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 2024-01-16.

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 7% 6% 4 5% 5% 6% 7% 35%	74685
	Night	16% 8% 3 6% 6% 3 5% 5% 5% 7% 35%	75098
Transparency	Day	47% 8% 13% 20% 11% 1	74399
	Night	48% 8% 12% 20% 11% 1	74918
Seeing	Day	17% 6% 22% 31% 19% 5%	13321
	Night	19% 7% 27% 30% 15% 2	22841

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	15% 11% 5% 8% 7% 4 6% 5% 5% 7% 26%	1517
	11th to 20th	Day	17% 15% 5% 9% 7% 4 5% 6% 5% 5% 22%	1599
	21st to end	Day	13% 13% 6% 12% 7% 5% 6% 6% 5% 7% 22%	1782
February	1st to 10th	Day	16% 14% 6% 10% 7% 5% 7% 4 4 5% 23%	1620
	11th to 20th	Day	13% 12% 6% 10% 7% 4 5% 4 4 6% 28%	1573
	21st to end	Day	19% 11% 5% 8% 7% 4 4 5% 5% 5% 28%	1379
March	1st to 10th	Day	14% 10% 3 9% 7% 4 5% 5% 5% 8% 30%	1977
	11th to 20th	Day	14% 9% 5% 7% 7% 4 5% 5% 4 6% 33%	1980
	21st to end	Day	18% 9% 3 7% 6% 3 5% 4 4 6% 35%	2178
April	1st to 10th	Day	17% 10% 4 7% 6% 3 6% 5% 5% 6% 31%	2136
	11th to 20th	Day	18% 8% 3 5% 5% 3 5% 4 5% 7% 36%	2186
	21st to end	Day	20% 8% 3 6% 6% 3 4 5% 6% 6% 34%	2340
May	1st to 10th	Day	15% 8% 4 7% 6% 4 5% 4 6% 7% 34%	2340
	11th to 20th	Day	15% 5% 3 6% 5% 3 5% 4 5% 6% 42%	2478
	21st to end	Day	11% 4 3 5% 5% 3 5% 5% 5% 8% 46%	2725
June	1st to 10th	Day	14% 5% 3 5% 6% 3 5% 6% 8% 8% 38%	2520
	11th to 20th	Day	12% 4 2 5% 5% 4 6% 5% 7% 10% 41%	2520
	21st to end	Day	13% 5% 2 5% 5% 3 5% 6% 6% 9% 42%	2520
July	1st to 10th	Day	7% 4 2 5% 6% 3 6% 6% 7% 8% 47%	2520
	11th to 20th	Day	11% 4 2 4 5% 3 5% 5% 7% 9% 43%	2520
	21st to end	Day	10% 5% 2 5% 5% 5% 7% 7% 7% 10% 38%	2592
August	1st to 10th	Day	8% 5% 2 6% 6% 4 5% 6% 7% 9% 42%	2327
	11th to 20th	Day	8% 5% 3 6% 6% 4 5% 5% 7% 10% 42%	2303
	21st to end	Day	10% 5% 2 5% 6% 3 5% 5% 7% 8% 43%	2466
September	1st to 10th	Day	13% 5% 2 5% 5% 3 6% 4 6% 8% 43%	2160
	11th to 20th	Day	12% 5% 3 4 4 3 4 5% 6% 8% 47%	2070
	21st to end	Day	12% 5% 3 6% 7% 3 6% 5% 5% 9% 39%	1933
October	1st to 10th	Day	18% 7% 3 6% 6% 3 5% 5% 6% 6% 37%	1800
	11th to 20th	Day	20% 8% 3 6% 7% 4 5% 5% 5% 7% 31%	1800
	21st to end	Day	24% 9% 4 8% 6% 4 5% 4 5% 6% 26%	1980
November	1st to 10th	Day	14% 8% 3 7% 6% 3 4 6% 6% 8% 35%	1606
	11th to 20th	Day	16% 10% 4 7% 7% 4 6% 5% 7% 8% 27%	1520
	21st to end	Day	19% 12% 3 8% 7% 4 4 5% 6% 7% 26%	1520
December	1st to 10th	Day	17% 14% 5% 10% 7% 5% 5% 5% 5% 7% 20%	1520
	11th to 20th	Day	20% 11% 5% 7% 5% 3 5% 5% 6% 7% 25%	1906
	21st to end	Day	29% 15% 5% 10% 6% 4 4 3 4 5% 14%	2772
January	1st to 10th	Night	18% 13% 5% 8% 7% 3 4 5% 6% 6% 25%	2649
	11th to 20th	Night	20% 13% 6% 8% 7% 4 6% 5% 5% 5% 21%	2468
	21st to end	Night	14% 11% 5% 8% 7% 4 6% 5% 5% 6% 29%	2574
February	1st to 10th	Night	15% 15% 5% 7% 7% 4 6% 5% 5% 6% 25%	2340
	11th to 20th	Night	16% 11% 4 8% 8% 3 6% 5% 5% 7% 27%	2273
