Drummond Island Airport
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Drummond Island Airport clear sky chart from 2005-11-06 to 2025-09-19.

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	14% 10% 4 8% 5% 4 4 3 5% 7% 36%	126088
	Night	15% 12% 4 10% 5% 5% 4 3 5% 6% 31%	123153
Transparency	Day	47% 6% 12% 22% 11% 1	129560
	Night	53% 6% 11% 19% 10% 1	127755
Seeing	Day	17% 6% 20% 33% 20% 5%	16691
	Night	21% 6% 25% 31% 15% 2	26007

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	11% 16% 6% 16% 6% 5% 5% 4 5% 6% 20%	3412
	11th to 20th	Day	11% 16% 6% 13% 10% 7% 5% 4 5% 6% 16%	3589
	21st to end	Day	12% 20% 8% 15% 6% 6% 4 3 5% 5% 16%	3982
February	1st to 10th	Day	9% 14% 6% 13% 8% 6% 5% 4 5% 6% 25%	1773
	11th to 20th	Day	11% 13% 5% 10% 7% 5% 5% 3 6% 6% 29%	1828
	21st to end	Day	11% 13% 5% 12% 7% 4 6% 4 5% 6% 28%	1650
March	1st to 10th	Day	13% 11% 4 8% 4 4 4 4 6% 7% 36%	4511
	11th to 20th	Day	11% 12% 4 10% 6% 5% 5% 4 5% 7% 31%	4980
	21st to end	Day	17% 10% 6% 9% 5% 5% 4 3 5% 6% 31%	5465
April	1st to 10th	Day	17% 11% 4 7% 5% 4 4 4 6% 6% 33%	2390
	11th to 20th	Day	18% 9% 2 7% 4 4 4 4 5% 5% 37%	2570
	21st to end	Day	17% 9% 3 7% 5% 4 4 4 6% 7% 35%	2600
May	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 4 6% 43%	5888
	11th to 20th	Day	12% 10% 3 7% 4 3 4 3 5% 6% 43%	5945
	21st to end	Day	13% 8% 3 7% 4 4 4 3 4 7% 43%	7439
June	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 7% 8% 43%	3000
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 4 6% 8% 41%	3000
	21st to end	Day	15% 7% 3 5% 4 4 4 4 6% 7% 43%	3000
July	1st to 10th	Day	14% 6% 2 5% 4 3 3 4 5% 8% 44%	6819
	11th to 20th	Day	9% 5% 2 5% 4 3 4 3 5% 9% 52%	6410
	21st to end	Day	9% 5% 2 5% 3 3 4 3 5% 9% 52%	6926
August	1st to 10th	Day	8% 5% 2 7% 4 4 5% 4 6% 9% 47%	4283
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 6% 7% 46%	4174
	21st to end	Day	12% 6% 2 5% 4 4 4 3 6% 7% 47%	4675
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 3 5% 8% 44%	2333
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 6% 8% 46%	2178
	21st to end	Day	16% 8% 2 7% 4 4 4 4 5% 8% 39%	2090
October	1st to 10th	Day	19% 8% 3 5% 4 3 4 3 5% 7% 38%	2024
	11th to 20th	Day	20% 10% 4 9% 6% 6% 5% 4 6% 8% 23%	1859
	21st to end	Day	22% 13% 4 7% 5% 4 5% 4 5% 7% 23%	1881
November	1st to 10th	Day	18% 13% 4 9% 5% 5% 4 3 6% 7% 26%	1762
	11th to 20th	Day	14% 18% 5% 11% 7% 6% 4 5% 5% 6% 20%	1675
	21st to end	Day	20% 20% 5% 12% 7% 5% 4 3 4 5% 16%	1600
December	1st to 10th	Day	14% 21% 6% 11% 6% 6% 5% 4 5% 5% 17%	2280
	11th to 20th	Day	15% 18% 6% 16% 8% 5% 4 3 5% 5% 14%	2621
	21st to end	Day	25% 22% 7% 13% 5% 3 5% 2 4 3 12%	3476
January	1st to 10th	Night	12% 12% 6% 16% 6% 5% 5% 4 5% 5% 22%	6550
	11th to 20th	Night	12% 17% 6% 15% 8% 7% 5% 4 5% 5% 16%	6316
	21st to end	Night	10% 16% 7% 17% 7% 7% 6% 4 5% 5% 16%	6996
February	1st to 10th	Night	10% 16% 5% 11% 8% 6% 6% 5% 6% 5% 23%	2625
