Stratford
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stratford clear sky chart from 2005-11-06 to 2025-11-18.

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% 11% 4 10% 5% 5% 5% 4 5% 7% 30%	127430
	Night	13% 12% 5% 10% 6% 5% 4 3 5% 6% 30%	122834
Transparency	Day	51% 7% 12% 18% 10% 1	131010
	Night	52% 7% 12% 17% 11% 1	127342
Seeing	Day	18% 12% 30% 23% 12% 5%	16357
	Night	19% 12% 30% 27% 11% 1	25390

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% 8% 22% 8% 4 4 3 3 4 17%	3512
	11th to 20th	Day	10% 19% 9% 25% 8% 8% 5% 4 4 2 6%	4026
	21st to end	Day	15% 18% 7% 19% 8% 6% 5% 3 4 4 10%	4466
February	1st to 10th	Day	15% 19% 6% 15% 10% 7% 4 3 4 4 15%	1800
	11th to 20th	Day	13% 14% 8% 14% 9% 6% 5% 3 5% 6% 16%	1753
	21st to end	Day	15% 16% 5% 13% 7% 4 5% 4 4 5% 21%	1625
March	1st to 10th	Day	16% 12% 6% 14% 8% 4 3 2 4 5% 26%	4834
	11th to 20th	Day	11% 13% 6% 10% 5% 5% 4 3 6% 9% 28%	4980
	21st to end	Day	17% 13% 6% 12% 6% 4 3 3 4 5% 27%	5465
April	1st to 10th	Day	16% 15% 4 8% 4 4 4 4 6% 7% 27%	2290
	11th to 20th	Day	17% 12% 3 6% 4 4 4 3 5% 7% 34%	2543
	21st to end	Day	19% 11% 4 8% 4 4 4 3 4 7% 32%	2600
May	1st to 10th	Day	17% 10% 3 8% 5% 4 4 3 6% 7% 32%	5888
	11th to 20th	Day	16% 10% 3 8% 5% 5% 4 4 5% 8% 32%	5970
	21st to end	Day	17% 9% 2 6% 4 4 4 4 5% 7% 38%	6949
June	1st to 10th	Day	13% 7% 2 7% 5% 5% 5% 4 6% 8% 37%	2800
	11th to 20th	Day	10% 7% 2 7% 5% 5% 5% 5% 7% 9% 38%	2985
	21st to end	Day	13% 7% 3 6% 5% 5% 5% 4 7% 9% 38%	3000
July	1st to 10th	Day	10% 6% 2 7% 4 5% 6% 4 7% 9% 40%	6819
	11th to 20th	Day	8% 6% 2 6% 5% 5% 5% 5% 7% 8% 43%	6164
	21st to end	Day	8% 5% 2 6% 5% 4 5% 5% 6% 8% 46%	6424
August	1st to 10th	Day	13% 7% 2 7% 4 5% 5% 4 7% 9% 36%	4234
	11th to 20th	Day	11% 8% 3 7% 4 5% 7% 5% 7% 10% 34%	4174
	21st to end	Day	8% 6% 3 6% 5% 5% 6% 5% 7% 9% 39%	4595
September	1st to 10th	Day	11% 6% 2 6% 6% 5% 5% 5% 7% 8% 39%	2369
	11th to 20th	Day	11% 6% 2 5% 4 4 4 5% 6% 8% 45%	2200
	21st to end	Day	16% 7% 3 8% 5% 5% 4 4 6% 8% 34%	2200
October	1st to 10th	Day	17% 9% 3 7% 4 5% 5% 4 5% 6% 36%	2028
	11th to 20th	Day	21% 11% 4 8% 4 4 5% 4 6% 7% 28%	2000
	21st to end	Day	22% 14% 4 9% 5% 5% 5% 4 5% 6% 21%	2170
November	1st to 10th	Day	14% 15% 4 8% 5% 5% 4 4 5% 6% 30%	1852
	11th to 20th	Day	14% 16% 5% 10% 6% 5% 5% 4 7% 6% 22%	1658
	21st to end	Day	17% 19% 9% 11% 6% 4 4 3 6% 7% 15%	1600
December	1st to 10th	Day	17% 22% 9% 16% 7% 5% 5% 3 4 4 9%	2600
	11th to 20th	Day	20% 20% 7% 13% 7% 5% 4 3 3 4 14%	2986
	21st to end	Day	25% 22% 7% 12% 6% 4 4 2 3 4 9%	3871
January	1st to 10th	Night	12% 15% 7% 21% 7% 6% 4 3 4 5% 15%	6398
	11th to 20th	Night	11% 15% 8% 21% 11% 7% 5% 4 5% 5% 8%	5859
	21st to end	Night	15% 15% 8% 17% 7% 5% 6% 4 5% 6% 14%	6490
February	1st to 10th	Night	14% 18% 6% 13% 8% 5% 5% 4 4 6% 17%	2600
	11th to 20th	Night	13% 14% 6% 11% 8% 6% 4 4 6% 7% 20%	2533
