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 2024-01-22.

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	23% 13% 5% 8% 6% 3 5% 4 5% 6% 22%	74664
	Night	22% 13% 5% 8% 6% 3 4 4 5% 6% 24%	75278
Transparency	Day	64% 3 10% 16% 7% 1	74355
	Night	62% 4 10% 16% 7% 1	75122
Seeing	Day	32% 12% 29% 19% 7% 2	16970
	Night	31% 12% 31% 20% 5%	23829

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	17% 14% 8% 9% 9% 4 7% 5% 4 5% 17%	1326
	11th to 20th	Day	18% 13% 7% 11% 7% 4 5% 4 5% 7% 20%	1496
	21st to end	Day	16% 14% 5% 11% 9% 5% 6% 6% 5% 6% 18%	1593
February	1st to 10th	Day	16% 13% 5% 9% 9% 4 6% 5% 5% 6% 23%	1458
	11th to 20th	Day	18% 17% 6% 11% 7% 3 6% 5% 5% 4 17%	1616
	21st to end	Day	14% 14% 6% 10% 9% 5% 6% 5% 3 5% 23%	1480
March	1st to 10th	Day	18% 15% 6% 11% 6% 4 5% 4 5% 5% 22%	1796
	11th to 20th	Day	18% 16% 6% 10% 8% 4 5% 4 5% 5% 19%	1939
	21st to end	Day	25% 21% 8% 9% 6% 3 3 3 3 4 16%	2236
April	1st to 10th	Day	18% 17% 7% 11% 7% 4 5% 5% 4 5% 17%	2160
	11th to 20th	Day	22% 19% 6% 7% 5% 3 4 3 4 5% 22%	2311
	21st to end	Day	22% 22% 6% 7% 5% 3 4 4 3 4 19%	2340
May	1st to 10th	Day	27% 20% 5% 7% 5% 2 3 3 4 4 19%	2387
	11th to 20th	Day	26% 16% 4 6% 6% 3 4 5% 4 5% 21%	2568
	21st to end	Day	27% 13% 4 7% 6% 3 5% 4 5% 6% 21%	2918
June	1st to 10th	Day	33% 15% 5% 5% 5% 3 4 4 4 5% 17%	2700
	11th to 20th	Day	30% 11% 3 6% 5% 3 4 4 5% 6% 21%	2700
	21st to end	Day	24% 8% 4 7% 6% 3 5% 5% 6% 6% 24%	2700
July	1st to 10th	Day	22% 9% 3 6% 6% 4 5% 5% 5% 7% 28%	2700
	11th to 20th	Day	19% 8% 3 7% 6% 4 6% 5% 6% 8% 29%	2683
	21st to end	Day	22% 10% 4 8% 6% 4 5% 5% 6% 8% 22%	2745
August	1st to 10th	Day	22% 9% 3 7% 7% 4 6% 6% 6% 7% 23%	2485
	11th to 20th	Day	24% 8% 3 7% 6% 3 5% 5% 6% 7% 25%	2305
	21st to end	Day	21% 8% 4 6% 5% 4 5% 6% 6% 8% 28%	2574
September	1st to 10th	Day	22% 9% 3 6% 5% 3 4 4 6% 6% 32%	2163
	11th to 20th	Day	28% 8% 3 7% 6% 3 4 3 4 6% 28%	1980
	21st to end	Day	19% 10% 3 6% 6% 3 4 4 5% 8% 33%	1980
October	1st to 10th	Day	22% 11% 3 6% 6% 2 5% 4 4 6% 30%	1926
	11th to 20th	Day	25% 14% 4 7% 6% 3 4 4 5% 5% 22%	1670
	21st to end	Day	25% 17% 4 7% 6% 4 5% 4 4 4 20%	1782
November	1st to 10th	Day	24% 14% 4 8% 6% 4 6% 4 4 5% 21%	1661
	11th to 20th	Day	20% 15% 5% 9% 5% 3 5% 3 3 5% 26%	1516
	21st to end	Day	23% 16% 7% 8% 5% 3 3 3 3 5% 23%	1330
December	1st to 10th	Day	20% 16% 6% 10% 6% 4 5% 5% 4 5% 18%	1330
	11th to 20th	Day	26% 14% 6% 8% 6% 3 4 4 4 6% 19%	1668
	21st to end	Day	24% 17% 5% 10% 7% 3 5% 4 4 5% 16%	2442
January	1st to 10th	Night	17% 15% 6% 11% 7% 4 5% 4 5% 5% 21%	2839
	11th to 20th	Night	15% 15% 6% 11% 8% 4 5% 5% 5% 6% 20%	2679
	21st to end	Night	17% 14% 6% 10% 7% 4 4 4 4 5% 25%	2797
February	1st to 10th	Night	16% 12% 5% 10% 9% 4 6% 4 5% 5% 23%	2502
	11th to 20th	Night	21% 15% 6% 9% 7% 3 5% 6% 5% 4 20%	2228
