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 2025-01-22.

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	21% 14% 4 9% 5% 4 4 3 5% 6% 23%	105121
	Night	20% 14% 5% 10% 5% 4 4 3 5% 6% 24%	104416
Transparency	Day	63% 3 10% 16% 6% 1	108579
	Night	61% 4 11% 16% 7% 1	109071
Seeing	Day	32% 12% 29% 19% 7% 2	18376
	Night	31% 12% 31% 20% 5%	25210

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% 18% 8% 13% 8% 5% 6% 4 4 4 18%	2286
	11th to 20th	Day	19% 14% 6% 15% 5% 4 4 3 4 5% 20%	2561
	21st to end	Day	10% 15% 5% 11% 8% 7% 6% 5% 6% 5% 21%	2791
February	1st to 10th	Day	17% 14% 4 11% 8% 6% 5% 3 5% 6% 22%	1539
	11th to 20th	Day	18% 19% 6% 12% 6% 4 5% 4 5% 4 16%	1708
	21st to end	Day	13% 15% 4 13% 7% 6% 6% 4 3 6% 23%	1570
March	1st to 10th	Day	17% 16% 5% 13% 7% 5% 4 3 4 6% 21%	3114
	11th to 20th	Day	23% 20% 8% 11% 7% 5% 4 3 4 4 12%	3411
	21st to end	Day	26% 20% 7% 10% 5% 4 2 2 2 4 18%	3929
April	1st to 10th	Day	17% 20% 6% 12% 6% 6% 4 4 4 5% 16%	2280
	11th to 20th	Day	22% 20% 5% 8% 4 3 4 3 4 5% 21%	2441
	21st to end	Day	22% 23% 5% 8% 5% 4 4 3 3 4 20%	2470
May	1st to 10th	Day	29% 24% 6% 8% 4 3 3 3 4 4 14%	4194
	11th to 20th	Day	26% 17% 4 9% 5% 4 4 3 4 5% 18%	4566
	21st to end	Day	28% 17% 4 8% 4 4 4 3 4 6% 19%	5173
June	1st to 10th	Day	33% 16% 4 6% 5% 4 4 4 4 4 16%	2850
	11th to 20th	Day	29% 12% 3 7% 5% 4 4 4 5% 6% 22%	2850
	21st to end	Day	24% 10% 3 8% 5% 4 5% 4 6% 6% 24%	2850
July	1st to 10th	Day	21% 10% 3 9% 6% 5% 5% 4 5% 7% 24%	4750
	11th to 20th	Day	15% 8% 3 8% 6% 5% 6% 5% 7% 9% 28%	4720
	21st to end	Day	15% 7% 2 6% 4 4 5% 4 7% 9% 36%	4815
August	1st to 10th	Day	21% 8% 3 9% 7% 6% 6% 5% 7% 8% 22%	4376
	11th to 20th	Day	19% 7% 2 6% 4 4 4 3 5% 6% 39%	4046
	21st to end	Day	21% 7% 3 7% 4 4 5% 4 5% 8% 33%	4532
September	1st to 10th	Day	22% 9% 2 6% 4 5% 5% 3 7% 7% 32%	2282
	11th to 20th	Day	27% 9% 3 8% 5% 5% 4 3 4 6% 28%	2090
	21st to end	Day	19% 11% 3 8% 5% 3 4 3 5% 8% 31%	2090
October	1st to 10th	Day	22% 12% 3 7% 5% 4 4 3 5% 6% 30%	2033
	11th to 20th	Day	24% 14% 4 9% 5% 4 4 3 5% 5% 22%	1762
	21st to end	Day	25% 17% 4 9% 5% 5% 5% 3 4 4 20%	1881
November	1st to 10th	Day	23% 15% 4 9% 5% 6% 5% 4 4 5% 21%	1750
	11th to 20th	Day	22% 15% 5% 9% 5% 4 4 2 3 5% 25%	1595
	21st to end	Day	24% 18% 6% 8% 4 4 3 2 3 5% 22%	1400
December	1st to 10th	Day	13% 12% 4 9% 5% 4 4 4 6% 6% 32%	2290
	11th to 20th	Day	22% 14% 5% 11% 5% 4 4 3 4 6% 23%	2628
	21st to end	Day	20% 16% 5% 16% 5% 5% 4 3 4 5% 17%	3498
January	1st to 10th	Night	14% 17% 7% 15% 5% 5% 4 3 4 5% 20%	4889
	11th to 20th	Night	12% 15% 6% 14% 7% 5% 5% 4 5% 5% 22%	4612
	21st to end	Night	10% 13% 5% 12% 8% 6% 5% 4 5% 6% 26%	4899
February	1st to 10th	Night	17% 13% 4 12% 7% 6% 4 4 5% 5% 23%	2641
	11th to 20th	Night	21% 17% 5% 10% 6% 5% 5% 4 5% 4 19%	2356
