Thetford Mines
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thetford Mines 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	18% 12% 4 10% 6% 5% 5% 4 5% 6% 25%	131290
	Night	16% 14% 5% 12% 6% 5% 4 3 4 5% 25%	131534
Transparency	Day	59% 5% 10% 17% 8% 1	134758
	Night	61% 5% 10% 16% 8% 1	136154
Seeing	Day	28% 18% 29% 17% 6% 1	16974
	Night	26% 16% 28% 22% 8% 1	25745

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% 8% 20% 9% 6% 8% 4 3 3 11%	4661
	11th to 20th	Day	7% 20% 9% 22% 14% 7% 3 2 2 2 13%	4650
	21st to end	Day	5% 11% 9% 17% 9% 11% 9% 4 6% 5% 15%	5124
February	1st to 10th	Day	11% 17% 7% 15% 10% 7% 5% 3 3 4 19%	1708
	11th to 20th	Day	12% 19% 8% 15% 7% 5% 5% 3 4 5% 17%	2048
	21st to end	Day	12% 19% 5% 13% 8% 5% 4 3 4 5% 21%	1730
March	1st to 10th	Day	17% 18% 7% 12% 7% 4 4 2 4 4 21%	4576
	11th to 20th	Day	13% 18% 5% 11% 5% 4 5% 3 4 5% 26%	4853
	21st to end	Day	19% 17% 5% 10% 5% 4 3 2 3 4 27%	5932
April	1st to 10th	Day	21% 17% 4 10% 6% 3 3 3 4 5% 24%	2520
	11th to 20th	Day	20% 14% 4 8% 4 4 3 2 4 5% 32%	2581
	21st to end	Day	21% 13% 4 8% 5% 4 4 3 4 6% 27%	2730
May	1st to 10th	Day	27% 11% 4 9% 6% 5% 4 3 5% 6% 21%	6098
	11th to 20th	Day	21% 9% 3 8% 5% 5% 5% 5% 5% 7% 27%	6402
	21st to end	Day	20% 9% 3 8% 6% 5% 6% 5% 6% 6% 27%	6949
June	1st to 10th	Day	24% 9% 2 6% 5% 5% 5% 4 5% 8% 26%	2925
	11th to 20th	Day	19% 7% 2 7% 5% 4 6% 5% 6% 9% 31%	3000
	21st to end	Day	25% 9% 3 8% 6% 5% 4 4 5% 6% 25%	2926
July	1st to 10th	Day	18% 8% 3 9% 6% 6% 7% 6% 7% 8% 23%	6360
	11th to 20th	Day	16% 7% 3 7% 5% 5% 5% 5% 7% 9% 32%	6360
	21st to end	Day	15% 6% 2 5% 4 4 5% 4 6% 9% 40%	6926
August	1st to 10th	Day	18% 7% 2 8% 5% 5% 6% 5% 7% 9% 27%	4504
	11th to 20th	Day	19% 8% 3 8% 6% 6% 6% 5% 6% 7% 26%	4175
	21st to end	Day	21% 7% 2 6% 5% 5% 5% 3 6% 6% 34%	4444
September	1st to 10th	Day	17% 8% 3 7% 4 4 4 5% 5% 7% 36%	2400
	11th to 20th	Day	19% 7% 2 5% 3 3 3 3 6% 6% 42%	2219
	21st to end	Day	20% 8% 2 6% 3 4 5% 4 6% 9% 34%	2200
October	1st to 10th	Day	21% 11% 3 6% 4 4 4 3 5% 7% 32%	2200
	11th to 20th	Day	21% 15% 3 8% 5% 5% 5% 3 5% 5% 24%	1845
	21st to end	Day	25% 19% 5% 9% 4 3 4 2 4 4 21%	1980
November	1st to 10th	Day	19% 19% 5% 9% 5% 4 3 3 3 4 27%	1851
	11th to 20th	Day	17% 19% 8% 13% 5% 3 3 3 4 4 21%	1890
	21st to end	Day	19% 21% 7% 12% 6% 4 4 3 3 5% 17%	1617
December	1st to 10th	Day	16% 19% 8% 15% 9% 5% 4 2 4 4 13%	2350
	11th to 20th	Day	19% 16% 8% 12% 6% 4 4 3 3 3 22%	2688
	21st to end	Day	18% 20% 6% 13% 6% 4 4 3 3 5% 19%	3868
January	1st to 10th	Night	11% 16% 8% 21% 9% 7% 5% 3 4 4 13%	8234
	11th to 20th	Night	9% 21% 10% 22% 12% 6% 4 3 2 3 10%	8235
	21st to end	Night	5% 11% 7% 17% 9% 11% 8% 5% 5% 4 17%	9057
February	1st to 10th	Night	10% 16% 6% 13% 9% 6% 5% 3 4 4 23%	2908
	11th to 20th	Night	10% 18% 7% 15% 7% 6% 5% 3 4 3 21%	2458