	21st to end	Night	13% 13% 6% 9% 7% 5% 6% 4 5% 6% 27%	1776
March	1st to 10th	Night	19% 16% 6% 8% 6% 3 4 5% 4 5% 26%	2151
	11th to 20th	Night	14% 14% 8% 10% 8% 4 5% 5% 5% 6% 20%	2020
	21st to end	Night	25% 16% 5% 8% 7% 4 3 3 3 4 21%	2116
April	1st to 10th	Night	24% 15% 8% 7% 6% 3 3 4 4 5% 23%	1800
	11th to 20th	Night	23% 16% 6% 8% 5% 2 3 4 4 5% 24%	1598
	21st to end	Night	27% 15% 4 7% 6% 2 4 3 3 6% 23%	1620
May	1st to 10th	Night	29% 17% 5% 7% 5% 3 4 4 5% 6% 17%	1571
	11th to 20th	Night	26% 14% 5% 6% 5% 2 4 3 5% 6% 25%	1325
	21st to end	Night	29% 12% 3 6% 6% 2 5% 4 6% 6% 22%	1356
June	1st to 10th	Night	32% 12% 4 6% 5% 3 4 5% 5% 6% 19%	1260
	11th to 20th	Night	32% 9% 4 6% 6% 2 3 4 5% 6% 22%	1260
	21st to end	Night	24% 9% 3 7% 6% 4 5% 5% 6% 7% 26%	1260
July	1st to 10th	Night	22% 8% 3 6% 6% 2 5% 5% 6% 7% 33%	1260
	11th to 20th	Night	16% 8% 3 5% 7% 4 6% 5% 5% 8% 33%	1273
	21st to end	Night	22% 9% 4 7% 4 4 5% 5% 6% 6% 29%	1573
August	1st to 10th	Night	20% 7% 4 6% 7% 3 6% 5% 6% 8% 27%	1454
	11th to 20th	Night	19% 9% 4 6% 6% 4 4 5% 7% 6% 31%	1595
	21st to end	Night	21% 7% 3 6% 5% 4 3 5% 4 8% 34%	1782
September	1st to 10th	Night	21% 8% 3 6% 5% 3 4 3 4 7% 36%	1794
	11th to 20th	Night	24% 9% 3 5% 5% 3 4 4 4 6% 32%	1980
	21st to end	Night	21% 11% 3 7% 4 3 5% 4 6% 6% 31%	1980
October	1st to 10th	Night	22% 12% 3 6% 5% 3 4 4 5% 6% 30%	2034
	11th to 20th	Night	25% 13% 3 6% 5% 3 4 5% 5% 6% 27%	2286
	21st to end	Night	29% 14% 3 6% 5% 3 4 3 4 5% 24%	2574
November	1st to 10th	Night	21% 16% 4 7% 6% 4 5% 5% 5% 7% 21%	2415
	11th to 20th	Night	23% 13% 5% 7% 6% 3 5% 5% 5% 6% 23%	2664
	21st to end	Night	19% 16% 6% 8% 5% 3 6% 4 5% 5% 22%	2850
December	1st to 10th	Night	21% 14% 6% 10% 8% 4 4 4 4 5% 19%	2850
	11th to 20th	Night	29% 17% 6% 10% 6% 3 3 3 3 4 15%	3531
	21st to end	Night	22% 15% 6% 9% 6% 3 4 4 5% 5% 20%	5225

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% 5% 9% 12% 6% 1	1327
	11th to 20th	Day	65% 4 9% 14% 8%	1496
	21st to end	Day	66% 4 7% 13% 9%	1593
February	1st to 10th	Day	61% 4 10% 13% 9% 2	1458
	11th to 20th	Day	69% 4 7% 13% 6%	1623
	21st to end	Day	64% 4 7% 13% 11% 2	1480
March	1st to 10th	Day	64% 4 7% 14% 11% 1	1800
	11th to 20th	Day	68% 4 7% 12% 8% 1	1939
	21st to end	Day	74% 3 6% 11% 6% 1	2236
April	1st to 10th	Day	69% 4 7% 13% 7% 1	2160
	11th to 20th	Day	67% 2 7% 14% 9% 1	2311
	21st to end	Day	70% 2 7% 12% 8% 1	2340
May	1st to 10th	Day	71% 2 8% 12% 6%	2387
	11th to 20th	Day	67% 3 8% 14% 8%	2572
	21st to end	Day	64% 4 11% 16% 5%	2918
June	1st to 10th	Day	71% 3 9% 14% 4	2700
	11th to 20th	Day	65% 4 9% 17% 5% 1	2700
	21st to end	Day	60% 5% 13% 16% 7% 1	2700
July	1st to 10th	Day	55% 3 14% 21% 6%	2625
	11th to 20th	Day	54% 3 15% 21% 6% 1	2601
	21st to end	Day	60% 4 14% 16% 5%	2592
