 4 3 5% 5% 22%	1855
	21st to end	Day	25% 17% 4 8% 5% 5% 4 3 4 4 20%	1980
November	1st to 10th	Day	23% 15% 4 9% 5% 6% 4 4 5% 5% 20%	1839
	11th to 20th	Day	21% 15% 5% 9% 5% 4 5% 2 3 5% 25%	1675
	21st to end	Day	23% 18% 6% 8% 4 4 3 2 3 5% 23%	1470
December	1st to 10th	Day	13% 12% 4 9% 5% 4 4 4 5% 6% 33%	2360
	11th to 20th	Day	22% 13% 5% 11% 5% 4 4 3 4 6% 23%	2698
	21st to end	Day	20% 16% 5% 16% 5% 5% 4 3 4 5% 17%	3575
January	1st to 10th	Night	18% 19% 7% 13% 5% 4 4 3 4 5% 18%	8802
	11th to 20th	Night	14% 21% 7% 14% 6% 5% 5% 4 4 4 16%	8291
	21st to end	Night	7% 13% 5% 10% 8% 6% 5% 4 6% 6% 30%	9056
February	1st to 10th	Night	16% 13% 5% 12% 7% 6% 4 4 5% 5% 23%	2919
	11th to 20th	Night	20% 19% 5% 10% 6% 5% 5% 4 5% 4 18%	2610
	21st to end	Night	13% 14% 5% 10% 6% 6% 5% 3 5% 5% 27%	2076
March	1st to 10th	Night	19% 13% 4 12% 6% 5% 4 4 5% 6% 22%	5486
	11th to 20th	Night	18% 15% 6% 14% 7% 6% 5% 4 5% 5% 15%	5188
	21st to end	Night	22% 17% 5% 10% 5% 4 4 3 5% 6% 18%	5411
April	1st to 10th	Night	23% 17% 6% 9% 5% 3 3 3 3 5% 22%	2100
	11th to 20th	Night	23% 17% 5% 9% 5% 3 3 3 4 5% 23%	1868
	21st to end	Night	27% 16% 5% 8% 4 3 3 2 3 5% 23%	1890
May	1st to 10th	Night	31% 19% 6% 8% 4 3 3 2 4 4 16%	4029
	11th to 20th	Night	24% 18% 7% 10% 5% 3 3 2 3 5% 19%	3410
	21st to end	Night	28% 14% 5% 7% 4 3 4 3 4 6% 21%	3468
June	1st to 10th	Night	31% 13% 3 6% 5% 4 4 4 5% 6% 19%	1400
	11th to 20th	Night	30% 10% 3 7% 5% 3 3 4 6% 7% 23%	1400
	21st to end	Night	23% 10% 3 7% 6% 5% 5% 4 6% 7% 25%	1400
July	1st to 10th	Night	22% 11% 3 8% 5% 4 5% 4 6% 7% 25%	3179
	11th to 20th	Night	11% 9% 2 8% 7% 6% 6% 5% 7% 9% 30%	3219
	21st to end	Night	21% 10% 3 6% 4 4 4 3 6% 7% 33%	3948
August	1st to 10th	Night	15% 7% 3 7% 5% 5% 5% 4 7% 9% 32%	2645
	11th to 20th	Night	16% 8% 3 6% 4 4 4 4 5% 6% 40%	2881
	21st to end	Night	22% 8% 3 7% 4 4 4 3 4 6% 36%	3234
September	1st to 10th	Night	20% 8% 3 7% 4 4 4 2 5% 7% 37%	1996
	11th to 20th	Night	25% 10% 3 7% 4 4 4 3 4 6% 31%	2200
	21st to end	Night	21% 12% 3 7% 4 4 4 3 6% 6% 31%	2200
October	1st to 10th	Night	22% 12% 3 7% 4 4 4 3 5% 6% 30%	2260
	11th to 20th	Night	24% 14% 3 7% 4 4 4 3 5% 5% 27%	2541
	21st to end	Night	27% 15% 3 7% 4 3 3 3 4 5% 25%	2860
November	1st to 10th	Night	22% 18% 4 8% 5% 4 4 3 5% 6% 20%	2677
	11th to 20th	Night	24% 14% 4 8% 5% 4 5% 3 5% 6% 22%	2945
	21st to end	Night	20% 18% 5% 8% 5% 5% 5% 3 5% 5% 21%	3150
December	1st to 10th	Night	15% 11% 4 10% 6% 5% 4 3 5% 6% 31%	5050
	11th to 20th	Night	21% 14% 6% 11% 6% 5% 4 3 4 5% 21%	5731
	21st to end	Night	17% 14% 6% 14% 6% 5% 4 3 5% 5% 21%	7645

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	72% 3 6% 14% 3 1	4123
	11th to 20th	Day	70% 3 6% 11% 9%	4636
	21st to end	Day	57% 3 9% 18% 10% 2	5157
February	1st to 10th	Day	63% 3 10% 12% 9% 2	1701
	11th to 20th	Day	72% 3 6% 12% 6%	1901
	21st to end	Day	64% 4 6% 13% 11% 2	1730
March	1st to 10th	Day	67% 2 6% 12% 9% 2	4578
	11th to 20th	Day	70% 3 7% 10% 8% 2	4991
	21st to end	Day	75% 2 5% 11% 7%	5759
April	1st to 10th	Day	70% 4 6% 12% 7% 1	2520