	11th to 20th	Night	11% 13% 5% 11% 8% 5% 5% 4 5% 6% 26%	2457
	21st to end	Night	12% 15% 5% 10% 6% 5% 5% 4 5% 6% 26%	1980
March	1st to 10th	Night	16% 11% 3 7% 4 4 4 3 4 7% 37%	5328
	11th to 20th	Night	12% 12% 4 9% 6% 4 4 4 5% 7% 33%	4979
	21st to end	Night	19% 10% 6% 8% 4 4 4 3 4 6% 33%	5467
April	1st to 10th	Night	19% 11% 3 8% 5% 4 5% 3 5% 6% 31%	2025
	11th to 20th	Night	20% 8% 3 6% 4 4 4 3 6% 5% 38%	1778
	21st to end	Night	19% 9% 3 7% 4 4 4 3 6% 5% 35%	1800
May	1st to 10th	Night	16% 7% 3 7% 5% 5% 4 3 5% 5% 39%	4053
	11th to 20th	Night	14% 10% 3 9% 5% 4 4 3 4 5% 38%	3950
	21st to end	Night	15% 10% 3 8% 4 4 4 3 4 6% 39%	3456
June	1st to 10th	Night	13% 6% 3 4 5% 4 4 4 6% 8% 42%	1400
	11th to 20th	Night	11% 6% 3 6% 5% 5% 4 5% 5% 8% 42%	1400
	21st to end	Night	14% 7% 2 7% 5% 4 3 4 5% 7% 42%	1400
July	1st to 10th	Night	14% 6% 2 5% 4 4 5% 4 6% 8% 43%	3160
	11th to 20th	Night	9% 4 2 6% 4 4 4 3 5% 8% 52%	3566
	21st to end	Night	9% 5% 2 4 3 3 4 3 6% 9% 53%	3948
August	1st to 10th	Night	8% 6% 2 7% 5% 5% 5% 4 6% 7% 45%	2865
	11th to 20th	Night	13% 5% 2 6% 3 4 4 3 5% 9% 45%	2875
	21st to end	Night	11% 5% 2 6% 4 4 4 4 4 6% 52%	3223
September	1st to 10th	Night	13% 6% 2 6% 5% 4 5% 4 5% 6% 45%	2043
	11th to 20th	Night	14% 6% 2 6% 3 3 4 3 5% 8% 47%	2179
	21st to end	Night	18% 9% 3 6% 4 4 3 4 5% 6% 37%	2090
October	1st to 10th	Night	19% 8% 2 7% 4 3 3 3 5% 7% 37%	2156
	11th to 20th	Night	22% 11% 3 9% 4 4 4 4 5% 6% 28%	2318
	21st to end	Night	27% 13% 3 8% 4 4 4 3 4 5% 24%	2717
November	1st to 10th	Night	17% 13% 4 8% 6% 4 4 3 6% 7% 29%	2536
	11th to 20th	Night	19% 16% 5% 10% 6% 6% 4 3 5% 5% 21%	2724
	21st to end	Night	20% 20% 6% 11% 5% 4 4 3 4 5% 19%	2800
December	1st to 10th	Night	14% 19% 6% 12% 6% 4 5% 4 5% 6% 19%	4905
	11th to 20th	Night	16% 18% 5% 14% 6% 5% 5% 3 4 4 19%	5586
	21st to end	Night	25% 23% 8% 11% 4 4 4 2 3 3 13%	7502

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	63% 9% 9% 11% 5% 2	3413
	11th to 20th	Day	67% 8% 7% 11% 7%	3620
	21st to end	Day	69% 7% 6% 10% 7%	3982
February	1st to 10th	Day	59% 7% 14% 14% 5%	1773
	11th to 20th	Day	56% 6% 10% 17% 10% 1	1837
	21st to end	Day	56% 6% 10% 18% 9%	1650
March	1st to 10th	Day	47% 7% 13% 14% 16% 3	4513
	11th to 20th	Day	52% 5% 13% 19% 9% 1	4980
	21st to end	Day	53% 7% 10% 17% 11% 1	5465
April	1st to 10th	Day	51% 7% 10% 19% 12% 1	2390
	11th to 20th	Day	47% 6% 9% 22% 14% 2	2570
	21st to end	Day	47% 5% 10% 17% 16% 4	2600
May	1st to 10th	Day	42% 6% 8% 24% 18% 3	5897
	11th to 20th	Day	43% 7% 8% 22% 18% 2	5949
	21st to end	Day	42% 6% 11% 21% 16% 4	7439
June	1st to 10th	Day	38% 7% 12% 27% 14% 2	3000
	11th to 20th	Day	39% 7% 12% 25% 15% 1	3000
	21st to end	Day	41% 6% 12% 26% 13% 2	3000
July	1st to 10th	Day	38% 5% 14% 29% 14% 1	6746
	11th to 20th	Day	30% 5% 16% 39% 10% 1	6333
	21st to end	Day	30% 5% 13% 36% 16%	6728