	21st to end	Night	16% 11% 4 8% 7% 4 5% 4 5% 6% 30%	1873
March	1st to 10th	Night	13% 7% 4 8% 6% 4 5% 5% 5% 5% 37%	1970
	11th to 20th	Night	17% 10% 6% 7% 6% 2 5% 3 4 5% 35%	1979
	21st to end	Night	16% 8% 3 7% 6% 3 4 4 4 5% 40%	2178
April	1st to 10th	Night	19% 8% 3 6% 6% 3 5% 4 6% 7% 34%	1823
	11th to 20th	Night	16% 6% 3 6% 6% 4 5% 4 5% 6% 39%	1738
	21st to end	Night	17% 7% 3 6% 6% 3 5% 5% 5% 7% 36%	1620
May	1st to 10th	Night	16% 7% 3 5% 5% 3 4 5% 6% 7% 39%	1620
	11th to 20th	Night	14% 4 2 5% 4 3 4 5% 6% 7% 46%	1415
	21st to end	Night	11% 4 2 4 4 3 3 4 6% 8% 52%	1550
June	1st to 10th	Night	12% 4 2 5% 5% 3 6% 5% 6% 8% 43%	1440
	11th to 20th	Night	9% 5% 2 6% 5% 4 5% 5% 5% 9% 45%	1440
	21st to end	Night	14% 4 2 4 4 3 5% 5% 6% 8% 46%	1440
July	1st to 10th	Night	6% 3 1 4 5% 3 4 5% 5% 9% 56%	1440
	11th to 20th	Night	10% 4 1 5% 4 2 4 5% 5% 9% 50%	1440
	21st to end	Night	9% 3 1 4 4 3 4 5% 7% 9% 50%	1724
August	1st to 10th	Night	8% 4 2 5% 5% 3 5% 5% 6% 8% 49%	1611
	11th to 20th	Night	7% 4 2 5% 4 3 4 5% 6% 8% 52%	1598
	21st to end	Night	10% 4 2 4 5% 3 4 5% 6% 7% 49%	1871
September	1st to 10th	Night	10% 4 3 5% 5% 3 6% 5% 6% 7% 46%	1800
	11th to 20th	Night	12% 3 2 4 4 2 4 4 5% 6% 55%	1885
	21st to end	Night	15% 5% 2 6% 5% 3 4 5% 5% 7% 44%	2026
October	1st to 10th	Night	17% 7% 3 6% 5% 3 4 5% 5% 7% 39%	2160
	11th to 20th	Night	20% 7% 3 7% 6% 3 4 5% 5% 7% 34%	2160
	21st to end	Night	22% 9% 4 6% 5% 3 5% 4 6% 6% 30%	2376
November	1st to 10th	Night	17% 6% 2 5% 5% 2 6% 4 5% 7% 40%	2465
	11th to 20th	Night	17% 8% 3 6% 6% 4 5% 5% 7% 7% 32%	2660
	21st to end	Night	20% 9% 4 6% 6% 4 6% 5% 6% 7% 27%	2660
December	1st to 10th	Night	21% 12% 4 8% 6% 4 5% 4 5% 6% 24%	2660
	11th to 20th	Night	17% 14% 5% 8% 7% 4 6% 5% 5% 6% 24%	3288
	21st to end	Night	32% 12% 5% 7% 6% 3 4 5% 4 6% 15%	4884

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	59% 9% 14% 11% 7%	1518
	11th to 20th	Day	64% 10% 11% 10% 5%	1599
	21st to end	Day	62% 9% 11% 12% 7% 1	1782
February	1st to 10th	Day	64% 7% 11% 14% 3 1	1620
	11th to 20th	Day	57% 7% 13% 15% 8% 1	1582
	21st to end	Day	57% 6% 14% 15% 6% 1	1379
March	1st to 10th	Day	52% 7% 14% 16% 10% 1	1980
	11th to 20th	Day	52% 7% 10% 17% 12% 2	1980
	21st to end	Day	49% 6% 11% 18% 14% 3	2178
April	1st to 10th	Day	51% 6% 13% 17% 11% 1	2136
	11th to 20th	Day	47% 6% 11% 20% 15% 2	2186
	21st to end	Day	49% 7% 11% 18% 12% 3	2340
May	1st to 10th	Day	47% 6% 12% 19% 15% 2	2340
	11th to 20th	Day	41% 6% 12% 24% 13% 2	2482
	21st to end	Day	35% 10% 15% 25% 14% 2	2725
June	1st to 10th	Day	40% 9% 13% 21% 15% 1	2520
	11th to 20th	Day	37% 8% 12% 24% 17% 2	2520
	21st to end	Day	36% 9% 16% 24% 15%	2520
July	1st to 10th	Day	30% 9% 14% 25% 19% 3	2450
	11th to 20th	Day	34% 8% 17% 29% 11% 1	2445
	21st to end	Day	37% 11% 16% 22% 13% 1	2447
August	1st to 10th	Day	34% 10% 17% 23% 14% 2	2314