	21st to end	Night	12% 17% 6% 11% 7% 5% 4 3 4 4 27%	2076
March	1st to 10th	Night	18% 16% 5% 12% 6% 5% 6% 3 3 4 22%	5484
	11th to 20th	Night	13% 15% 6% 13% 7% 5% 5% 4 4 5% 24%	5325
	21st to end	Night	16% 17% 5% 11% 6% 4 3 3 4 4 26%	5224
April	1st to 10th	Night	21% 15% 3 8% 4 4 4 3 4 5% 28%	2100
	11th to 20th	Night	19% 12% 4 7% 5% 4 3 3 5% 6% 31%	1988
	21st to end	Night	22% 12% 3 7% 4 3 5% 3 5% 5% 31%	1890
May	1st to 10th	Night	24% 11% 4 8% 6% 5% 5% 4 5% 6% 24%	4106
	11th to 20th	Night	22% 9% 3 8% 5% 4 4 3 4 6% 31%	3623
	21st to end	Night	19% 9% 2 6% 4 4 5% 3 6% 8% 33%	3926
June	1st to 10th	Night	23% 10% 3 7% 4 4 5% 4 4 7% 29%	1470
	11th to 20th	Night	18% 6% 3 6% 5% 4 5% 4 7% 8% 35%	1400
	21st to end	Night	21% 8% 2 7% 5% 5% 5% 4 5% 7% 30%	1467
July	1st to 10th	Night	16% 9% 3 8% 6% 5% 5% 4 6% 8% 29%	3600
	11th to 20th	Night	21% 8% 3 6% 4 4 4 3 5% 7% 36%	3600
	21st to end	Night	12% 7% 2 7% 5% 5% 5% 4 6% 8% 39%	3926
August	1st to 10th	Night	22% 7% 3 6% 4 4 4 3 6% 8% 33%	2612
	11th to 20th	Night	17% 11% 3 9% 6% 4 5% 4 5% 6% 30%	2872
	21st to end	Night	18% 7% 3 8% 4 4 4 3 5% 6% 38%	3450
September	1st to 10th	Night	19% 6% 2 6% 4 3 4 3 5% 6% 40%	2000
	11th to 20th	Night	15% 7% 3 5% 3 3 4 4 4 5% 47%	2180
	21st to end	Night	17% 7% 2 6% 4 5% 4 3 4 7% 41%	2200
October	1st to 10th	Night	18% 9% 2 6% 4 4 4 3 5% 7% 38%	2200
	11th to 20th	Night	20% 11% 3 7% 5% 5% 5% 3 5% 6% 30%	2551
	21st to end	Night	24% 14% 4 7% 5% 4 4 3 5% 4 25%	2860
November	1st to 10th	Night	20% 16% 5% 7% 5% 5% 3 3 5% 5% 26%	2667
	11th to 20th	Night	17% 14% 6% 12% 6% 4 3 3 4 4 26%	2730
	21st to end	Night	17% 20% 7% 12% 7% 4 3 3 4 4 20%	2994
December	1st to 10th	Night	15% 18% 10% 20% 10% 4 3 2 3 4 11%	5060
	11th to 20th	Night	18% 19% 6% 15% 8% 4 3 2 4 4 17%	5741
	21st to end	Night	18% 20% 6% 11% 5% 4 3 3 3 4 24%	7320

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	79% 4 5% 9% 3	4662
	11th to 20th	Day	81% 2 3 9% 4 1	4680
	21st to end	Day	70% 4 5% 9% 11% 1	5124
February	1st to 10th	Day	70% 3 5% 13% 7% 2	1708
	11th to 20th	Day	71% 3 8% 10% 7% 1	2057
	21st to end	Day	66% 4 7% 15% 6% 2	1730
March	1st to 10th	Day	68% 4 6% 10% 9% 3	4579
	11th to 20th	Day	61% 4 12% 12% 9% 1	4853
	21st to end	Day	62% 3 5% 15% 13% 2	5932
April	1st to 10th	Day	64% 3 8% 15% 8% 2	2520
	11th to 20th	Day	56% 4 9% 19% 12% 1	2581
	21st to end	Day	60% 4 8% 16% 11% 2	2730
May	1st to 10th	Day	62% 5% 8% 16% 7% 1	6109
	11th to 20th	Day	54% 9% 14% 15% 6% 2	6406
	21st to end	Day	55% 7% 11% 16% 11% 1	6949
June	1st to 10th	Day	55% 7% 10% 20% 7% 1	2925
	11th to 20th	Day	49% 6% 14% 21% 10% 1	3000
	21st to end	Day	59% 7% 9% 16% 8%	2926
July	1st to 10th	Day	56% 8% 13% 19% 4	6290
	11th to 20th	Day	46% 8% 15% 26% 5%	6285
	21st to end	Day	41% 6% 15% 30% 8%	6728
August	1st to 10th	Day	51% 8% 16% 20% 5%	4490