August	1st to 10th	Day	59% 3 14% 20% 4	2471
	11th to 20th	Day	58% 3 15% 17% 6%	2305
	21st to end	Day	54% 5% 17% 20% 4	2574
September	1st to 10th	Day	52% 3 11% 24% 8% 2	2163
	11th to 20th	Day	60% 3 10% 19% 7% 1	1980
	21st to end	Day	51% 3 17% 21% 7% 1	1980
October	1st to 10th	Day	56% 3 10% 21% 9% 1	1926
	11th to 20th	Day	66% 3 10% 17% 5%	1670
	21st to end	Day	69% 3 8% 14% 6% 1	1782
November	1st to 10th	Day	65% 4 13% 12% 6%	1661
	11th to 20th	Day	63% 3 10% 15% 8% 1	1516
	21st to end	Day	65% 4 11% 11% 8% 1	1330
December	1st to 10th	Day	69% 3 8% 13% 6%	1330
	11th to 20th	Day	68% 3 11% 12% 6%	1667
	21st to end	Day	72% 2 10% 12% 4	2442
January	1st to 10th	Night	65% 3 8% 14% 8% 1	2839
	11th to 20th	Night	65% 4 9% 13% 8% 1	2679
	21st to end	Night	61% 3 8% 17% 10% 1	2797
February	1st to 10th	Night	61% 4 9% 15% 10% 1	2502
	11th to 20th	Night	67% 4 8% 14% 6% 1	2232
	21st to end	Night	59% 4 7% 18% 11% 1	1776
March	1st to 10th	Night	62% 3 8% 16% 10% 1	2160
	11th to 20th	Night	66% 3 8% 14% 8% 2	2021
	21st to end	Night	70% 2 5% 14% 8% 1	2116
April	1st to 10th	Night	64% 4 6% 14% 11% 1	1800
	11th to 20th	Night	65% 3 7% 17% 8% 1	1598
	21st to end	Night	66% 3 7% 14% 9% 1	1620
May	1st to 10th	Night	68% 3 8% 14% 5% 1	1571
	11th to 20th	Night	63% 3 8% 18% 8% 1	1326
	21st to end	Night	63% 4 12% 13% 7% 1	1357
June	1st to 10th	Night	67% 4 12% 12% 5%	1260
	11th to 20th	Night	63% 3 13% 16% 6%	1260
	21st to end	Night	57% 5% 13% 17% 6% 1	1260
July	1st to 10th	Night	51% 5% 17% 22% 6%	1226
	11th to 20th	Night	49% 6% 15% 22% 8%	1231
	21st to end	Night	55% 4 15% 21% 4	1486
August	1st to 10th	Night	54% 5% 19% 17% 5%	1443
	11th to 20th	Night	53% 5% 15% 19% 8%	1595
	21st to end	Night	50% 4 16% 23% 6%	1782
September	1st to 10th	Night	51% 3 14% 23% 10% 1	1794
	11th to 20th	Night	54% 3 13% 19% 11% 1	1980
	21st to end	Night	53% 4 13% 21% 8% 1	1980
October	1st to 10th	Night	55% 3 11% 20% 9% 1	2034
	11th to 20th	Night	59% 3 12% 18% 7%	2286
	21st to end	Night	65% 3 9% 15% 7% 1	2574
November	1st to 10th	Night	63% 5% 12% 13% 7%	2415
	11th to 20th	Night	63% 4 11% 16% 6%	2664
	21st to end	Night	63% 4 11% 13% 8% 1	2850
December	1st to 10th	Night	68% 4 8% 14% 6%	2850
	11th to 20th	Night	75% 3 6% 11% 5%	3533
	21st to end	Night	67% 3 11% 15% 4	5225

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	28% 14% 37% 14% 6% 1	370
	11th to 20th	Day	28% 20% 33% 15% 4	370
	21st to end	Day	28% 16% 35% 18% 3	378
February	1st to 10th	Day	28% 19% 35% 14% 3	340
	11th to 20th	Day	28% 22% 34% 14% 2 1	336
	21st to end	Day	23% 17% 39% 15% 4 2	280
March	1st to 10th	Day	27% 19% 33% 15% 4 1	340
	11th to 20th	Day	32% 16% 25% 17% 7% 3	340
	21st to end	Day	39% 12% 31% 12% 5% 2	400
April	1st to 10th	Day	31% 16% 29% 18% 5%	420
	11th to 20th	Day	38% 10% 29% 18% 4 1	589