	11th to 20th	Day	34% 10% 16% 27% 12% 1	2303
	21st to end	Day	36% 8% 16% 27% 12% 1	2466
September	1st to 10th	Day	37% 9% 15% 27% 11% 1	2160
	11th to 20th	Day	33% 9% 14% 29% 14% 1	2070
	21st to end	Day	40% 10% 14% 27% 9% 1	1933
October	1st to 10th	Day	47% 7% 16% 18% 11% 1	1800
	11th to 20th	Day	52% 8% 14% 19% 7% 1	1800
	21st to end	Day	59% 7% 13% 16% 5% 1	1980
November	1st to 10th	Day	46% 8% 16% 19% 9% 1	1606
	11th to 20th	Day	54% 9% 15% 14% 8% 1	1520
	21st to end	Day	57% 9% 15% 15% 3	1520
December	1st to 10th	Day	64% 9% 12% 12% 3	1520
	11th to 20th	Day	58% 11% 14% 11% 6%	1906
	21st to end	Day	73% 8% 9% 8% 2	2772
January	1st to 10th	Night	61% 8% 12% 14% 6%	2649
	11th to 20th	Night	66% 9% 10% 10% 5% 1	2468
	21st to end	Night	57% 9% 12% 14% 7% 1	2574
February	1st to 10th	Night	60% 9% 10% 16% 4 1	2340
	11th to 20th	Night	58% 7% 11% 15% 8% 1	2275
	21st to end	Night	55% 5% 13% 18% 8%	1873
March	1st to 10th	Night	49% 7% 13% 20% 10% 2	1980
	11th to 20th	Night	52% 5% 9% 20% 12% 2	1980
	21st to end	Night	47% 5% 9% 22% 15% 3	2178
April	1st to 10th	Night	49% 5% 11% 19% 14% 1	1823
	11th to 20th	Night	47% 5% 9% 21% 16% 2	1738
	21st to end	Night	46% 8% 10% 20% 14% 2	1620
May	1st to 10th	Night	43% 7% 10% 21% 16% 3	1620
	11th to 20th	Night	37% 6% 10% 25% 17% 4	1416
	21st to end	Night	31% 8% 14% 25% 18% 4	1551
June	1st to 10th	Night	37% 8% 11% 24% 16% 3	1440
	11th to 20th	Night	36% 6% 12% 22% 22% 2	1440
	21st to end	Night	37% 8% 12% 25% 18% 1	1440
July	1st to 10th	Night	26% 7% 11% 27% 24% 4	1400
	11th to 20th	Night	31% 8% 11% 29% 19% 2	1392
	21st to end	Night	29% 9% 15% 28% 17% 3	1628
August	1st to 10th	Night	31% 9% 16% 28% 14% 2	1600
	11th to 20th	Night	28% 10% 14% 33% 14% 1	1598
	21st to end	Night	33% 8% 12% 29% 16% 1	1871
September	1st to 10th	Night	34% 10% 14% 27% 14% 2	1800
	11th to 20th	Night	29% 9% 10% 35% 17%	1885
	21st to end	Night	39% 8% 12% 28% 12%	2026
October	1st to 10th	Night	44% 8% 11% 22% 14% 1	2160
	11th to 20th	Night	49% 9% 12% 22% 7%	2160
	21st to end	Night	54% 7% 12% 17% 10% 1	2376
November	1st to 10th	Night	43% 7% 13% 24% 12% 1	2465
	11th to 20th	Night	51% 8% 14% 17% 10% 1	2660
	21st to end	Night	55% 10% 16% 15% 4	2660
December	1st to 10th	Night	61% 8% 12% 14% 5%	2660
	11th to 20th	Night	60% 9% 13% 14% 4	3288
	21st to end	Night	71% 9% 11% 7% 2	4884

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% 13% 27% 31% 7% 2	180
	11th to 20th	Day	20% 16% 36% 17% 9% 1	224
	21st to end	Day	20% 8% 35% 30% 6% 1	264
February	1st to 10th	Day	22% 13% 41% 19% 4	240
	11th to 20th	Day	20% 13% 38% 24% 5%	240
	21st to end	Day	24% 20% 28% 25% 4	225
March	1st to 10th	Day	19% 9% 34% 30% 8%	420
	11th to 20th	Day	18% 12% 30% 30% 9% 1	420
	21st to end	Day	24% 6% 29% 30% 9% 1	462
April	1st to 10th	Day	18% 8% 29% 35% 9% 1	420
	11th to 20th	Day	23% 6% 28% 26% 13% 3	412
