Scarborough
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Scarborough clear sky chart from 2005-11-06 to 2025-11-03.

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	15% 8% 3 8% 5% 5% 5% 4 6% 7% 35%	126994
	Night	16% 9% 3 7% 5% 4 5% 4 5% 7% 35%	123037
Transparency	Day	45% 8% 13% 20% 12% 1	130586
	Night	47% 8% 12% 20% 12% 1	127531
Seeing	Day	17% 7% 23% 30% 18% 5%	15720
	Night	18% 7% 27% 30% 15% 2	25062

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% 10% 4 9% 5% 5% 5% 4 5% 7% 34%	3597
	11th to 20th	Day	10% 16% 6% 16% 8% 7% 5% 4 5% 5% 19%	3866
	21st to end	Day	17% 16% 5% 12% 6% 5% 5% 4 4 5% 21%	4466
February	1st to 10th	Day	15% 14% 6% 10% 6% 6% 6% 4 4 5% 25%	1800
	11th to 20th	Day	13% 13% 5% 12% 7% 5% 4 3 5% 6% 27%	1753
	21st to end	Day	17% 12% 4 9% 6% 5% 4 4 5% 5% 28%	1539
March	1st to 10th	Day	20% 9% 3 8% 5% 4 5% 4 5% 6% 33%	4932
	11th to 20th	Day	11% 10% 4 9% 6% 5% 5% 4 6% 8% 32%	4980
	21st to end	Day	17% 10% 3 8% 5% 4 4 3 5% 5% 36%	5465
April	1st to 10th	Day	18% 11% 4 8% 5% 4 5% 4 5% 6% 31%	2374
	11th to 20th	Day	19% 10% 3 6% 4 4 4 3 5% 6% 36%	2432
	21st to end	Day	19% 9% 3 7% 5% 4 4 4 6% 6% 34%	2600
May	1st to 10th	Day	19% 7% 3 7% 5% 4 5% 3 6% 7% 33%	5888
	11th to 20th	Day	17% 8% 3 8% 4 4 4 3 5% 7% 36%	6318
	21st to end	Day	18% 6% 2 6% 4 4 4 4 5% 8% 40%	6949
June	1st to 10th	Day	14% 6% 2 6% 6% 5% 5% 5% 8% 8% 37%	2800
	11th to 20th	Day	11% 5% 2 6% 5% 6% 5% 4 7% 10% 40%	2800
	21st to end	Day	13% 5% 2 6% 5% 4 5% 5% 7% 9% 40%	2800
July	1st to 10th	Day	9% 5% 2 7% 5% 5% 6% 4 7% 9% 42%	6360
	11th to 20th	Day	8% 4 2 5% 4 5% 5% 4 6% 8% 49%	6360
	21st to end	Day	7% 4 2 6% 4 5% 5% 5% 7% 8% 48%	6525
August	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 6% 7% 40%	4234
	11th to 20th	Day	10% 6% 3 7% 5% 5% 5% 4 7% 10% 39%	4174
	21st to end	Day	9% 5% 2 6% 4 5% 5% 4 7% 8% 45%	4477
September	1st to 10th	Day	12% 5% 2 6% 4 5% 5% 4 6% 8% 45%	2400
	11th to 20th	Day	10% 5% 2 5% 4 3 4 4 6% 8% 49%	2300
	21st to end	Day	13% 6% 2 7% 6% 5% 5% 4 5% 9% 38%	2147
October	1st to 10th	Day	17% 7% 2 6% 4 4 5% 3 6% 6% 39%	2000
	11th to 20th	Day	19% 8% 3 7% 5% 5% 4 4 5% 7% 33%	2000
	21st to end	Day	22% 9% 4 9% 5% 5% 5% 3 5% 6% 26%	2200
November	1st to 10th	Day	14% 8% 3 8% 6% 5% 5% 4 6% 8% 35%	1712
	11th to 20th	Day	16% 11% 3 8% 6% 6% 6% 4 7% 7% 27%	1600
	21st to end	Day	18% 12% 4 9% 6% 5% 4 4 7% 7% 25%	1600
December	1st to 10th	Day	19% 14% 4 11% 6% 6% 6% 5% 6% 7% 17%	2600
	11th to 20th	Day	21% 14% 4 9% 5% 4 4 4 5% 6% 25%	2986
	21st to end	Day	27% 15% 6% 12% 5% 4 4 3 4 4 15%	3960
January	1st to 10th	Night	15% 11% 3 9% 5% 5% 5% 4 6% 7% 30%	6349
	11th to 20th	Night	13% 13% 6% 12% 8% 6% 6% 4 6% 6% 21%	6015
	21st to end	Night	17% 13% 4 9% 6% 5% 6% 4 5% 6% 26%	6490
February	1st to 10th	Night	14% 16% 4 8% 6% 6% 5% 4 6% 6% 27%	2600
	11th to 20th	Night	15% 12% 4 10% 7% 4 5% 4 5% 7% 28%	2533
