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 2024-01-12.

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% 9% 4 7% 6% 3 4 4 5% 7% 36%	74586
	Night	16% 12% 4 8% 6% 3 5% 4 5% 6% 31%	75133
Transparency	Day	48% 7% 12% 21% 11% 1	74284
	Night	54% 7% 11% 18% 9% 1	74970
Seeing	Day	17% 4 19% 33% 21% 5%	14331
	Night	21% 5% 25% 31% 16% 2	23882

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	12% 17% 7% 12% 8% 4 5% 4 5% 6% 19%	1433
	11th to 20th	Day	15% 14% 7% 11% 8% 5% 5% 4 5% 5% 21%	1453
	21st to end	Day	13% 15% 6% 11% 7% 5% 4 4 5% 7% 21%	1584
February	1st to 10th	Day	9% 13% 6% 12% 9% 4 6% 5% 5% 7% 24%	1596
	11th to 20th	Day	12% 12% 5% 9% 8% 4 5% 4 5% 6% 30%	1641
	21st to end	Day	11% 12% 5% 11% 8% 3 5% 5% 5% 5% 29%	1480
March	1st to 10th	Day	11% 11% 4 7% 5% 3 5% 4 4 6% 39%	1803
	11th to 20th	Day	12% 10% 4 7% 7% 3 4 4 4 7% 39%	1980
	21st to end	Day	15% 8% 3 6% 5% 3 5% 4 5% 6% 39%	2178
April	1st to 10th	Day	17% 10% 5% 7% 5% 3 4 4 6% 7% 32%	2150
	11th to 20th	Day	18% 9% 3 6% 5% 3 4 4 5% 6% 37%	2310
	21st to end	Day	18% 8% 4 6% 6% 3 4 5% 5% 7% 35%	2340
May	1st to 10th	Day	15% 7% 2 5% 6% 3 5% 5% 5% 8% 39%	2340
	11th to 20th	Day	17% 6% 3 5% 4 2 4 5% 6% 7% 43%	2329
	21st to end	Day	11% 5% 3 4 5% 3 5% 5% 5% 8% 46%	2907
June	1st to 10th	Day	13% 5% 2 5% 5% 3 4 5% 6% 8% 44%	2700
	11th to 20th	Day	12% 5% 2 5% 5% 3 5% 5% 6% 8% 43%	2700
	21st to end	Day	13% 6% 3 5% 4 2 5% 5% 6% 7% 45%	2700
July	1st to 10th	Day	8% 4 2 4 4 3 4 5% 6% 8% 52%	2700
	11th to 20th	Day	10% 6% 2 4 4 3 5% 5% 5% 9% 48%	2542
	21st to end	Day	10% 5% 2 5% 4 2 4 5% 5% 9% 48%	2743
August	1st to 10th	Day	11% 4 2 5% 5% 3 5% 4 5% 9% 47%	2361
	11th to 20th	Day	10% 4 2 5% 5% 2 5% 4 6% 9% 47%	2303
	21st to end	Day	12% 6% 2 4 5% 2 4 4 5% 8% 47%	2574
September	1st to 10th	Day	14% 6% 2 5% 5% 4 4 4 4 8% 44%	2101
	11th to 20th	Day	14% 5% 3 5% 5% 3 4 4 6% 8% 44%	1980
	21st to end	Day	16% 7% 3 6% 6% 3 4 5% 5% 8% 37%	1980
October	1st to 10th	Day	20% 8% 3 5% 5% 3 4 4 5% 7% 36%	1918
	11th to 20th	Day	22% 9% 4 7% 7% 4 5% 5% 6% 8% 22%	1762
	21st to end	Day	22% 12% 5% 7% 6% 3 5% 5% 5% 7% 23%	1782
November	1st to 10th	Day	18% 13% 4 8% 6% 4 5% 4 5% 8% 25%	1672
	11th to 20th	Day	14% 17% 6% 10% 8% 4 5% 6% 5% 6% 20%	1592
	21st to end	Day	20% 18% 6% 10% 9% 3 4 4 4 4 16%	1520
December	1st to 10th	Day	15% 20% 7% 9% 8% 3 5% 5% 3 6% 19%	1330
	11th to 20th	Day	16% 18% 7% 13% 7% 4 4 4 4 6% 17%	1671
	21st to end	Day	25% 23% 6% 12% 7% 2 3 3 3 3 13%	2431
January	1st to 10th	Night	16% 14% 6% 11% 8% 4 5% 4 5% 6% 20%	2729
	11th to 20th	Night	16% 18% 6% 10% 8% 4 5% 4 4 5% 20%	2547
	21st to end	Night	14% 15% 7% 11% 8% 4 5% 5% 5% 5% 22%	2772
February	1st to 10th	Night	11% 14% 6% 9% 9% 5% 6% 6% 6% 6% 22%	2362
	11th to 20th	Night	12% 12% 5% 9% 9% 4 5% 5% 5% 6% 27%	2205
	21st to end	Night	13% 15% 6% 9% 7% 4 6% 5% 5% 6% 26%	1776