	21st to end	Day	38% 13% 28% 18% 3 1	600
May	1st to 10th	Day	43% 11% 21% 17% 6% 2	603
	11th to 20th	Day	33% 10% 28% 17% 9% 3	624
	21st to end	Day	37% 10% 31% 17% 5% 1	680
June	1st to 10th	Day	46% 6% 24% 15% 8% 1	630
	11th to 20th	Day	39% 8% 25% 18% 8% 2	630
	21st to end	Day	28% 8% 30% 17% 11% 5%	630
July	1st to 10th	Day	29% 10% 29% 21% 8% 3	620
	11th to 20th	Day	24% 7% 31% 28% 9% 1	618
	21st to end	Day	29% 6% 24% 31% 8% 1	685
August	1st to 10th	Day	25% 7% 29% 26% 11% 2	625
	11th to 20th	Day	26% 7% 25% 24% 15% 2	617
	21st to end	Day	26% 5% 30% 24% 10% 4	693
September	1st to 10th	Day	26% 10% 28% 24% 8% 2	464
	11th to 20th	Day	38% 9% 26% 21% 4 1	360
	21st to end	Day	25% 16% 34% 18% 6% 1	360
October	1st to 10th	Day	28% 14% 33% 18% 6% 2	360
	11th to 20th	Day	37% 9% 26% 18% 8% 3	360
	21st to end	Day	36% 17% 25% 14% 6% 3	396
November	1st to 10th	Day	36% 13% 30% 18% 3	365
	11th to 20th	Day	35% 17% 25% 17% 6% 1	370
	21st to end	Day	37% 20% 29% 12% 1	370
December	1st to 10th	Day	32% 12% 31% 18% 7% 1	370
	11th to 20th	Day	34% 21% 29% 14% 4	370
	21st to end	Day	26% 16% 35% 18% 4 1	407
January	1st to 10th	Night	29% 17% 32% 16% 5%	850
	11th to 20th	Night	27% 19% 34% 17% 3	850
	21st to end	Night	27% 16% 39% 15% 3	889
February	1st to 10th	Night	26% 21% 36% 15% 2	800
	11th to 20th	Night	32% 19% 33% 14% 2	763
	21st to end	Night	23% 14% 40% 15% 6% 2	592
March	1st to 10th	Night	29% 18% 33% 14% 5% 1	720
	11th to 20th	Night	25% 15% 29% 23% 8%	581
	21st to end	Night	36% 14% 28% 19% 4	594
April	1st to 10th	Night	35% 16% 27% 17% 5% 1	540
	11th to 20th	Night	33% 9% 31% 22% 5% 1	534
	21st to end	Night	38% 11% 28% 18% 4	540
May	1st to 10th	Night	38% 8% 23% 24% 6% 1	540
	11th to 20th	Night	37% 7% 31% 17% 8%	532
	21st to end	Night	35% 8% 32% 19% 6% 1	583
June	1st to 10th	Night	38% 9% 28% 21% 4	540
	11th to 20th	Night	39% 7% 24% 26% 5%	540
	21st to end	Night	29% 7% 32% 22% 9% 1	540
July	1st to 10th	Night	26% 7% 35% 25% 7%	526
	11th to 20th	Night	22% 7% 35% 30% 6%	522
	21st to end	Night	28% 5% 31% 31% 5%	590
August	1st to 10th	Night	25% 5% 28% 32% 10%	538
	11th to 20th	Night	24% 8% 29% 30% 9%	532
	21st to end	Night	26% 6% 26% 32% 9% 1	594
September	1st to 10th	Night	27% 6% 32% 28% 7%	540
	11th to 20th	Night	29% 10% 30% 27% 4	540
	21st to end	Night	31% 9% 33% 22% 3	540
October	1st to 10th	Night	32% 11% 32% 21% 4	567
	11th to 20th	Night	35% 10% 25% 22% 7% 1	756
	21st to end	Night	37% 12% 25% 20% 5% 1	891
November	1st to 10th	Night	34% 14% 31% 16% 4	830
	11th to 20th	Night	34% 13% 29% 17% 6% 1	850
	21st to end	Night	33% 18% 32% 15% 2	850
December	1st to 10th	Night	32% 13% 31% 17% 6%	850
	11th to 20th	Night	36% 17% 30% 14% 4	850
	21st to end	Night	25% 18% 32% 20% 4 1	935

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