	21st to end	Night	15% 11% 4 9% 6% 5% 5% 4 6% 6% 30%	2087
March	1st to 10th	Night	17% 8% 4 8% 6% 5% 5% 3 5% 5% 34%	4908
	11th to 20th	Night	11% 8% 3 7% 5% 4 5% 4 6% 8% 38%	4979
	21st to end	Night	20% 7% 3 7% 4 4 4 3 4 5% 40%	5467
April	1st to 10th	Night	19% 10% 3 7% 5% 4 4 3 5% 6% 33%	2025
	11th to 20th	Night	16% 7% 3 6% 5% 5% 5% 3 5% 6% 38%	1933
	21st to end	Night	16% 7% 2 7% 5% 4 5% 4 6% 7% 37%	1800
May	1st to 10th	Night	23% 8% 3 6% 4 4 4 4 5% 5% 35%	4053
	11th to 20th	Night	16% 7% 2 7% 5% 5% 5% 3 5% 7% 37%	3575
	21st to end	Night	17% 5% 2 5% 4 3 4 3 5% 7% 44%	3926
June	1st to 10th	Night	12% 5% 2 6% 5% 4 5% 4 6% 8% 43%	1600
	11th to 20th	Night	8% 6% 2 6% 5% 5% 5% 4 6% 8% 44%	1600
	21st to end	Night	13% 5% 2 5% 4 4 5% 4 6% 8% 44%	1600
July	1st to 10th	Night	7% 5% 2 6% 5% 4 5% 4 6% 9% 47%	3600
	11th to 20th	Night	10% 4 1 5% 4 3 4 3 5% 8% 50%	3600
	21st to end	Night	9% 4 1 5% 3 3 4 4 6% 8% 53%	4325
August	1st to 10th	Night	10% 5% 2 5% 4 4 5% 4 6% 8% 48%	2915
	11th to 20th	Night	12% 5% 2 5% 3 4 3 3 5% 7% 51%	2875
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 6% 7% 47%	3387
September	1st to 10th	Night	10% 5% 2 5% 4 4 5% 4 6% 7% 48%	2000
	11th to 20th	Night	11% 3 1 4 3 3 3 3 5% 6% 57%	2095
	21st to end	Night	16% 6% 2 7% 4 4 4 3 5% 6% 42%	2252
October	1st to 10th	Night	16% 6% 3 6% 4 3 5% 4 5% 7% 42%	2400
	11th to 20th	Night	19% 7% 2 7% 5% 4 4 4 5% 7% 35%	2400
	21st to end	Night	21% 9% 3 7% 4 5% 5% 3 6% 6% 33%	2640
November	1st to 10th	Night	16% 7% 2 6% 5% 4 5% 3 5% 7% 39%	2624
	11th to 20th	Night	17% 9% 3 7% 5% 5% 4 4 6% 7% 32%	2800
	21st to end	Night	20% 10% 3 6% 6% 5% 5% 4 7% 8% 27%	2800
December	1st to 10th	Night	18% 14% 4 9% 6% 5% 6% 4 6% 7% 22%	4580
	11th to 20th	Night	17% 15% 4 8% 5% 4 5% 3 5% 6% 26%	5208
	21st to end	Night	26% 13% 7% 10% 6% 5% 4 4 4 5% 16%	6996

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	49% 9% 16% 15% 11% 1	3598
	11th to 20th	Day	68% 7% 10% 10% 4 1	3900
	21st to end	Day	63% 8% 11% 10% 8%	4466
February	1st to 10th	Day	62% 8% 11% 15% 3 1	1800
	11th to 20th	Day	57% 7% 13% 15% 7% 1	1762
	21st to end	Day	56% 7% 14% 16% 6% 1	1539
March	1st to 10th	Day	50% 7% 14% 14% 14% 1	4935
	11th to 20th	Day	48% 6% 18% 17% 8% 2	4980
	21st to end	Day	48% 5% 12% 21% 13% 1	5465
April	1st to 10th	Day	52% 6% 12% 17% 12% 1	2374
	11th to 20th	Day	48% 6% 10% 21% 14% 1	2432
	21st to end	Day	49% 7% 11% 18% 13% 3	2600
May	1st to 10th	Day	48% 8% 12% 15% 12% 4	5897
	11th to 20th	Day	46% 13% 9% 15% 15% 1	6322
	21st to end	Day	41% 8% 12% 24% 13% 3	6949
June	1st to 10th	Day	41% 9% 12% 21% 16% 1	2800
	11th to 20th	Day	37% 9% 13% 24% 16% 1	2800
	21st to end	Day	37% 9% 15% 24% 15%	2800
July	1st to 10th	Day	34% 9% 15% 26% 15% 2	6290
	11th to 20th	Day	30% 8% 13% 33% 16% 1	6285
	21st to end	Day	31% 9% 14% 29% 17% 1	6338
August	1st to 10th	Day	41% 8% 14% 21% 15% 1	4221