	11th to 20th	Day	66% 2 8% 15% 9% 1	2701
	21st to end	Day	70% 2 7% 12% 8% 1	2730
May	1st to 10th	Day	74% 2 9% 11% 4 1	6161
	11th to 20th	Day	66% 2 9% 16% 6%	6640
	21st to end	Day	64% 3 9% 19% 5%	7428
June	1st to 10th	Day	70% 2 8% 14% 5%	3000
	11th to 20th	Day	63% 4 10% 18% 5% 1	3000
	21st to end	Day	60% 4 12% 16% 6% 1	3000
July	1st to 10th	Day	58% 4 12% 19% 6%	6725
	11th to 20th	Day	49% 6% 17% 22% 6%	6675
	21st to end	Day	52% 5% 16% 22% 4	6730
August	1st to 10th	Day	58% 6% 14% 18% 4	4501
	11th to 20th	Day	46% 3 15% 24% 11% 1	4215
	21st to end	Day	49% 4 16% 25% 5%	4675
September	1st to 10th	Day	51% 3 11% 25% 7% 2	2401
	11th to 20th	Day	60% 3 12% 18% 7% 1	2200
	21st to end	Day	53% 4 15% 21% 7% 1	2200
October	1st to 10th	Day	60% 6% 9% 19% 6%	3494
	11th to 20th	Day	61% 4 11% 18% 6%	3003
	21st to end	Day	66% 3 9% 12% 9% 1	3191
November	1st to 10th	Day	66% 4 13% 11% 6%	1839
	11th to 20th	Day	64% 3 10% 15% 7% 1	1675
	21st to end	Day	66% 3 10% 11% 8% 1	1470
December	1st to 10th	Day	51% 3 10% 24% 12%	2360
	11th to 20th	Day	61% 5% 16% 11% 7%	2697
	21st to end	Day	71% 2 9% 14% 4	3575
January	1st to 10th	Night	69% 4 7% 11% 7% 1	8802
	11th to 20th	Night	69% 5% 8% 11% 7% 1	8337
	21st to end	Night	52% 3 11% 13% 19% 2	9056
February	1st to 10th	Night	62% 4 9% 14% 10% 1	2919
	11th to 20th	Night	69% 4 8% 13% 6% 1	2614
	21st to end	Night	59% 4 7% 18% 10% 2	2076
March	1st to 10th	Night	62% 3 9% 14% 10% 2	5495
	11th to 20th	Night	71% 2 9% 10% 7% 1	5189
	21st to end	Night	68% 3 8% 14% 6%	5411
April	1st to 10th	Night	66% 3 6% 13% 11% 1	2100
	11th to 20th	Night	65% 3 7% 17% 7% 1	1868
	21st to end	Night	66% 3 7% 14% 10% 1	1890
May	1st to 10th	Night	74% 3 8% 13% 3	4034
	11th to 20th	Night	71% 2 8% 13% 6%	3411
	21st to end	Night	66% 3 10% 16% 5%	3469
June	1st to 10th	Night	66% 4 12% 12% 6%	1400
	11th to 20th	Night	62% 3 12% 17% 6%	1400
	21st to end	Night	57% 5% 13% 17% 6% 1	1400
July	1st to 10th	Night	58% 5% 15% 19% 3	3146
	11th to 20th	Night	49% 7% 15% 22% 7%	3177
	21st to end	Night	52% 5% 17% 23% 3	3831
August	1st to 10th	Night	48% 8% 18% 22% 5%	2634
	11th to 20th	Night	45% 5% 15% 26% 9% 1	2908
	21st to end	Night	50% 4 14% 27% 5%	3233
September	1st to 10th	Night	49% 3 14% 24% 10% 1	1996
	11th to 20th	Night	55% 3 13% 17% 10% 1	2200
	21st to end	Night	54% 4 13% 21% 8%	2200
October	1st to 10th	Night	60% 4 13% 17% 6% 1	3689
	11th to 20th	Night	57% 3 16% 16% 8%	4142
	21st to end	Night	57% 3 12% 18% 9% 1	4610
November	1st to 10th	Night	65% 5% 11% 13% 6%	2677
	11th to 20th	Night	64% 4 11% 15% 5%	2945
	21st to end	Night	65% 4 10% 13% 8% 1	3150
December	1st to 10th	Night	54% 4 8% 23% 12%	5050
	11th to 20th	Night	65% 5% 14% 11% 5%	5733
	21st to end	Night	65% 4 10% 16% 5%	7645

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	30% 15% 33% 14% 6% 1	430
	11th to 20th	Day	30% 19% 30% 16% 5%	430
	21st to end	Day	25% 21% 33% 16% 4	473
February	1st to 10th	Day	29% 18% 33% 14% 4 1	430
	11th to 20th	Day	31% 22% 31% 13% 3	426
	21st to end	Day	23% 18% 38% 15% 5% 1	355
