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 2024-11-08.

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 9% 5% 5% 5% 4 6% 7% 30%	101866
	Night	14% 12% 4 9% 5% 5% 4 4 5% 6% 31%	97177
Transparency	Day	50% 8% 13% 19% 10% 1	105456
	Night	50% 8% 12% 18% 10% 1	101676
Seeing	Day	18% 12% 29% 23% 12% 5%	15023
	Night	19% 11% 30% 28% 11% 1	24083

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% 19% 10% 15% 7% 4 4 3 3 4 18%	2454
	11th to 20th	Day	11% 19% 10% 22% 6% 6% 6% 5% 4 3 7%	2840
	21st to end	Day	19% 22% 8% 15% 7% 5% 5% 2 3 3 9%	3124
February	1st to 10th	Day	15% 19% 6% 14% 10% 7% 5% 3 4 4 14%	1710
	11th to 20th	Day	13% 14% 8% 14% 9% 6% 5% 3 5% 6% 17%	1663
	21st to end	Day	15% 16% 5% 13% 7% 4 5% 3 4 5% 22%	1546
March	1st to 10th	Day	18% 14% 7% 11% 6% 4 3 3 4 5% 26%	3362
	11th to 20th	Day	11% 15% 7% 10% 5% 6% 4 3 5% 7% 27%	3480
	21st to end	Day	16% 12% 4 11% 7% 5% 4 3 5% 5% 29%	3815
April	1st to 10th	Day	16% 16% 5% 8% 5% 4 4 4 6% 7% 27%	2175
	11th to 20th	Day	18% 12% 3 6% 4 4 4 3 5% 7% 34%	2414
	21st to end	Day	20% 11% 4 8% 4 4 3 2 4 7% 31%	2470
May	1st to 10th	Day	18% 9% 3 7% 4 5% 5% 4 6% 8% 32%	4108
	11th to 20th	Day	16% 9% 3 8% 5% 5% 4 4 6% 9% 30%	4162
	21st to end	Day	13% 6% 2 5% 4 4 4 4 5% 8% 44%	4837
June	1st to 10th	Day	14% 7% 2 7% 5% 5% 5% 4 7% 8% 36%	2660
	11th to 20th	Day	11% 6% 2 7% 4 4 5% 5% 7% 8% 39%	2836
	21st to end	Day	13% 7% 3 6% 4 5% 5% 4 7% 9% 38%	2850
July	1st to 10th	Day	11% 5% 2 6% 4 5% 6% 5% 7% 10% 38%	4760
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 5% 7% 8% 41%	4300
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 9% 41%	4466
August	1st to 10th	Day	14% 7% 2 7% 4 5% 5% 4 7% 9% 36%	4104
	11th to 20th	Day	11% 8% 3 7% 4 5% 7% 5% 7% 10% 34%	4044
	21st to end	Day	8% 6% 3 7% 5% 5% 6% 5% 7% 9% 39%	4454
September	1st to 10th	Day	11% 6% 2 7% 6% 5% 5% 5% 7% 8% 38%	2251
	11th to 20th	Day	11% 6% 2 5% 4 4 4 5% 6% 8% 43%	2090
	21st to end	Day	16% 7% 3 8% 5% 5% 4 4 6% 8% 34%	2090
October	1st to 10th	Day	17% 9% 3 8% 4 5% 5% 4 5% 6% 34%	1927
	11th to 20th	Day	21% 11% 4 8% 4 4 5% 4 6% 6% 27%	1900
	21st to end	Day	22% 14% 4 9% 5% 5% 5% 4 5% 6% 21%	2062
November	1st to 10th	Day	13% 15% 4 8% 5% 4 4 4 5% 6% 30%	1735
	11th to 20th	Day	14% 16% 5% 10% 6% 5% 5% 4 7% 7% 22%	1522
	21st to end	Day	18% 18% 8% 11% 6% 4 4 3 6% 7% 15%	1520
December	1st to 10th	Day	14% 22% 9% 14% 6% 5% 5% 3 4 4 13%	1520
	11th to 20th	Day	18% 18% 8% 15% 7% 5% 4 4 4 4 14%	1906
	21st to end	Day	28% 21% 8% 13% 6% 4 4 3 4 4 6%	2709
January	1st to 10th	Night	15% 19% 8% 17% 7% 6% 4 3 4 4 13%	4478
	11th to 20th	Night	13% 17% 8% 17% 9% 7% 5% 4 6% 5% 10%	4120
	21st to end	Night	19% 17% 9% 16% 6% 4 4 3 4 4 14%	4532
February	1st to 10th	Night	14% 18% 6% 13% 8% 5% 5% 4 4 6% 17%	2470
	11th to 20th	Night	12% 15% 6% 11% 8% 5% 4 4 6% 7% 21%	2403