	11th to 20th	Day	38% 8% 16% 27% 11% 1	4213
	21st to end	Day	34% 7% 14% 34% 10% 1	4477
September	1st to 10th	Day	36% 9% 14% 29% 11% 1	2400
	11th to 20th	Day	32% 8% 14% 30% 14% 1	2300
	21st to end	Day	41% 9% 13% 26% 10%	2147
October	1st to 10th	Day	34% 7% 15% 27% 15% 1	3260
	11th to 20th	Day	37% 6% 17% 24% 15%	3240
	21st to end	Day	49% 7% 15% 21% 6%	3538
November	1st to 10th	Day	46% 8% 16% 20% 9% 1	1712
	11th to 20th	Day	54% 9% 14% 14% 8% 1	1600
	21st to end	Day	58% 9% 15% 15% 3	1600
December	1st to 10th	Day	64% 9% 15% 9% 3	2600
	11th to 20th	Day	59% 9% 13% 9% 9%	2986
	21st to end	Day	71% 7% 9% 8% 4	3960
January	1st to 10th	Night	51% 8% 14% 21% 5%	6349
	11th to 20th	Night	64% 11% 10% 11% 4	6056
	21st to end	Night	58% 8% 13% 11% 10% 1	6490
February	1st to 10th	Night	57% 9% 11% 17% 5% 1	2600
	11th to 20th	Night	57% 7% 11% 16% 9% 1	2535
	21st to end	Night	54% 5% 14% 19% 8%	2087
March	1st to 10th	Night	51% 6% 10% 18% 12% 2	4918
	11th to 20th	Night	42% 7% 19% 21% 10% 2	4980
	21st to end	Night	46% 4 9% 20% 18% 2	5467
April	1st to 10th	Night	51% 5% 11% 18% 14% 1	2025
	11th to 20th	Night	47% 5% 9% 21% 15% 2	1933
	21st to end	Night	45% 8% 11% 20% 16% 2	1800
May	1st to 10th	Night	50% 7% 10% 14% 13% 5%	4060
	11th to 20th	Night	44% 12% 11% 18% 13% 3	3576
	21st to end	Night	40% 6% 13% 24% 15% 2	3927
June	1st to 10th	Night	37% 8% 11% 25% 17% 3	1600
	11th to 20th	Night	36% 6% 12% 23% 21% 2	1600
	21st to end	Night	37% 8% 12% 24% 18% 1	1600
July	1st to 10th	Night	33% 7% 14% 25% 19% 2	3560
	11th to 20th	Night	31% 7% 12% 31% 17% 1	3552
	21st to end	Night	29% 7% 12% 28% 22% 1	4197
August	1st to 10th	Night	34% 7% 13% 28% 15% 2	2904
	11th to 20th	Night	32% 7% 14% 33% 13% 1	2902
	21st to end	Night	34% 7% 13% 32% 14% 1	3387
September	1st to 10th	Night	32% 9% 14% 30% 14% 1	2000
	11th to 20th	Night	28% 8% 10% 36% 18%	2095
	21st to end	Night	41% 8% 11% 27% 12% 1	2252
October	1st to 10th	Night	33% 6% 9% 27% 23% 1	3920
	11th to 20th	Night	42% 7% 14% 27% 10%	3899
	21st to end	Night	39% 7% 15% 28% 11% 1	4252
November	1st to 10th	Night	43% 7% 13% 23% 12% 1	2624
	11th to 20th	Night	51% 7% 14% 17% 10% 1	2800
	21st to end	Night	55% 10% 16% 15% 4	2800
December	1st to 10th	Night	60% 9% 13% 14% 4	4580
	11th to 20th	Night	58% 8% 11% 13% 10% 1	5208
	21st to end	Night	69% 9% 10% 8% 4	6996

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% 15% 29% 29% 7% 2	200
	11th to 20th	Day	20% 17% 35% 19% 9% 1	266
	21st to end	Day	22% 11% 33% 28% 5% 1	330
February	1st to 10th	Day	20% 15% 38% 21% 5%	300
	11th to 20th	Day	21% 15% 36% 22% 5%	300
	21st to end	Day	23% 19% 28% 27% 4	280
March	1st to 10th	Day	20% 11% 32% 29% 8%	500
	11th to 20th	Day	17% 13% 32% 28% 9% 1	500
	21st to end	Day	25% 7% 30% 29% 9% 1	550
April	1st to 10th	Day	20% 10% 27% 33% 8% 1	500
	11th to 20th	Day	25% 8% 29% 25% 11% 3	492