	21st to end	Night	13% 14% 5% 10% 6% 6% 5% 3 5% 5% 27%	1884
March	1st to 10th	Night	21% 14% 4 11% 5% 5% 4 4 5% 5% 22%	3726
	11th to 20th	Night	19% 16% 7% 13% 7% 6% 5% 4 5% 5% 13%	3548
	21st to end	Night	26% 15% 5% 8% 5% 3 4 3 4 6% 20%	3699
April	1st to 10th	Night	23% 17% 7% 8% 5% 3 3 3 3 5% 21%	1900
	11th to 20th	Night	23% 17% 4 9% 5% 3 3 3 4 5% 23%	1688
	21st to end	Night	26% 16% 4 9% 4 3 3 2 3 5% 23%	1710
May	1st to 10th	Night	33% 20% 6% 8% 3 3 3 2 4 5% 13%	2754
	11th to 20th	Night	26% 18% 7% 11% 5% 3 3 2 4 5% 16%	2337
	21st to end	Night	29% 14% 6% 7% 4 3 4 3 4 5% 21%	2412
June	1st to 10th	Night	32% 13% 3 7% 5% 4 3 4 5% 6% 18%	1330
	11th to 20th	Night	32% 10% 3 6% 5% 3 3 4 6% 7% 22%	1330
	21st to end	Night	24% 10% 3 7% 5% 5% 5% 4 6% 6% 25%	1330
July	1st to 10th	Night	23% 11% 3 7% 5% 4 4 4 5% 6% 26%	2220
	11th to 20th	Night	13% 11% 3 8% 7% 7% 5% 5% 6% 8% 28%	2246
	21st to end	Night	17% 7% 2 6% 4 4 4 3 6% 7% 39%	2750
August	1st to 10th	Night	15% 7% 3 7% 5% 5% 5% 4 7% 9% 30%	2564
	11th to 20th	Night	16% 9% 3 6% 4 4 4 4 6% 6% 40%	2791
	21st to end	Night	22% 7% 3 7% 4 4 4 3 4 6% 36%	3135
September	1st to 10th	Night	21% 8% 2 7% 4 4 4 2 5% 7% 36%	1895
	11th to 20th	Night	25% 10% 3 7% 4 4 4 3 4 6% 32%	2090
	21st to end	Night	21% 12% 3 7% 4 4 4 3 6% 6% 30%	2090
October	1st to 10th	Night	23% 12% 3 8% 4 4 4 3 5% 6% 29%	2147
	11th to 20th	Night	24% 14% 3 7% 4 4 4 3 5% 5% 27%	2414
	21st to end	Night	28% 15% 3 8% 5% 3 3 3 4 5% 25%	2717
November	1st to 10th	Night	21% 17% 3 8% 6% 4 5% 4 5% 6% 20%	2546
	11th to 20th	Night	24% 14% 5% 8% 5% 4 5% 3 5% 6% 21%	2805
	21st to end	Night	20% 19% 5% 8% 5% 5% 5% 3 5% 5% 21%	3000
December	1st to 10th	Night	15% 11% 4 9% 6% 5% 4 3 5% 6% 31%	4900
	11th to 20th	Night	21% 14% 5% 11% 6% 5% 4 3 4 5% 21%	5581
	21st to end	Night	17% 14% 6% 14% 6% 5% 4 3 5% 5% 21%	7480

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	69% 4 6% 15% 4 1	2287
	11th to 20th	Day	67% 3 6% 14% 11%	2561
	21st to end	Day	62% 3 6% 16% 11% 2	2791
February	1st to 10th	Day	63% 4 10% 12% 9% 2	1539
	11th to 20th	Day	70% 4 7% 12% 6%	1715
	21st to end	Day	63% 4 7% 14% 11% 2	1570
March	1st to 10th	Day	65% 3 6% 14% 9% 2	3118
	11th to 20th	Day	77% 2 6% 9% 5%	3411
	21st to end	Day	73% 2 6% 13% 5%	3929
April	1st to 10th	Day	70% 4 6% 12% 7% 1	2280
	11th to 20th	Day	67% 2 7% 14% 9% 1	2441
	21st to end	Day	70% 2 7% 12% 9% 1	2470
May	1st to 10th	Day	77% 1 9% 9% 4	4209
	11th to 20th	Day	70% 2 9% 14% 5%	4570
	21st to end	Day	68% 2 8% 17% 5%	5173
June	1st to 10th	Day	72% 2 8% 13% 4	2850
	11th to 20th	Day	64% 4 10% 17% 5% 1	2850
	21st to end	Day	60% 5% 13% 16% 6% 1	2850
July	1st to 10th	Day	60% 4 12% 18% 6%	4675
	11th to 20th	Day	52% 5% 18% 20% 5%	4638
	21st to end	Day	44% 5% 17% 27% 6%	4662
August	1st to 10th	Day	60% 6% 15% 17% 3	4362