	21st to end	Night	15% 13% 5% 13% 8% 5% 4 4 5% 5% 22%	2003
March	1st to 10th	Night	13% 10% 4 10% 7% 6% 4 4 5% 6% 30%	5004
	11th to 20th	Night	9% 9% 5% 10% 5% 5% 5% 4 7% 9% 33%	4979
	21st to end	Night	17% 11% 4 8% 4 4 4 3 4 5% 35%	5467
April	1st to 10th	Night	18% 11% 3 9% 5% 5% 4 3 5% 5% 31%	2110
	11th to 20th	Night	16% 10% 4 7% 5% 5% 4 4 5% 7% 34%	1822
	21st to end	Night	14% 10% 3 7% 5% 5% 5% 3 6% 7% 34%	1800
May	1st to 10th	Night	18% 9% 3 6% 4 4 4 3 4 7% 37%	4053
	11th to 20th	Night	13% 9% 3 8% 5% 5% 5% 4 6% 6% 36%	3906
	21st to end	Night	17% 7% 2 5% 3 3 3 3 5% 7% 45%	3926
June	1st to 10th	Night	11% 6% 3 5% 5% 5% 5% 4 6% 7% 44%	1600
	11th to 20th	Night	9% 6% 2 6% 5% 5% 5% 3 7% 10% 41%	1409
	21st to end	Night	9% 7% 2 6% 4 4 4 4 6% 8% 46%	1400
July	1st to 10th	Night	6% 5% 2 6% 3 5% 4 4 7% 10% 48%	3160
	11th to 20th	Night	9% 7% 2 5% 4 3 4 3 6% 7% 50%	3780
	21st to end	Night	8% 5% 2 6% 4 4 4 4 6% 7% 50%	4428
August	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 6% 7% 46%	2915
	11th to 20th	Night	11% 8% 2 7% 4 3 4 3 6% 7% 44%	2875
	21st to end	Night	8% 6% 3 6% 4 4 4 4 6% 7% 48%	3256
September	1st to 10th	Night	9% 6% 2 7% 4 5% 5% 4 6% 7% 46%	2029
	11th to 20th	Night	10% 4 1 4 3 3 3 3 5% 8% 55%	2200
	21st to end	Night	16% 7% 2 8% 4 4 4 3 5% 7% 40%	2200
October	1st to 10th	Night	14% 8% 3 7% 4 5% 4 3 5% 7% 41%	2371
	11th to 20th	Night	18% 12% 3 8% 5% 4 4 3 5% 7% 33%	2400
	21st to end	Night	22% 10% 4 9% 5% 4 5% 4 6% 6% 27%	2670
November	1st to 10th	Night	14% 11% 4 8% 5% 4 5% 4 6% 6% 34%	2666
	11th to 20th	Night	15% 16% 4 12% 5% 5% 5% 4 5% 6% 22%	2894
	21st to end	Night	19% 16% 7% 10% 5% 5% 4 3 5% 7% 19%	2800
December	1st to 10th	Night	12% 21% 10% 16% 7% 6% 5% 4 4 4 11%	4580
	11th to 20th	Night	13% 22% 8% 14% 7% 5% 4 3 4 4 16%	5208
	21st to end	Night	22% 20% 8% 12% 5% 4 4 3 4 4 13%	7043

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	70% 8% 7% 10% 6%	3513
	11th to 20th	Day	82% 8% 6% 3 2	4060
	21st to end	Day	76% 7% 8% 4 5%	4466
February	1st to 10th	Day	73% 7% 7% 10% 3 1	1800
	11th to 20th	Day	67% 8% 9% 12% 4	1762
	21st to end	Day	64% 7% 10% 12% 6% 1	1625
March	1st to 10th	Day	61% 5% 14% 14% 5% 1	4837
	11th to 20th	Day	51% 7% 17% 17% 7% 2	4980
	21st to end	Day	58% 6% 11% 16% 8% 1	5465
April	1st to 10th	Day	55% 7% 12% 17% 10%	2290
	11th to 20th	Day	48% 8% 12% 20% 11% 1	2543
	21st to end	Day	52% 6% 11% 19% 10% 2	2600
May	1st to 10th	Day	48% 7% 12% 18% 10% 5%	5897
	11th to 20th	Day	48% 9% 12% 18% 13% 1	5974
	21st to end	Day	44% 7% 11% 18% 16% 4	6949
June	1st to 10th	Day	42% 8% 14% 21% 14% 1	2800
	11th to 20th	Day	40% 8% 13% 22% 15% 1	2985
	21st to end	Day	42% 8% 12% 24% 14%	3000
July	1st to 10th	Day	39% 6% 13% 25% 17% 2	6746
	11th to 20th	Day	35% 6% 13% 31% 14% 1	6088
	21st to end	Day	34% 6% 14% 27% 18% 1	6240