	21st to end	Day	21% 7% 27% 32% 11% 2	500
May	1st to 10th	Day	20% 5% 23% 30% 18% 5%	503
	11th to 20th	Day	14% 3 19% 33% 25% 5%	527
	21st to end	Day	15% 2 16% 34% 27% 7%	573
June	1st to 10th	Day	13% 4 21% 36% 23% 3	530
	11th to 20th	Day	12% 4 19% 34% 27% 3	530
	21st to end	Day	15% 4 17% 36% 24% 5%	530
July	1st to 10th	Day	8% 2 14% 33% 34% 8%	525
	11th to 20th	Day	14% 2 13% 31% 31% 10%	524
	21st to end	Day	8% 2 12% 43% 27% 8%	576
August	1st to 10th	Day	10% 3 11% 33% 31% 12%	526
	11th to 20th	Day	8% 4 14% 31% 29% 14%	519
	21st to end	Day	11% 3 16% 33% 28% 10%	583
September	1st to 10th	Day	11% 4 17% 30% 27% 11%	530
	11th to 20th	Day	8% 2 11% 32% 29% 17%	530
	21st to end	Day	15% 5% 20% 32% 18% 10%	530
October	1st to 10th	Day	19% 5% 24% 27% 17% 8%	530
	11th to 20th	Day	18% 8% 33% 28% 10% 2	530
	21st to end	Day	24% 10% 29% 26% 10% 1	583
November	1st to 10th	Day	18% 11% 26% 29% 14% 2	303
	11th to 20th	Day	18% 14% 29% 30% 8% 2	200
	21st to end	Day	22% 14% 35% 24% 6% 1	200
December	1st to 10th	Day	22% 12% 36% 23% 8% 1	200
	11th to 20th	Day	23% 16% 36% 19% 5% 1	200
	21st to end	Day	29% 10% 39% 17% 5%	220
January	1st to 10th	Night	23% 12% 38% 19% 8%	800
	11th to 20th	Night	25% 12% 32% 25% 5%	800
	21st to end	Night	19% 11% 36% 27% 7%	880
February	1st to 10th	Night	21% 13% 38% 24% 5%	800
	11th to 20th	Night	20% 12% 37% 24% 6%	783
	21st to end	Night	20% 10% 39% 26% 5%	660
March	1st to 10th	Night	18% 10% 34% 31% 8%	800
	11th to 20th	Night	20% 10% 36% 27% 7% 1	800
	21st to end	Night	20% 7% 29% 32% 11% 1	880
April	1st to 10th	Night	24% 7% 27% 33% 8% 1	800
	11th to 20th	Night	18% 7% 28% 34% 11% 3	749
	21st to end	Night	20% 5% 25% 36% 14% 1	600
May	1st to 10th	Night	19% 4 19% 38% 20% 2	600
	11th to 20th	Night	15% 2 19% 40% 22% 2	394
	21st to end	Night	11% 3 17% 37% 28% 3	433
June	1st to 10th	Night	14% 3 16% 42% 24% 3	400
	11th to 20th	Night	11% 4 17% 38% 28% 2	400
	21st to end	Night	14% 2 16% 42% 24% 3	400
July	1st to 10th	Night	6% 2 10% 47% 30% 5%	390
	11th to 20th	Night	11% 3 12% 36% 32% 4	388
	21st to end	Night	8% 2 11% 44% 32% 4	606
August	1st to 10th	Night	9% 2 10% 36% 37% 6%	597
	11th to 20th	Night	8% 2 13% 34% 36% 7%	594
	21st to end	Night	10% 3 19% 33% 31% 5%	759
September	1st to 10th	Night	11% 3 19% 34% 26% 7%	800
	11th to 20th	Night	12% 3 14% 29% 33% 9%	800
	21st to end	Night	18% 4 18% 30% 25% 6%	800
October	1st to 10th	Night	19% 4 26% 25% 21% 5%	800
	11th to 20th	Night	21% 9% 28% 30% 11% 2	800
	21st to end	Night	25% 7% 28% 31% 8% 1	880
November	1st to 10th	Night	21% 7% 27% 32% 12% 2	789
	11th to 20th	Night	21% 10% 33% 26% 9% 2	800
	21st to end	Night	23% 10% 32% 28% 7% 1	800
December	1st to 10th	Night	27% 10% 35% 22% 6% 1	800
	11th to 20th	Night	21% 11% 41% 21% 5%	800
	21st to end	Night	30% 9% 34% 22% 4	880

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