	11th to 20th	Day	54% 9% 16% 16% 5%	4214
	21st to end	Day	50% 5% 14% 19% 12% 1	4444
September	1st to 10th	Day	46% 7% 14% 24% 9% 1	2400
	11th to 20th	Day	43% 5% 12% 28% 12%	2219
	21st to end	Day	46% 7% 13% 24% 10%	2200
October	1st to 10th	Day	51% 6% 13% 24% 6%	3590
	11th to 20th	Day	57% 4 12% 23% 3	2983
	21st to end	Day	58% 4 15% 16% 7%	3180
November	1st to 10th	Day	62% 5% 9% 17% 7%	1851
	11th to 20th	Day	67% 4 9% 12% 7% 1	1890
	21st to end	Day	72% 5% 9% 11% 3	1617
December	1st to 10th	Day	76% 4 8% 8% 5%	2350
	11th to 20th	Day	68% 4 6% 14% 8%	2688
	21st to end	Day	70% 3 5% 10% 10% 2	3868
January	1st to 10th	Night	75% 5% 7% 9% 4	8234
	11th to 20th	Night	82% 4 4 7% 3	8280
	21st to end	Night	67% 4 7% 10% 11%	9057
February	1st to 10th	Night	65% 4 8% 14% 8% 1	2908
	11th to 20th	Night	69% 4 7% 12% 8% 1	2460
	21st to end	Night	62% 4 7% 15% 11% 2	2076
March	1st to 10th	Night	65% 5% 8% 11% 9% 2	5494
	11th to 20th	Night	63% 5% 12% 12% 7% 1	5326
	21st to end	Night	62% 3 6% 18% 10% 1	5224
April	1st to 10th	Night	59% 4 9% 15% 12% 2	2100
	11th to 20th	Night	53% 5% 9% 20% 11% 2	1988
	21st to end	Night	55% 5% 9% 17% 12% 2	1890
May	1st to 10th	Night	61% 5% 10% 14% 8% 1	4112
	11th to 20th	Night	53% 8% 10% 20% 7% 1	3624
	21st to end	Night	48% 6% 15% 16% 14%	3927
June	1st to 10th	Night	53% 7% 12% 20% 8%	1470
	11th to 20th	Night	46% 6% 11% 24% 12% 2	1400
	21st to end	Night	53% 7% 12% 19% 9%	1467
July	1st to 10th	Night	51% 7% 18% 18% 5%	3560
	11th to 20th	Night	47% 6% 18% 23% 6%	3552
	21st to end	Night	42% 7% 17% 27% 7%	3809
August	1st to 10th	Night	49% 7% 16% 24% 5%	2602
	11th to 20th	Night	54% 8% 15% 18% 6% 1	2899
	21st to end	Night	47% 6% 13% 21% 13%	3450
September	1st to 10th	Night	44% 6% 14% 26% 9% 1	2000
	11th to 20th	Night	39% 6% 14% 31% 9% 1	2180
	21st to end	Night	43% 8% 11% 24% 13% 1	2200
October	1st to 10th	Night	49% 5% 11% 25% 10%	3590
	11th to 20th	Night	55% 4 10% 26% 5%	4152
	21st to end	Night	53% 5% 16% 16% 9% 1	4610
November	1st to 10th	Night	61% 5% 12% 17% 5%	2667
	11th to 20th	Night	62% 4 10% 14% 8% 1	2730
	21st to end	Night	69% 5% 10% 12% 5%	2994
December	1st to 10th	Night	79% 3 7% 8% 2	5060
	11th to 20th	Night	73% 4 5% 13% 5%	5742
	21st to end	Night	66% 3 6% 14% 10%	7320

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	31% 20% 37% 11% 1	220
	11th to 20th	Day	33% 26% 28% 10% 3	220
	21st to end	Day	26% 26% 32% 14% 2 1	242
February	1st to 10th	Day	34% 24% 28% 10% 4 1	245
	11th to 20th	Day	29% 28% 32% 10% 1	536
	21st to end	Day	31% 22% 29% 15% 2	446
March	1st to 10th	Day	29% 24% 29% 13% 4	540
	11th to 20th	Day	32% 24% 28% 12% 3 1	540
	21st to end	Day	34% 23% 27% 10% 6% 1	594
April	1st to 10th	Day	39% 21% 25% 10% 4 1	540
	11th to 20th	Day	36% 20% 26% 12% 5% 1	532
	21st to end	Day	33% 23% 27% 12% 4 2	540