	11th to 20th	Day	46% 3 15% 24% 11% 1	4085
	21st to end	Day	49% 4 16% 26% 5%	4532
September	1st to 10th	Day	52% 3 11% 24% 8% 2	2282
	11th to 20th	Day	60% 3 11% 19% 7% 1	2090
	21st to end	Day	53% 3 16% 20% 7% 1	2090
October	1st to 10th	Day	60% 6% 9% 18% 6%	3387
	11th to 20th	Day	61% 4 11% 18% 6%	2910
	21st to end	Day	66% 3 9% 12% 9% 1	3092
November	1st to 10th	Day	66% 4 13% 11% 6%	1750
	11th to 20th	Day	64% 3 10% 15% 7% 1	1595
	21st to end	Day	67% 4 11% 11% 8% 1	1400
December	1st to 10th	Day	52% 3 9% 24% 12%	2290
	11th to 20th	Day	62% 5% 15% 11% 7%	2627
	21st to end	Day	70% 2 9% 14% 4	3498
January	1st to 10th	Night	67% 4 7% 13% 7% 1	4889
	11th to 20th	Night	63% 4 7% 16% 10% 1	4612
	21st to end	Night	59% 4 11% 13% 11% 2	4899
February	1st to 10th	Night	62% 4 9% 14% 10% 1	2641
	11th to 20th	Night	68% 4 8% 14% 7% 1	2360
	21st to end	Night	59% 4 7% 18% 11% 1	1884
March	1st to 10th	Night	65% 3 9% 16% 7% 1	3735
	11th to 20th	Night	74% 2 7% 11% 5% 1	3549
	21st to end	Night	67% 2 8% 17% 7% 1	3699
April	1st to 10th	Night	66% 4 6% 13% 10% 1	1900
	11th to 20th	Night	65% 2 7% 17% 7% 1	1688
	21st to end	Night	66% 3 7% 14% 10% 1	1710
May	1st to 10th	Night	76% 2 9% 9% 3	2759
	11th to 20th	Night	74% 2 7% 13% 5%	2338
	21st to end	Night	68% 3 9% 14% 6% 1	2413
June	1st to 10th	Night	67% 4 12% 12% 5%	1330
	11th to 20th	Night	62% 3 13% 16% 6%	1330
	21st to end	Night	57% 5% 13% 17% 6% 1	1330
July	1st to 10th	Night	58% 4 15% 19% 3	2186
	11th to 20th	Night	54% 7% 15% 16% 7%	2204
	21st to end	Night	45% 5% 20% 26% 4	2663
August	1st to 10th	Night	49% 8% 18% 21% 4	2553
	11th to 20th	Night	45% 4 15% 26% 9% 1	2818
	21st to end	Night	50% 4 14% 27% 5%	3134
September	1st to 10th	Night	51% 3 13% 23% 10% 1	1895
	11th to 20th	Night	54% 3 13% 18% 11% 1	2090
	21st to end	Night	54% 4 13% 20% 8%	2090
October	1st to 10th	Night	60% 4 12% 17% 6% 1	3576
	11th to 20th	Night	56% 3 16% 16% 8%	4015
	21st to end	Night	57% 3 11% 17% 10% 1	4467
November	1st to 10th	Night	64% 5% 12% 13% 6%	2546
	11th to 20th	Night	65% 3 11% 15% 5%	2805
	21st to end	Night	65% 4 10% 13% 8% 1	3000
December	1st to 10th	Night	54% 4 8% 23% 12%	4900
	11th to 20th	Night	65% 5% 14% 11% 5%	5583
	21st to end	Night	65% 4 10% 16% 5%	7480

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% 14% 35% 14% 6% 2	400
	11th to 20th	Day	28% 19% 32% 16% 5% 1	400
	21st to end	Day	26% 17% 34% 17% 4	410
February	1st to 10th	Day	29% 18% 33% 14% 4 1	370
	11th to 20th	Day	29% 22% 33% 14% 2 1	366
	21st to end	Day	21% 18% 39% 16% 4 2	307
March	1st to 10th	Day	28% 19% 34% 14% 4 2	370
	11th to 20th	Day	32% 15% 25% 18% 8% 3	370
	21st to end	Day	40% 12% 30% 12% 4 2	437
April	1st to 10th	Day	32% 16% 30% 18% 4	460
	11th to 20th	Day	38% 10% 29% 18% 5% 1	639
	21st to end	Day	38% 13% 29% 17% 3 1	650