August	1st to 10th	Day	34% 6% 14% 33% 12% 1	4268
	11th to 20th	Day	37% 6% 15% 29% 12% 1	4213
	21st to end	Day	37% 6% 16% 31% 9% 1	4674
September	1st to 10th	Day	39% 8% 14% 27% 11% 1	2333
	11th to 20th	Day	36% 6% 16% 32% 9%	2178
	21st to end	Day	44% 7% 15% 24% 9%	2090
October	1st to 10th	Day	34% 5% 13% 30% 17%	3362
	11th to 20th	Day	50% 7% 11% 27% 5%	3062
	21st to end	Day	58% 6% 12% 17% 7%	3081
November	1st to 10th	Day	58% 6% 11% 18% 6%	1762
	11th to 20th	Day	64% 7% 12% 12% 5%	1675
	21st to end	Day	72% 7% 9% 9% 2	1600
December	1st to 10th	Day	69% 8% 11% 9% 4	2280
	11th to 20th	Day	69% 8% 9% 10% 4	2621
	21st to end	Day	78% 6% 5% 8% 4	3476
January	1st to 10th	Night	63% 8% 9% 11% 7% 2	6550
	11th to 20th	Night	69% 8% 9% 9% 5%	6360
	21st to end	Night	67% 8% 8% 11% 5% 1	6996
February	1st to 10th	Night	60% 7% 11% 15% 7%	2625
	11th to 20th	Night	57% 8% 11% 17% 6% 1	2459
	21st to end	Night	58% 6% 11% 16% 8%	1980
March	1st to 10th	Night	48% 5% 15% 16% 15% 2	5339
	11th to 20th	Night	49% 6% 17% 18% 8% 1	4980
	21st to end	Night	52% 7% 9% 18% 12% 2	5467
April	1st to 10th	Night	53% 5% 10% 18% 15%	2025
	11th to 20th	Night	47% 5% 7% 24% 15% 2	1778
	21st to end	Night	50% 6% 8% 17% 16% 3	1800
May	1st to 10th	Night	45% 5% 8% 22% 15% 4	4060
	11th to 20th	Night	48% 5% 6% 24% 15% 2	3951
	21st to end	Night	47% 6% 8% 20% 13% 6%	3457
June	1st to 10th	Night	40% 6% 10% 25% 19% 2	1400
	11th to 20th	Night	40% 7% 11% 24% 16% 3	1400
	21st to end	Night	41% 6% 12% 26% 13% 2	1400
July	1st to 10th	Night	39% 5% 13% 27% 15% 2	3125
	11th to 20th	Night	32% 5% 14% 34% 15%	3520
	21st to end	Night	29% 5% 13% 35% 17% 1	3832
August	1st to 10th	Night	37% 6% 12% 31% 13% 1	2856
	11th to 20th	Night	36% 8% 14% 29% 12% 1	2902
	21st to end	Night	34% 5% 13% 34% 14% 1	3223
September	1st to 10th	Night	39% 7% 13% 26% 13% 2	2043
	11th to 20th	Night	36% 6% 16% 33% 9%	2179
	21st to end	Night	47% 8% 14% 22% 9%	2090
October	1st to 10th	Night	41% 5% 13% 28% 12%	3598
	11th to 20th	Night	44% 6% 17% 23% 10%	3841
	21st to end	Night	58% 6% 16% 17% 4	4467
November	1st to 10th	Night	54% 7% 12% 19% 8%	2536
	11th to 20th	Night	65% 8% 10% 11% 6%	2724
	21st to end	Night	69% 8% 10% 10% 3	2800
December	1st to 10th	Night	66% 7% 13% 12% 2	4905
	11th to 20th	Night	66% 7% 9% 10% 7% 1	5585
	21st to end	Night	78% 4 6% 8% 4	7502

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	20% 10% 25% 29% 14% 2	256
	11th to 20th	Day	22% 5% 22% 36% 12% 2	300
	21st to end	Day	22% 7% 20% 34% 15% 2	330
February	1st to 10th	Day	17% 10% 32% 30% 10% 1	300
	11th to 20th	Day	17% 5% 33% 34% 10% 1	300
	21st to end	Day	16% 9% 36% 29% 10%	249
March	1st to 10th	Day	18% 8% 28% 33% 12% 2	307
	11th to 20th	Day	18% 8% 28% 31% 14% 2	500
	21st to end	Day	23% 7% 21% 32% 15% 2	550
April	1st to 10th	Day	20% 6% 22% 35% 15% 2	500
	11th to 20th	Day	22% 6% 25% 28% 15% 3	492
	21st to end	Day	21% 5% 23% 32% 15% 3	500