May	1st to 10th	Day	36% 17% 29% 13% 4 1	543
	11th to 20th	Day	25% 16% 31% 19% 7% 1	551
	21st to end	Day	25% 21% 34% 14% 6% 1	573
June	1st to 10th	Day	30% 15% 31% 16% 8% 1	655
	11th to 20th	Day	23% 15% 28% 23% 10% 1	730
	21st to end	Day	28% 13% 30% 19% 8% 2	656
July	1st to 10th	Day	20% 14% 32% 26% 7% 1	525
	11th to 20th	Day	19% 14% 41% 18% 8% 1	524
	21st to end	Day	22% 11% 34% 25% 6% 1	576
August	1st to 10th	Day	22% 13% 31% 23% 10% 2	526
	11th to 20th	Day	21% 9% 29% 28% 12% 2	519
	21st to end	Day	24% 14% 26% 20% 11% 5%	583
September	1st to 10th	Day	20% 15% 29% 22% 9% 4	530
	11th to 20th	Day	22% 11% 24% 23% 15% 5%	530
	21st to end	Day	23% 16% 26% 21% 10% 5%	530
October	1st to 10th	Day	24% 17% 28% 19% 9% 3	530
	11th to 20th	Day	31% 16% 29% 15% 7% 2	442
	21st to end	Day	39% 19% 23% 16% 3	462
November	1st to 10th	Day	36% 17% 28% 14% 4	425
	11th to 20th	Day	30% 22% 27% 14% 6% 1	430
	21st to end	Day	34% 23% 30% 11% 1	257
December	1st to 10th	Day	33% 19% 30% 13% 4 1	220
	11th to 20th	Day	37% 24% 28% 9% 2	220
	21st to end	Day	34% 22% 28% 12% 2 1	242
January	1st to 10th	Night	28% 22% 33% 14% 3	840
	11th to 20th	Night	28% 22% 30% 14% 5%	840
	21st to end	Night	19% 20% 35% 20% 4 1	924
February	1st to 10th	Night	21% 23% 32% 18% 5%	840
	11th to 20th	Night	25% 22% 35% 15% 3	823
	21st to end	Night	25% 17% 35% 18% 4	692
March	1st to 10th	Night	27% 19% 34% 18% 3	840
	11th to 20th	Night	27% 21% 30% 18% 4 1	840
	21st to end	Night	25% 18% 28% 23% 5% 1	924
April	1st to 10th	Night	33% 17% 27% 19% 4	840
	11th to 20th	Night	28% 16% 30% 20% 6%	744
	21st to end	Night	30% 17% 27% 19% 6%	630
May	1st to 10th	Night	29% 13% 29% 24% 5% 1	618
	11th to 20th	Night	23% 9% 31% 29% 6% 1	406
	21st to end	Night	24% 9% 33% 27% 8% 1	433
June	1st to 10th	Night	26% 12% 26% 28% 8% 1	400
	11th to 20th	Night	22% 9% 24% 33% 12%	400
	21st to end	Night	22% 9% 30% 29% 10% 1	400
July	1st to 10th	Night	15% 7% 32% 34% 12% 1	390
	11th to 20th	Night	16% 7% 25% 39% 13% 1	388
	21st to end	Night	18% 7% 25% 41% 9%	438
August	1st to 10th	Night	19% 4 19% 39% 18% 1	411
	11th to 20th	Night	18% 7% 22% 32% 19% 1	592
	21st to end	Night	21% 7% 23% 30% 16% 3	796
September	1st to 10th	Night	21% 8% 26% 31% 13% 2	800
	11th to 20th	Night	20% 8% 22% 28% 21% 2	800
	21st to end	Night	22% 9% 28% 25% 15% 1	800
October	1st to 10th	Night	23% 12% 27% 26% 11% 1	800
	11th to 20th	Night	27% 15% 26% 22% 9% 1	800
	21st to end	Night	34% 16% 25% 20% 5%	880
November	1st to 10th	Night	32% 18% 25% 21% 5%	820
	11th to 20th	Night	28% 18% 27% 18% 9%	840
	21st to end	Night	32% 21% 26% 17% 5%	891
December	1st to 10th	Night	33% 20% 27% 16% 4	950
	11th to 20th	Night	30% 23% 25% 16% 5% 1	950
	21st to end	Night	35% 20% 26% 14% 5%	965

Page generated at 2026-05-14 01:58:56. Version ac07

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