	21st to end	Day	22% 4 26% 34% 12% 2	420
May	1st to 10th	Day	20% 5% 21% 30% 19% 5%	423
	11th to 20th	Day	15% 2 18% 36% 25% 5%	447
	21st to end	Day	14% 1 14% 35% 28% 8%	485
June	1st to 10th	Day	12% 3 18% 39% 25% 3	450
	11th to 20th	Day	14% 3 16% 34% 29% 4	450
	21st to end	Day	14% 2 13% 38% 28% 5%	450
July	1st to 10th	Day	8% 1 9% 33% 39% 9%	445
	11th to 20th	Day	14% 1 9% 31% 33% 11%	444
	21st to end	Day	8% 2 10% 41% 30% 8%	488
August	1st to 10th	Day	8% 2 9% 34% 33% 13%	446
	11th to 20th	Day	8% 3 11% 31% 32% 15%	439
	21st to end	Day	11% 2 16% 30% 30% 11%	495
September	1st to 10th	Day	11% 2 16% 32% 28% 11%	450
	11th to 20th	Day	10% 2 12% 34% 25% 16%	450
	21st to end	Day	13% 4 21% 33% 18% 11%	450
October	1st to 10th	Day	22% 5% 22% 26% 17% 8%	450
	11th to 20th	Day	20% 8% 31% 29% 10% 2	450
	21st to end	Day	26% 9% 26% 27% 10% 1	495
November	1st to 10th	Day	19% 9% 26% 28% 15% 2	269
	11th to 20th	Day	18% 12% 27% 32% 9% 2	180
	21st to end	Day	22% 11% 34% 26% 7% 1	180
December	1st to 10th	Day	21% 10% 38% 24% 7% 1	180
	11th to 20th	Day	20% 15% 38% 20% 6% 1	180
	21st to end	Day	28% 10% 40% 18% 5%	198
January	1st to 10th	Night	23% 12% 37% 20% 8%	760
	11th to 20th	Night	27% 11% 31% 26% 5%	741
	21st to end	Night	18% 9% 36% 28% 7% 1	792
February	1st to 10th	Night	22% 12% 39% 23% 4	720
	11th to 20th	Night	21% 11% 37% 24% 6%	703
	21st to end	Night	21% 10% 40% 25% 5%	592
March	1st to 10th	Night	18% 8% 34% 31% 8%	720
	11th to 20th	Night	21% 9% 36% 25% 8% 2	720
	21st to end	Night	19% 7% 28% 33% 12% 2	792
April	1st to 10th	Night	23% 6% 28% 33% 9% 1	720
	11th to 20th	Night	17% 7% 28% 33% 12% 3	674
	21st to end	Night	20% 4 24% 36% 14% 1	540
May	1st to 10th	Night	18% 2 19% 38% 21% 2	540
	11th to 20th	Night	16% 1 19% 42% 21% 2	354
	21st to end	Night	10% 2 16% 39% 29% 4	389
June	1st to 10th	Night	13% 2 16% 42% 24% 3	360
	11th to 20th	Night	12% 2 17% 38% 29% 2	360
	21st to end	Night	14% 2 13% 42% 26% 3	360
July	1st to 10th	Night	6% 2 8% 47% 31% 6%	350
	11th to 20th	Night	12% 3 12% 35% 33% 5%	348
	21st to end	Night	8% 2 10% 44% 31% 4	544
August	1st to 10th	Night	10% 2 8% 37% 37% 6%	537
	11th to 20th	Night	7% 2 11% 36% 36% 8%	534
	21st to end	Night	10% 2 18% 33% 31% 6%	683
September	1st to 10th	Night	11% 3 19% 33% 26% 8%	720
	11th to 20th	Night	13% 3 15% 30% 30% 9%	720
	21st to end	Night	17% 3 19% 29% 26% 6%	720
October	1st to 10th	Night	20% 3 25% 25% 21% 5%	720
	11th to 20th	Night	21% 8% 27% 30% 11% 2	720
	21st to end	Night	27% 6% 26% 32% 8% 1	792
November	1st to 10th	Night	21% 7% 26% 32% 13% 2	740
	11th to 20th	Night	21% 9% 34% 27% 9% 2	760
	21st to end	Night	23% 10% 31% 29% 7% 1	760
December	1st to 10th	Night	27% 9% 35% 22% 6% 1	760
	11th to 20th	Night	21% 10% 41% 22% 6%	760
	21st to end	Night	31% 9% 34% 22% 4	836

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