May	1st to 10th	Day	43% 10% 20% 18% 7% 2	653
	11th to 20th	Day	34% 9% 27% 18% 9% 3	674
	21st to end	Day	37% 9% 31% 16% 5% 1	735
June	1st to 10th	Day	47% 6% 24% 14% 8% 1	680
	11th to 20th	Day	38% 9% 25% 18% 8% 2	680
	21st to end	Day	29% 9% 30% 17% 11% 4	680
July	1st to 10th	Day	29% 10% 29% 21% 8% 3	670
	11th to 20th	Day	23% 8% 31% 28% 9% 1	668
	21st to end	Day	28% 6% 26% 31% 8% 1	740
August	1st to 10th	Day	24% 8% 28% 26% 11% 2	675
	11th to 20th	Day	25% 8% 25% 25% 15% 2	667
	21st to end	Day	26% 5% 30% 25% 10% 3	748
September	1st to 10th	Day	25% 11% 29% 25% 8% 2	503
	11th to 20th	Day	37% 10% 27% 22% 4 1	390
	21st to end	Day	25% 16% 34% 18% 6% 2	390
October	1st to 10th	Day	27% 13% 33% 18% 6% 2	390
	11th to 20th	Day	36% 9% 27% 17% 7% 3	390
	21st to end	Day	36% 17% 25% 14% 6% 3	429
November	1st to 10th	Day	35% 14% 31% 16% 3	395
	11th to 20th	Day	37% 17% 24% 16% 6% 1	400
	21st to end	Day	40% 20% 28% 12% 1	400
December	1st to 10th	Day	30% 13% 32% 19% 7% 1	400
	11th to 20th	Day	32% 21% 30% 13% 4	400
	21st to end	Day	26% 15% 34% 18% 6% 1	440
January	1st to 10th	Night	30% 17% 31% 16% 5% 1	900
	11th to 20th	Night	28% 18% 33% 17% 4	900
	21st to end	Night	26% 16% 39% 16% 3	941
February	1st to 10th	Night	27% 20% 34% 16% 3	850
	11th to 20th	Night	33% 18% 33% 14% 2	811
	21st to end	Night	23% 14% 40% 15% 6% 1	628
March	1st to 10th	Night	30% 17% 34% 14% 4 1	760
	11th to 20th	Night	25% 15% 29% 22% 9%	613
	21st to end	Night	36% 14% 28% 19% 4	627
April	1st to 10th	Night	35% 15% 27% 17% 4 1	570
	11th to 20th	Night	33% 9% 31% 21% 5% 1	564
	21st to end	Night	37% 12% 28% 19% 4	570
May	1st to 10th	Night	39% 7% 23% 24% 6% 1	570
	11th to 20th	Night	38% 6% 31% 17% 8%	562
	21st to end	Night	35% 9% 31% 19% 5%	616
June	1st to 10th	Night	39% 8% 28% 21% 4	570
	11th to 20th	Night	38% 8% 25% 25% 5%	570
	21st to end	Night	30% 8% 32% 22% 9% 1	570
July	1st to 10th	Night	26% 7% 34% 25% 7%	556
	11th to 20th	Night	22% 7% 35% 30% 5%	552
	21st to end	Night	27% 6% 32% 31% 4	623
August	1st to 10th	Night	24% 5% 29% 32% 10%	568
	11th to 20th	Night	23% 8% 30% 29% 10%	562
	21st to end	Night	25% 7% 26% 32% 9% 1	627
September	1st to 10th	Night	27% 6% 32% 28% 6%	570
	11th to 20th	Night	29% 10% 30% 26% 4	570
	21st to end	Night	32% 10% 32% 22% 4	570
October	1st to 10th	Night	33% 10% 31% 21% 5% 1	600
	11th to 20th	Night	34% 10% 27% 22% 7% 1	804
	21st to end	Night	36% 13% 26% 19% 5% 1	946
November	1st to 10th	Night	34% 14% 32% 16% 4	880
	11th to 20th	Night	36% 13% 29% 16% 6% 1	900
	21st to end	Night	34% 18% 31% 15% 2	900
December	1st to 10th	Night	31% 14% 31% 18% 6%	900
	11th to 20th	Night	34% 18% 30% 14% 4	900
	21st to end	Night	25% 17% 32% 21% 5% 1	990

Page generated at 2025-01-19 04:06:47. Version ac07

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