August	1st to 10th	Day	42% 7% 14% 23% 13% 1	4221
	11th to 20th	Day	42% 8% 17% 24% 9%	4213
	21st to end	Day	38% 7% 13% 31% 11% 1	4595
September	1st to 10th	Day	39% 10% 15% 25% 11% 1	2369
	11th to 20th	Day	34% 7% 15% 29% 14% 1	2200
	21st to end	Day	46% 8% 13% 23% 10% 1	2200
October	1st to 10th	Day	40% 6% 13% 23% 16% 1	3301
	11th to 20th	Day	42% 6% 13% 22% 17%	3240
	21st to end	Day	54% 6% 15% 18% 7%	3484
November	1st to 10th	Day	54% 7% 11% 20% 7% 1	1852
	11th to 20th	Day	58% 9% 12% 12% 7% 1	1658
	21st to end	Day	68% 8% 12% 9% 2	1600
December	1st to 10th	Day	79% 6% 7% 6% 2	2600
	11th to 20th	Day	75% 5% 7% 9% 4	2986
	21st to end	Day	77% 8% 6% 6% 3	3871
January	1st to 10th	Night	68% 9% 7% 11% 4	6398
	11th to 20th	Night	77% 8% 8% 5% 2	5900
	21st to end	Night	70% 8% 8% 8% 6% 1	6490
February	1st to 10th	Night	68% 7% 10% 12% 3	2600
	11th to 20th	Night	61% 8% 12% 13% 5%	2535
	21st to end	Night	61% 7% 12% 13% 6%	2003
March	1st to 10th	Night	52% 7% 15% 17% 8% 1	5014
	11th to 20th	Night	43% 8% 20% 19% 9% 1	4980
	21st to end	Night	49% 6% 10% 18% 14% 2	5467
April	1st to 10th	Night	54% 5% 11% 17% 13% 1	2110
	11th to 20th	Night	48% 7% 10% 18% 16% 2	1822
	21st to end	Night	47% 8% 10% 17% 15% 3	1800
May	1st to 10th	Night	46% 6% 12% 15% 13% 8%	4060
	11th to 20th	Night	46% 9% 12% 19% 12% 3	3907
	21st to end	Night	38% 7% 13% 23% 17% 3	3927
June	1st to 10th	Night	37% 7% 13% 25% 15% 3	1600
	11th to 20th	Night	37% 7% 13% 24% 16% 2	1409
	21st to end	Night	35% 7% 15% 22% 19% 1	1400
July	1st to 10th	Night	30% 5% 16% 25% 19% 4	3125
	11th to 20th	Night	33% 5% 11% 28% 20% 3	3733
	21st to end	Night	33% 6% 11% 26% 22% 3	4297
August	1st to 10th	Night	37% 6% 14% 25% 16% 2	2904
	11th to 20th	Night	38% 7% 16% 25% 14% 1	2902
	21st to end	Night	34% 7% 12% 32% 14% 1	3256
September	1st to 10th	Night	35% 9% 12% 27% 14% 2	2029
	11th to 20th	Night	28% 7% 14% 30% 20% 1	2200
	21st to end	Night	44% 7% 12% 24% 13%	2200
October	1st to 10th	Night	33% 6% 9% 28% 23% 1	3878
	11th to 20th	Night	45% 6% 14% 20% 15% 1	3899
	21st to end	Night	44% 6% 14% 24% 11%	4306
November	1st to 10th	Night	48% 8% 13% 21% 10% 1	2666
	11th to 20th	Night	58% 9% 11% 13% 6% 1	2894
	21st to end	Night	63% 9% 14% 11% 3	2800
December	1st to 10th	Night	76% 7% 9% 7% 1	4580
	11th to 20th	Night	71% 7% 8% 10% 5%	5208
	21st to end	Night	74% 6% 9% 8% 4	7043

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% 22% 33% 18% 6% 1	240
	11th to 20th	Day	19% 20% 45% 12% 4 1	300
	21st to end	Day	24% 18% 36% 20% 2 1	330
February	1st to 10th	Day	24% 21% 38% 12% 4	300
	11th to 20th	Day	24% 23% 39% 11% 2	300
	21st to end	Day	27% 27% 29% 15% 1	249
March	1st to 10th	Day	22% 15% 40% 17% 6%	450
	11th to 20th	Day	21% 17% 35% 21% 5% 1	500
	21st to end	Day	26% 14% 35% 20% 5% 1	550
April	1st to 10th	Day	22% 18% 35% 19% 5% 1	500
	11th to 20th	Day	26% 16% 27% 17% 10% 4	492