March	1st to 10th	Night	12% 10% 4 7% 7% 3 5% 4 4 7% 38%	2143
	11th to 20th	Night	13% 10% 3 7% 7% 3 4 6% 5% 7% 36%	1979
	21st to end	Night	15% 8% 4 6% 5% 3 6% 5% 5% 7% 38%	2178
April	1st to 10th	Night	19% 10% 3 7% 6% 4 4 4 5% 6% 31%	1823
	11th to 20th	Night	18% 7% 3 6% 4 2 5% 4 6% 6% 38%	1598
	21st to end	Night	19% 8% 3 7% 5% 3 5% 4 5% 6% 36%	1620
May	1st to 10th	Night	13% 6% 4 8% 6% 4 5% 6% 6% 7% 36%	1620
	11th to 20th	Night	16% 7% 3 6% 6% 4 5% 4 5% 6% 37%	1562
	21st to end	Night	12% 6% 2 5% 5% 3 4 4 5% 8% 46%	1366
June	1st to 10th	Night	13% 5% 3 3 6% 3 5% 5% 6% 8% 43%	1260
	11th to 20th	Night	11% 6% 2 5% 5% 3 5% 6% 6% 8% 44%	1260
	21st to end	Night	13% 7% 2 6% 5% 3 4 4 5% 7% 44%	1260
July	1st to 10th	Night	10% 4 2 3 4 2 5% 5% 6% 8% 52%	1260
	11th to 20th	Night	6% 5% 2 6% 5% 3 5% 6% 7% 8% 48%	1414
	21st to end	Night	8% 6% 3 5% 4 3 5% 4 6% 8% 48%	1575
August	1st to 10th	Night	8% 4 2 5% 5% 3 5% 5% 5% 7% 52%	1575
	11th to 20th	Night	9% 4 2 5% 4 4 5% 5% 5% 8% 50%	1598
	21st to end	Night	12% 5% 3 5% 5% 3 4 5% 4 7% 47%	1782
September	1st to 10th	Night	14% 5% 2 5% 6% 2 5% 5% 5% 5% 45%	1837
	11th to 20th	Night	15% 6% 2 5% 4 2 4 4 5% 8% 46%	1980
	21st to end	Night	18% 9% 4 6% 5% 3 4 5% 5% 6% 36%	1980
October	1st to 10th	Night	20% 8% 3 6% 5% 3 4 4 5% 7% 37%	2042
	11th to 20th	Night	23% 11% 4 8% 5% 3 4 5% 5% 6% 27%	2196
	21st to end	Night	28% 13% 3 6% 5% 3 4 4 4 5% 24%	2574
November	1st to 10th	Night	17% 13% 4 7% 6% 3 5% 4 6% 7% 29%	2406
	11th to 20th	Night	20% 14% 7% 8% 7% 4 4 4 5% 5% 21%	2587
	21st to end	Night	20% 19% 7% 9% 6% 3 4 3 4 5% 19%	2660
December	1st to 10th	Night	16% 19% 7% 10% 6% 3 5% 4 4 5% 20%	2845
	11th to 20th	Night	18% 16% 5% 9% 7% 4 5% 4 4 4 23%	3526
	21st to end	Night	26% 23% 7% 10% 5% 2 3 2 3 3 14%	5236

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	66% 6% 12% 11% 4	1434
	11th to 20th	Day	66% 8% 7% 13% 5% 1	1453
	21st to end	Day	63% 7% 9% 12% 8% 1	1584
February	1st to 10th	Day	60% 7% 15% 13% 5%	1596
	11th to 20th	Day	56% 6% 10% 17% 9% 1	1650
	21st to end	Day	57% 6% 10% 18% 10%	1480
March	1st to 10th	Day	46% 6% 13% 17% 16% 2	1805
	11th to 20th	Day	47% 5% 11% 25% 9% 3	1980
	21st to end	Day	46% 6% 11% 20% 13% 3	2178
April	1st to 10th	Day	52% 7% 11% 18% 13% 1	2150
	11th to 20th	Day	48% 5% 9% 22% 14% 2	2310
	21st to end	Day	48% 5% 10% 17% 16% 4	2340
May	1st to 10th	Day	44% 5% 10% 24% 13% 4	2340
	11th to 20th	Day	42% 6% 9% 23% 18% 2	2333
	21st to end	Day	37% 8% 14% 25% 14% 2	2907
June	1st to 10th	Day	37% 7% 12% 28% 14% 2	2700
	11th to 20th	Day	39% 7% 12% 25% 16% 1	2700
	21st to end	Day	39% 6% 13% 27% 13% 2	2700
July	1st to 10th	Day	30% 6% 14% 31% 18% 1	2626
	11th to 20th	Day	34% 7% 12% 31% 15% 1	2465
	21st to end	Day	32% 8% 14% 29% 15% 1	2591
August	1st to 10th	Day	34% 8% 16% 29% 12% 1	2346