	21st to end	Night	15% 13% 5% 12% 8% 5% 4 4 5% 5% 22%	1906
March	1st to 10th	Night	15% 11% 4 9% 5% 5% 4 4 5% 7% 32%	3476
	11th to 20th	Night	10% 10% 5% 9% 5% 5% 5% 4 7% 8% 33%	3479
	21st to end	Night	15% 10% 3 8% 5% 5% 4 3 4 5% 37%	3817
April	1st to 10th	Night	18% 11% 3 8% 5% 5% 4 3 5% 6% 32%	2005
	11th to 20th	Night	16% 10% 3 6% 5% 5% 4 4 5% 7% 34%	1730
	21st to end	Night	14% 11% 3 8% 5% 5% 5% 3 5% 7% 34%	1710
May	1st to 10th	Night	17% 8% 2 6% 4 4 4 3 4 7% 41%	2833
	11th to 20th	Night	12% 8% 3 8% 5% 5% 5% 4 6% 7% 35%	2728
	21st to end	Night	10% 5% 1 5% 4 4 4 4 5% 8% 51%	2738
June	1st to 10th	Night	11% 6% 3 5% 5% 5% 4 4 6% 7% 43%	1520
	11th to 20th	Night	9% 5% 2 6% 5% 6% 5% 3 8% 10% 41%	1339
	21st to end	Night	9% 7% 2 6% 4 4 4 4 6% 8% 46%	1330
July	1st to 10th	Night	5% 4 2 6% 3 4 4 4 7% 10% 52%	2210
	11th to 20th	Night	9% 6% 1 4 4 4 3 3 6% 7% 53%	2641
	21st to end	Night	6% 5% 1 7% 5% 4 5% 4 7% 8% 48%	3087
August	1st to 10th	Night	11% 6% 2 6% 4 4 4 4 6% 7% 45%	2825
	11th to 20th	Night	11% 8% 2 7% 5% 3 4 3 6% 7% 43%	2785
	21st to end	Night	9% 6% 3 6% 4 4 4 4 6% 7% 48%	3156
September	1st to 10th	Night	10% 7% 2 7% 4 5% 5% 3 6% 7% 46%	1927
	11th to 20th	Night	11% 4 1 4 3 3 3 3 6% 8% 53%	2090
	21st to end	Night	15% 7% 2 8% 4 4 4 3 6% 7% 39%	2090
October	1st to 10th	Night	14% 8% 3 7% 5% 5% 4 3 5% 7% 39%	2252
	11th to 20th	Night	18% 11% 2 7% 5% 4 4 3 5% 7% 32%	2280
	21st to end	Night	22% 10% 4 9% 5% 4 5% 4 6% 6% 26%	2536
November	1st to 10th	Night	14% 11% 4 8% 5% 4 4 3 6% 6% 34%	2504
	11th to 20th	Night	15% 16% 4 11% 5% 5% 5% 4 6% 7% 22%	2655
	21st to end	Night	18% 16% 7% 10% 5% 5% 4 3 5% 7% 20%	2660
December	1st to 10th	Night	17% 22% 8% 12% 6% 5% 4 3 4 4 14%	2660
	11th to 20th	Night	13% 21% 7% 14% 6% 5% 4 4 5% 5% 16%	3288
	21st to end	Night	26% 21% 9% 12% 5% 4 4 3 4 4 9%	4917

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	68% 9% 9% 9% 4	2455
	11th to 20th	Day	77% 10% 7% 3 2	2840
	21st to end	Day	78% 8% 6% 5% 3	3124
February	1st to 10th	Day	73% 8% 7% 9% 3 1	1710
	11th to 20th	Day	65% 8% 9% 12% 5%	1672
	21st to end	Day	64% 7% 10% 12% 6% 1	1546
March	1st to 10th	Day	60% 6% 11% 18% 5% 1	3365
	11th to 20th	Day	54% 8% 15% 15% 7% 1	3480
	21st to end	Day	52% 8% 13% 15% 10% 2	3815
April	1st to 10th	Day	55% 7% 12% 16% 10%	2175
	11th to 20th	Day	48% 8% 12% 20% 11% 1	2414
	21st to end	Day	53% 6% 11% 18% 10% 2	2470
May	1st to 10th	Day	45% 9% 13% 19% 11% 2	4120
	11th to 20th	Day	47% 11% 16% 19% 7% 1	4166
	21st to end	Day	35% 9% 13% 19% 19% 5%	4837
June	1st to 10th	Day	42% 8% 14% 21% 13% 1	2660
	11th to 20th	Day	39% 8% 13% 22% 16% 1	2836
	21st to end	Day	42% 8% 12% 24% 14%	2850
July	1st to 10th	Day	39% 6% 15% 24% 14% 2	4686
	11th to 20th	Day	36% 7% 13% 32% 11% 1	4224
	21st to end	Day	37% 7% 15% 23% 17% 1	4324
August	1st to 10th	Day	43% 7% 14% 22% 13% 1	4091