	21st to end	Day	24% 14% 32% 21% 7% 2	500
May	1st to 10th	Day	23% 11% 31% 23% 10% 3	503
	11th to 20th	Day	19% 9% 29% 23% 15% 6%	527
	21st to end	Day	18% 7% 27% 26% 16% 6%	573
June	1st to 10th	Day	15% 9% 33% 28% 12% 4	530
	11th to 20th	Day	13% 9% 32% 31% 13% 3	715
	21st to end	Day	14% 9% 30% 31% 12% 3	730
July	1st to 10th	Day	9% 6% 25% 29% 23% 7%	721
	11th to 20th	Day	13% 5% 27% 26% 20% 8%	576
	21st to end	Day	9% 4 26% 33% 20% 7%	576
August	1st to 10th	Day	11% 6% 20% 29% 24% 11%	526
	11th to 20th	Day	10% 5% 19% 31% 20% 15%	519
	21st to end	Day	15% 6% 22% 27% 20% 11%	583
September	1st to 10th	Day	12% 7% 22% 26% 18% 14%	530
	11th to 20th	Day	8% 5% 17% 28% 28% 14%	530
	21st to end	Day	16% 10% 27% 24% 15% 8%	530
October	1st to 10th	Day	19% 11% 26% 22% 17% 6%	530
	11th to 20th	Day	20% 15% 35% 22% 7% 2	530
	21st to end	Day	26% 18% 31% 20% 5% 1	553
November	1st to 10th	Day	25% 21% 32% 14% 7% 1	330
	11th to 20th	Day	26% 21% 33% 13% 6% 1	214
	21st to end	Day	25% 21% 36% 18% 2	200
December	1st to 10th	Day	30% 24% 31% 11% 6%	200
	11th to 20th	Day	28% 26% 30% 15% 2	200
	21st to end	Day	32% 20% 33% 13% 2	220
January	1st to 10th	Night	22% 18% 38% 17% 5% 1	909
	11th to 20th	Night	28% 17% 33% 19% 3	800
	21st to end	Night	19% 12% 39% 24% 5%	880
February	1st to 10th	Night	23% 16% 38% 21% 2	800
	11th to 20th	Night	21% 16% 35% 23% 5%	783
	21st to end	Night	22% 17% 36% 22% 3	660
March	1st to 10th	Night	18% 15% 39% 22% 6%	800
	11th to 20th	Night	17% 18% 38% 21% 7%	800
	21st to end	Night	21% 14% 31% 24% 9% 1	880
April	1st to 10th	Night	23% 15% 31% 27% 4	800
	11th to 20th	Night	21% 13% 33% 23% 9% 1	638
	21st to end	Night	19% 13% 31% 30% 7% 1	600
May	1st to 10th	Night	19% 8% 30% 30% 12% 1	600
	11th to 20th	Night	17% 6% 29% 34% 13% 1	543
	21st to end	Night	13% 4 29% 36% 17%	433
June	1st to 10th	Night	13% 5% 31% 39% 13% 1	400
	11th to 20th	Night	12% 5% 31% 41% 11%	400
	21st to end	Night	12% 6% 26% 41% 14% 1	400
July	1st to 10th	Night	5% 2 22% 50% 21% 1	390
	11th to 20th	Night	10% 2 23% 39% 24% 2	516
	21st to end	Night	8% 3 19% 47% 21% 1	657
August	1st to 10th	Night	10% 2 16% 42% 27% 3	597
	11th to 20th	Night	8% 3 17% 40% 27% 5%	594
	21st to end	Night	10% 4 22% 37% 25% 2	680
September	1st to 10th	Night	12% 5% 24% 32% 22% 5%	800
	11th to 20th	Night	10% 4 19% 35% 28% 4	800
	21st to end	Night	18% 7% 23% 31% 17% 5%	800
October	1st to 10th	Night	19% 7% 27% 29% 17% 2	800
	11th to 20th	Night	22% 13% 29% 26% 9% 1	800
	21st to end	Night	27% 11% 33% 24% 5%	880
November	1st to 10th	Night	19% 15% 30% 28% 8% 1	820
	11th to 20th	Night	24% 15% 36% 18% 6% 1	829
	21st to end	Night	25% 15% 33% 22% 5%	800
December	1st to 10th	Night	26% 19% 32% 18% 4	800
	11th to 20th	Night	24% 21% 36% 16% 4	800
	21st to end	Night	27% 17% 35% 18% 2	901

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