March	1st to 10th	Day	29% 21% 33% 13% 3 1	430
	11th to 20th	Day	31% 17% 26% 17% 7% 3	430
	21st to end	Day	38% 13% 32% 11% 4 2	510
April	1st to 10th	Day	31% 18% 31% 16% 4	540
	11th to 20th	Day	36% 11% 30% 17% 4 1	739
	21st to end	Day	38% 12% 26% 18% 5% 1	750
May	1st to 10th	Day	41% 12% 22% 17% 6% 2	753
	11th to 20th	Day	33% 11% 27% 17% 8% 3	754
	21st to end	Day	37% 10% 31% 17% 5% 1	790
June	1st to 10th	Day	45% 7% 25% 15% 8% 1	730
	11th to 20th	Day	37% 10% 25% 18% 8% 2	730
	21st to end	Day	30% 9% 29% 18% 10% 4	730
July	1st to 10th	Day	29% 10% 29% 21% 8% 3	720
	11th to 20th	Day	22% 8% 31% 28% 9% 1	718
	21st to end	Day	29% 7% 27% 29% 8% 1	795
August	1st to 10th	Day	23% 8% 29% 27% 12% 2	725
	11th to 20th	Day	25% 8% 24% 25% 15% 2	717
	21st to end	Day	26% 7% 31% 24% 10% 3	803
September	1st to 10th	Day	24% 11% 29% 26% 8% 2	542
	11th to 20th	Day	36% 10% 27% 21% 5% 1	420
	21st to end	Day	25% 17% 34% 17% 5% 2	420
October	1st to 10th	Day	27% 13% 34% 18% 6% 2	420
	11th to 20th	Day	37% 10% 27% 17% 7% 3	420
	21st to end	Day	35% 17% 24% 15% 6% 3	462
November	1st to 10th	Day	36% 15% 31% 16% 3	425
	11th to 20th	Day	36% 17% 25% 16% 5% 1	430
	21st to end	Day	39% 20% 28% 12% 1	430
December	1st to 10th	Day	30% 14% 32% 18% 6% 1	430
	11th to 20th	Day	32% 23% 30% 13% 3	430
	21st to end	Day	26% 16% 34% 18% 5% 1	473
January	1st to 10th	Night	30% 17% 32% 16% 5% 1	950
	11th to 20th	Night	29% 19% 32% 16% 4	950
	21st to end	Night	26% 17% 38% 16% 3	1045
February	1st to 10th	Night	26% 20% 33% 17% 3	950
	11th to 20th	Night	33% 18% 32% 14% 3	905
	21st to end	Night	24% 13% 40% 16% 6% 1	692
March	1st to 10th	Night	30% 18% 33% 14% 4 1	840
	11th to 20th	Night	26% 15% 29% 22% 8%	677
	21st to end	Night	35% 13% 29% 19% 4	693
April	1st to 10th	Night	35% 17% 27% 17% 4 1	630
	11th to 20th	Night	33% 10% 31% 21% 5%	624
	21st to end	Night	38% 11% 27% 19% 5%	630
May	1st to 10th	Night	39% 7% 24% 23% 6% 1	630
	11th to 20th	Night	36% 7% 31% 17% 8%	611
	21st to end	Night	35% 9% 31% 20% 6% 1	649
June	1st to 10th	Night	38% 9% 28% 20% 5%	600
	11th to 20th	Night	37% 9% 25% 24% 5%	600
	21st to end	Night	29% 8% 33% 22% 8% 1	600
July	1st to 10th	Night	26% 8% 35% 25% 6%	586
	11th to 20th	Night	22% 7% 36% 30% 5%	582
	21st to end	Night	28% 6% 33% 30% 4	656
August	1st to 10th	Night	23% 5% 29% 32% 10% 1	598
	11th to 20th	Night	23% 7% 30% 29% 9%	592
	21st to end	Night	25% 7% 27% 31% 8% 1	660
September	1st to 10th	Night	26% 6% 33% 28% 7%	600
	11th to 20th	Night	30% 10% 31% 26% 4	600
	21st to end	Night	32% 11% 32% 22% 4	600
October	1st to 10th	Night	32% 10% 31% 22% 4	633
	11th to 20th	Night	35% 10% 27% 21% 6% 1	851
	21st to end	Night	35% 12% 26% 21% 6%	1001
November	1st to 10th	Night	35% 15% 31% 15% 3	930
	11th to 20th	Night	36% 13% 28% 16% 6% 1	950
	21st to end	Night	34% 18% 31% 15% 2	950
December	1st to 10th	Night	31% 14% 31% 17% 5%	950
	11th to 20th	Night	34% 19% 30% 14% 3	950
	21st to end	Night	25% 18% 31% 20% 4 1	1045

Page generated at 2026-05-14 02:58:58. Version ac07

Copyright © 2026 Attilla Danko.