	11th to 20th	Day	42% 8% 18% 24% 8%	4083
	21st to end	Day	39% 7% 13% 30% 11% 1	4454
September	1st to 10th	Day	39% 10% 15% 23% 11% 1	2251
	11th to 20th	Day	36% 7% 15% 27% 14% 1	2090
	21st to end	Day	46% 9% 13% 23% 9% 1	2090
October	1st to 10th	Day	41% 6% 13% 22% 16% 1	3200
	11th to 20th	Day	42% 6% 14% 21% 17%	3140
	21st to end	Day	54% 6% 15% 19% 6%	3376
November	1st to 10th	Day	54% 7% 12% 19% 7% 1	1735
	11th to 20th	Day	58% 9% 13% 12% 6% 1	1522
	21st to end	Day	67% 8% 13% 10% 3	1520
December	1st to 10th	Day	73% 8% 9% 8% 2	1520
	11th to 20th	Day	73% 8% 9% 8% 2	1906
	21st to end	Day	79% 9% 8% 4 1	2709
January	1st to 10th	Night	71% 10% 9% 7% 3	4478
	11th to 20th	Night	73% 10% 9% 5% 2	4120
	21st to end	Night	74% 7% 8% 7% 3 1	4532
February	1st to 10th	Night	67% 8% 9% 12% 3	2470
	11th to 20th	Night	61% 8% 12% 13% 5%	2405
	21st to end	Night	61% 7% 12% 13% 7%	1906
March	1st to 10th	Night	49% 8% 16% 21% 6% 1	3486
	11th to 20th	Night	43% 11% 17% 18% 9% 1	3480
	21st to end	Night	45% 7% 11% 22% 13% 2	3817
April	1st to 10th	Night	53% 5% 11% 18% 13% 1	2005
	11th to 20th	Night	47% 7% 10% 18% 16% 2	1730
	21st to end	Night	48% 8% 10% 17% 14% 3	1710
May	1st to 10th	Night	42% 7% 12% 19% 16% 4	2840
	11th to 20th	Night	43% 10% 15% 20% 10% 2	2729
	21st to end	Night	29% 10% 13% 22% 22% 4	2739
June	1st to 10th	Night	38% 7% 13% 25% 15% 2	1520
	11th to 20th	Night	37% 7% 13% 24% 16% 2	1339
	21st to end	Night	36% 8% 15% 23% 18% 1	1330
July	1st to 10th	Night	27% 6% 16% 26% 20% 6%	2175
	11th to 20th	Night	31% 5% 13% 31% 19% 1	2594
	21st to end	Night	33% 7% 15% 24% 18% 2	2988
August	1st to 10th	Night	37% 6% 14% 25% 16% 2	2814
	11th to 20th	Night	38% 7% 16% 25% 13% 1	2812
	21st to end	Night	34% 7% 12% 32% 15% 1	3156
September	1st to 10th	Night	36% 9% 12% 26% 15% 2	1927
	11th to 20th	Night	29% 8% 13% 29% 20% 1	2090
	21st to end	Night	44% 8% 12% 24% 12%	2090
October	1st to 10th	Night	34% 6% 9% 27% 23% 1	3759
	11th to 20th	Night	44% 6% 14% 20% 15%	3779
	21st to end	Night	44% 6% 15% 25% 10%	4172
November	1st to 10th	Night	48% 8% 13% 20% 10% 1	2504
	11th to 20th	Night	58% 10% 11% 14% 6% 1	2655
	21st to end	Night	62% 9% 14% 11% 3	2660
December	1st to 10th	Night	72% 9% 9% 8% 2	2660
	11th to 20th	Night	69% 8% 12% 10% 2	3288
	21st to end	Night	79% 7% 9% 5% 1	4917

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% 23% 30% 19% 7% 1	216
	11th to 20th	Day	19% 19% 46% 12% 4 1	270
	21st to end	Day	24% 15% 35% 22% 3 1	297
February	1st to 10th	Day	24% 19% 40% 13% 4	270
	11th to 20th	Day	21% 23% 41% 13% 2	270
	21st to end	Day	27% 29% 28% 14% 1	225
March	1st to 10th	Day	23% 13% 41% 17% 6%	412
	11th to 20th	Day	20% 16% 36% 21% 6% 1	460
	21st to end	Day	25% 13% 34% 21% 6% 1	506
April	1st to 10th	Day	20% 18% 36% 20% 6% 2	460
	11th to 20th	Day	27% 15% 27% 17% 10% 4	452