May	1st to 10th	Day	20% 3 20% 38% 17% 3	503
	11th to 20th	Day	16% 3 17% 36% 23% 4	559
	21st to end	Day	13% 3 17% 38% 23% 5%	789
June	1st to 10th	Day	12% 4 15% 42% 22% 4	730
	11th to 20th	Day	15% 4 18% 37% 21% 5%	730
	21st to end	Day	19% 5% 17% 34% 22% 4	730
July	1st to 10th	Day	10% 1 16% 39% 28% 6%	721
	11th to 20th	Day	11% 3 16% 35% 27% 7%	718
	21st to end	Day	11% 3 14% 36% 28% 8%	794
August	1st to 10th	Day	12% 2 12% 33% 32% 9%	563
	11th to 20th	Day	13% 3 9% 29% 34% 12%	519
	21st to end	Day	13% 5% 14% 34% 26% 8%	583
September	1st to 10th	Day	15% 6% 16% 28% 27% 9%	530
	11th to 20th	Day	14% 3 12% 29% 31% 11%	522
	21st to end	Day	21% 4 16% 27% 26% 6%	490
October	1st to 10th	Day	21% 8% 22% 25% 19% 5%	490
	11th to 20th	Day	21% 8% 27% 31% 12% 1	449
	21st to end	Day	26% 9% 25% 29% 10% 2	330
November	1st to 10th	Day	21% 13% 24% 29% 13% 1	300
	11th to 20th	Day	18% 14% 33% 27% 7%	237
	21st to end	Day	26% 11% 33% 26% 5%	200
December	1st to 10th	Day	25% 13% 26% 24% 12% 2	200
	11th to 20th	Day	24% 17% 30% 23% 6% 2	200
	21st to end	Day	29% 10% 37% 19% 5%	220
January	1st to 10th	Night	21% 9% 31% 30% 9% 1	1000
	11th to 20th	Night	24% 7% 28% 30% 11% 1	1000
	21st to end	Night	20% 6% 27% 35% 12% 1	1100
February	1st to 10th	Night	17% 7% 35% 31% 10% 1	825
	11th to 20th	Night	18% 5% 33% 32% 11% 1	783
	21st to end	Night	19% 5% 32% 34% 9% 1	660
March	1st to 10th	Night	18% 6% 31% 36% 9% 1	800
	11th to 20th	Night	19% 6% 29% 37% 8% 1	800
	21st to end	Night	19% 4 28% 37% 12% 1	880
April	1st to 10th	Night	25% 4 26% 34% 10% 1	625
	11th to 20th	Night	23% 5% 19% 38% 14% 2	593
	21st to end	Night	23% 4 24% 32% 16% 2	600
May	1st to 10th	Night	15% 4 22% 41% 17% 2	600
	11th to 20th	Night	19% 3 18% 36% 23% 2	591
	21st to end	Night	14% 3 21% 35% 24% 2	443
June	1st to 10th	Night	14% 4 19% 39% 24% 1	400
	11th to 20th	Night	12% 4 18% 42% 21% 3	400
	21st to end	Night	18% 3 17% 36% 24% 3	400
July	1st to 10th	Night	11% 1 15% 42% 29% 1	390
	11th to 20th	Night	8% 3 14% 38% 29% 7%	556
	21st to end	Night	11% 2 16% 37% 28% 5%	657
August	1st to 10th	Night	11% 1 13% 41% 29% 5%	597
	11th to 20th	Night	12% 1 16% 30% 32% 9%	594
	21st to end	Night	15% 2 14% 37% 28% 3	660
September	1st to 10th	Night	13% 6% 18% 30% 25% 7%	669
	11th to 20th	Night	15% 3 17% 32% 25% 8%	793
	21st to end	Night	22% 4 19% 26% 23% 6%	760
October	1st to 10th	Night	22% 7% 22% 28% 19% 3	760
	11th to 20th	Night	28% 8% 25% 28% 11% 1	760
	21st to end	Night	35% 8% 25% 24% 7% 1	836
November	1st to 10th	Night	23% 10% 29% 26% 11% 1	780
	11th to 20th	Night	26% 12% 30% 23% 8% 1	800
	21st to end	Night	29% 14% 29% 23% 5%	800
December	1st to 10th	Night	26% 12% 32% 23% 7%	995
	11th to 20th	Night	28% 11% 34% 19% 7% 1	1000
	21st to end	Night	33% 8% 28% 22% 8% 1	1100

Page generated at 2025-09-16 01:49:15. Version ac07

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