	11th to 20th	Day	34% 8% 16% 28% 12% 1	2303
	21st to end	Day	35% 8% 16% 28% 12% 1	2574
September	1st to 10th	Day	40% 8% 13% 27% 12% 1	2101
	11th to 20th	Day	38% 7% 17% 31% 8%	1980
	21st to end	Day	45% 8% 16% 24% 7%	1980
October	1st to 10th	Day	48% 7% 12% 24% 9%	1918
	11th to 20th	Day	59% 10% 11% 17% 4	1762
	21st to end	Day	60% 7% 13% 13% 6%	1782
November	1st to 10th	Day	58% 6% 12% 18% 6%	1672
	11th to 20th	Day	64% 7% 13% 11% 5%	1592
	21st to end	Day	72% 7% 10% 9% 2	1520
December	1st to 10th	Day	68% 9% 8% 10% 5%	1330
	11th to 20th	Day	67% 9% 10% 11% 3	1671
	21st to end	Day	77% 7% 6% 6% 5%	2431
January	1st to 10th	Night	66% 8% 9% 11% 6%	2729
	11th to 20th	Night	67% 7% 9% 10% 6%	2547
	21st to end	Night	64% 7% 9% 13% 7%	2772
February	1st to 10th	Night	61% 7% 12% 14% 6%	2362
	11th to 20th	Night	57% 7% 12% 17% 6% 1	2207
	21st to end	Night	60% 6% 10% 15% 8%	1776
March	1st to 10th	Night	47% 5% 13% 19% 15% 1	2154
	11th to 20th	Night	48% 7% 11% 21% 11% 2	1980
	21st to end	Night	45% 6% 11% 20% 14% 4	2178
April	1st to 10th	Night	53% 4 10% 18% 15%	1823
	11th to 20th	Night	46% 5% 8% 24% 15% 3	1598
	21st to end	Night	49% 5% 8% 18% 16% 4	1620
May	1st to 10th	Night	46% 6% 10% 21% 13% 4	1620
	11th to 20th	Night	48% 6% 7% 22% 15% 3	1563
	21st to end	Night	38% 7% 11% 23% 18% 4	1367
June	1st to 10th	Night	38% 6% 11% 25% 18% 2	1260
	11th to 20th	Night	38% 8% 12% 24% 15% 3	1260
	21st to end	Night	39% 6% 13% 27% 13% 2	1260
July	1st to 10th	Night	28% 6% 14% 28% 21% 3	1225
	11th to 20th	Night	33% 7% 12% 30% 17% 1	1368
	21st to end	Night	34% 7% 13% 27% 17% 1	1487
August	1st to 10th	Night	32% 7% 15% 33% 11% 2	1566
	11th to 20th	Night	33% 8% 16% 30% 12% 1	1598
	21st to end	Night	38% 7% 14% 26% 13% 1	1782
September	1st to 10th	Night	39% 7% 12% 25% 14% 2	1837
	11th to 20th	Night	38% 6% 16% 33% 7%	1980
	21st to end	Night	48% 8% 14% 22% 7%	1980
October	1st to 10th	Night	48% 6% 13% 23% 10%	2042
	11th to 20th	Night	58% 8% 14% 16% 4	2196
	21st to end	Night	63% 6% 11% 14% 5%	2574
November	1st to 10th	Night	55% 7% 11% 19% 8%	2406
	11th to 20th	Night	65% 8% 10% 11% 6%	2587
	21st to end	Night	69% 8% 11% 9% 3	2660
December	1st to 10th	Night	66% 8% 10% 13% 2	2845
	11th to 20th	Night	64% 8% 10% 10% 6% 2	3525
	21st to end	Night	77% 5% 7% 8% 3	5236

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	21% 10% 22% 29% 16% 2	230
	11th to 20th	Day	22% 3 21% 40% 12% 2	245
	21st to end	Day	24% 2 17% 38% 16% 2	264
February	1st to 10th	Day	18% 8% 33% 33% 8% 1	240
	11th to 20th	Day	18% 2 32% 37% 9% 2	240
	21st to end	Day	17% 8% 38% 27% 10%	198
March	1st to 10th	Day	17% 7% 28% 33% 13% 2	245
	11th to 20th	Day	17% 5% 29% 32% 15% 3	420
	21st to end	Day	23% 4 21% 32% 17% 2	462
April	1st to 10th	Day	20% 4 22% 36% 16% 2	420
	11th to 20th	Day	23% 4 22% 30% 17% 3	412