	21st to end	Day	24% 13% 32% 21% 7% 2	460
May	1st to 10th	Day	23% 10% 28% 25% 10% 4	463
	11th to 20th	Day	18% 9% 29% 23% 15% 6%	487
	21st to end	Day	15% 7% 26% 28% 17% 7%	529
June	1st to 10th	Day	14% 8% 32% 28% 13% 4	490
	11th to 20th	Day	13% 9% 30% 31% 14% 3	666
	21st to end	Day	15% 9% 30% 31% 12% 3	680
July	1st to 10th	Day	9% 6% 24% 29% 23% 8%	671
	11th to 20th	Day	14% 5% 27% 26% 20% 9%	533
	21st to end	Day	9% 4 26% 33% 20% 8%	532
August	1st to 10th	Day	11% 6% 21% 27% 24% 12%	486
	11th to 20th	Day	10% 5% 19% 31% 21% 15%	479
	21st to end	Day	15% 6% 22% 27% 20% 11%	539
September	1st to 10th	Day	12% 7% 22% 28% 18% 12%	490
	11th to 20th	Day	8% 5% 18% 28% 26% 14%	490
	21st to end	Day	15% 11% 28% 23% 14% 8%	490
October	1st to 10th	Day	20% 11% 27% 21% 16% 6%	490
	11th to 20th	Day	21% 15% 33% 22% 7% 2	490
	21st to end	Day	26% 17% 31% 20% 5% 1	511
November	1st to 10th	Day	21% 23% 31% 15% 8% 1	291
	11th to 20th	Day	23% 19% 34% 14% 7% 1	180
	21st to end	Day	24% 19% 36% 18% 2	180
December	1st to 10th	Day	26% 24% 33% 11% 6%	180
	11th to 20th	Day	27% 27% 29% 14% 2	180
	21st to end	Day	31% 22% 34% 12% 1	198
January	1st to 10th	Night	22% 18% 37% 17% 5% 1	864
	11th to 20th	Night	29% 16% 33% 19% 3	760
	21st to end	Night	19% 11% 39% 25% 6%	836
February	1st to 10th	Night	23% 16% 38% 22% 2	760
	11th to 20th	Night	21% 16% 35% 23% 5%	743
	21st to end	Night	23% 17% 35% 22% 3	628
March	1st to 10th	Night	18% 14% 39% 23% 6%	760
	11th to 20th	Night	17% 17% 38% 21% 7%	760
	21st to end	Night	21% 13% 32% 24% 9% 1	836
April	1st to 10th	Night	22% 14% 32% 28% 4	760
	11th to 20th	Night	20% 13% 33% 23% 10% 1	606
	21st to end	Night	18% 12% 31% 31% 7% 1	570
May	1st to 10th	Night	18% 8% 30% 30% 12% 1	570
	11th to 20th	Night	17% 6% 28% 34% 13% 1	516
	21st to end	Night	11% 4 29% 37% 18%	411
June	1st to 10th	Night	13% 5% 30% 39% 13% 1	380
	11th to 20th	Night	11% 5% 31% 41% 11%	380
	21st to end	Night	12% 6% 25% 42% 14% 1	380
July	1st to 10th	Night	5% 2 22% 49% 21% 1	370
	11th to 20th	Night	9% 2 24% 38% 24% 2	489
	21st to end	Night	8% 3 19% 47% 21% 1	624
August	1st to 10th	Night	11% 2 16% 42% 27% 2	567
	11th to 20th	Night	8% 3 17% 39% 28% 5%	564
	21st to end	Night	10% 4 22% 36% 25% 2	646
September	1st to 10th	Night	12% 5% 24% 33% 21% 5%	760
	11th to 20th	Night	11% 4 19% 34% 27% 4	760
	21st to end	Night	18% 7% 23% 31% 17% 5%	760
October	1st to 10th	Night	19% 7% 27% 28% 16% 2	760
	11th to 20th	Night	22% 13% 28% 27% 9% 1	760
	21st to end	Night	27% 11% 33% 24% 5%	836
November	1st to 10th	Night	19% 14% 29% 28% 8% 1	771
	11th to 20th	Night	23% 14% 36% 19% 6% 1	760
	21st to end	Night	25% 14% 33% 22% 5%	760
December	1st to 10th	Night	26% 19% 31% 18% 4	760
	11th to 20th	Night	23% 21% 36% 16% 4	760
	21st to end	Night	28% 17% 36% 18% 2	856

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