	21st to end	Day	22% 4 21% 33% 16% 4	420
May	1st to 10th	Day	21% 2 17% 39% 17% 3	423
	11th to 20th	Day	18% 3 16% 37% 22% 5%	475
	21st to end	Day	13% 2 16% 39% 25% 6%	679
June	1st to 10th	Day	12% 3 14% 43% 24% 4	630
	11th to 20th	Day	15% 2 15% 38% 23% 6%	630
	21st to end	Day	17% 2 14% 37% 25% 5%	630
July	1st to 10th	Day	10% 1 12% 39% 31% 7%	621
	11th to 20th	Day	12% 2 13% 35% 30% 8%	618
	21st to end	Day	11% 2 12% 35% 31% 9%	684
August	1st to 10th	Day	13% 1 10% 32% 33% 10%	480
	11th to 20th	Day	12% 2 9% 27% 36% 14%	439
	21st to end	Day	13% 4 12% 34% 29% 8%	495
September	1st to 10th	Day	15% 4 13% 29% 29% 9%	450
	11th to 20th	Day	16% 3 13% 30% 29% 10%	450
	21st to end	Day	21% 4 16% 26% 26% 6%	450
October	1st to 10th	Day	22% 5% 22% 26% 20% 6%	450
	11th to 20th	Day	23% 7% 27% 29% 13% 1	412
	21st to end	Day	26% 7% 24% 31% 11% 2	297
November	1st to 10th	Day	20% 11% 23% 31% 14% 1	270
	11th to 20th	Day	17% 13% 34% 29% 7%	214
	21st to end	Day	24% 9% 32% 29% 5%	180
December	1st to 10th	Day	24% 11% 27% 26% 12% 2	180
	11th to 20th	Day	24% 15% 28% 25% 6% 2	180
	21st to end	Day	30% 9% 37% 20% 5%	198
January	1st to 10th	Night	22% 8% 30% 30% 9% 1	950
	11th to 20th	Night	24% 6% 27% 29% 12% 1	910
	21st to end	Night	21% 4 27% 35% 12% 1	990
February	1st to 10th	Night	18% 4 36% 32% 9% 1	742
	11th to 20th	Night	19% 3 32% 33% 11% 1	703
	21st to end	Night	21% 4 33% 33% 9% 1	592
March	1st to 10th	Night	18% 4 32% 36% 10% 1	720
	11th to 20th	Night	19% 5% 28% 37% 9% 2	720
	21st to end	Night	18% 4 27% 37% 13% 1	792
April	1st to 10th	Night	25% 3 26% 35% 10% 2	563
	11th to 20th	Night	22% 4 19% 38% 15% 2	533
	21st to end	Night	23% 3 23% 33% 16% 1	540
May	1st to 10th	Night	14% 3 21% 41% 17% 2	540
	11th to 20th	Night	19% 3 17% 35% 24% 2	531
	21st to end	Night	14% 3 19% 37% 25% 3	399
June	1st to 10th	Night	15% 2 18% 39% 25% 1	360
	11th to 20th	Night	11% 3 17% 43% 23% 3	360
	21st to end	Night	17% 2 16% 36% 26% 3	360
July	1st to 10th	Night	11% 13% 43% 33% 1	350
	11th to 20th	Night	8% 2 13% 38% 31% 8%	498
	21st to end	Night	11% 2 14% 39% 29% 5%	591
August	1st to 10th	Night	11% 13% 41% 30% 5%	537
	11th to 20th	Night	12% 1 14% 30% 34% 10%	534
	21st to end	Night	15% 2 13% 36% 30% 4	594
September	1st to 10th	Night	13% 3 19% 31% 26% 7%	601
	11th to 20th	Night	16% 3 19% 33% 22% 7%	720
	21st to end	Night	23% 3 19% 26% 23% 6%	720
October	1st to 10th	Night	22% 5% 21% 29% 21% 3	720
	11th to 20th	Night	29% 7% 24% 28% 11% 1	720
	21st to end	Night	36% 7% 25% 25% 7% 1	792
November	1st to 10th	Night	23% 9% 29% 26% 12% 1	740
	11th to 20th	Night	25% 12% 31% 23% 8% 1	760
	21st to end	Night	29% 13% 29% 24% 5%	760
December	1st to 10th	Night	25% 12% 32% 23% 8%	945
	11th to 20th	Night	28% 9% 35% 20% 7% 1	950
	21st to end	Night	33% 8% 29% 22% 8%	1045

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