Fort Erie Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fort Erie Observatory clear sky chart from 2005-11-06 to 2026-06-16.

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	16% 11% 4 9% 5% 5% 4 4 5% 6% 30%	137917
	Night	17% 13% 4 10% 6% 5% 4 3 5% 6% 27%	134060
Transparency	Day	54% 5% 12% 19% 9% 1	141512
	Night	59% 5% 10% 16% 9% 1	138553
Seeing	Day	24% 17% 28% 19% 9% 3	16527
	Night	24% 15% 27% 24% 9% 1	25985

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	18% 21% 7% 15% 6% 5% 4 3 4 5% 12%	4661
	11th to 20th	Day	12% 24% 8% 21% 11% 7% 5% 2 3 2 5%	5125
	21st to end	Day	15% 16% 7% 16% 11% 10% 6% 4 4 3 9%	5780
February	1st to 10th	Day	17% 19% 5% 15% 8% 5% 7% 4 4 4 13%	1890
	11th to 20th	Day	14% 19% 7% 14% 8% 6% 5% 4 3 5% 16%	1843
	21st to end	Day	20% 14% 5% 12% 8% 5% 5% 4 5% 5% 17%	1638
March	1st to 10th	Day	17% 14% 6% 11% 6% 4 3 3 4 5% 27%	5042
	11th to 20th	Day	11% 16% 4 8% 5% 5% 5% 4 5% 6% 31%	5090
	21st to end	Day	21% 12% 4 9% 6% 4 4 3 5% 5% 27%	5586
April	1st to 10th	Day	19% 14% 3 8% 4 5% 3 3 4 6% 30%	2499
	11th to 20th	Day	20% 11% 3 7% 4 4 4 3 5% 6% 33%	2513
	21st to end	Day	21% 10% 3 7% 5% 4 4 3 5% 5% 34%	2730
May	1st to 10th	Day	22% 10% 3 7% 5% 5% 5% 4 6% 7% 29%	7642
	11th to 20th	Day	18% 9% 3 8% 4 4 4 4 6% 8% 32%	8162
	21st to end	Day	20% 7% 2 6% 4 4 4 4 5% 7% 38%	9060
June	1st to 10th	Day	15% 8% 2 7% 5% 4 4 4 6% 7% 39%	2940
	11th to 20th	Day	13% 5% 3 7% 5% 5% 5% 4 7% 9% 39%	2870
	21st to end	Day	14% 6% 2 5% 4 3 5% 3 6% 9% 43%	2800
July	1st to 10th	Day	10% 6% 2 6% 4 4 5% 4 6% 9% 45%	6360
	11th to 20th	Day	13% 6% 3 6% 4 5% 4 4 7% 8% 41%	6360
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 8% 47%	6456
August	1st to 10th	Day	16% 6% 2 6% 4 4 5% 3 6% 7% 40%	4234
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 4 7% 11% 34%	4174
	21st to end	Day	11% 5% 2 6% 4 4 4 3 6% 9% 45%	4422
September	1st to 10th	Day	14% 7% 2 5% 4 4 4 4 5% 7% 43%	2400
	11th to 20th	Day	11% 4 2 6% 4 4 4 4 6% 8% 47%	2315
	21st to end	Day	15% 8% 3 7% 5% 6% 5% 4 6% 8% 32%	2137
October	1st to 10th	Day	19% 8% 3 6% 6% 4 4 3 6% 6% 36%	2000
	11th to 20th	Day	21% 10% 3 8% 4 5% 5% 4 5% 7% 29%	2000
	21st to end	Day	26% 14% 3 10% 4 5% 4 3 4 5% 22%	2200
November	1st to 10th	Day	16% 12% 3 9% 5% 5% 5% 4 5% 6% 32%	1834
	11th to 20th	Day	15% 15% 3 11% 5% 5% 5% 4 5% 5% 28%	1680
	21st to end	Day	21% 19% 4 9% 5% 5% 5% 4 4 6% 18%	1680
December	1st to 10th	Day	21% 18% 7% 13% 7% 5% 5% 4 5% 4 11%	2680
	11th to 20th	Day	22% 21% 5% 10% 5% 5% 4 3 4 5% 16%	3066
	21st to end	Day	23% 23% 6% 12% 7% 6% 4 3 3 4 12%	4048
January	1st to 10th	Night	18% 21% 7% 13% 6% 4 4 3 3 4 16%	8234
	11th to 20th	Night	13% 22% 9% 22% 10% 7% 4 3 3 3 7%	7745
	21st to end	Night	17% 18% 6% 17% 10% 8% 5% 3 4 4 10%	8401
February	1st to 10th	Night	18% 19% 6% 13% 6% 5% 4 3 4 4 17%	2730
	11th to 20th	Night	16% 16% 6% 13% 7% 5% 5% 3 4 6% 18%	2663
	21st to end	Night	19% 17% 6% 13% 7% 4 4 3 4 5% 18%	2165
March	1st to 10th	Night	18% 14% 5% 9% 6% 5% 4 3 4 4 28%	5018
	11th to 20th	Night	16% 14% 3 6% 4 4 4 3 5% 7% 33%	5089
	21st to end	Night	18% 11% 4 9% 5% 4 4 3 5% 5% 33%	5588
April	1st to 10th	Night	21% 13% 4 8% 6% 4 4 2 5% 4 29%	2121
	11th to 20th	Night	19% 10% 3 8% 5% 4 4 4 5% 6% 33%	2074
	21st to end	Night	18% 9% 3 6% 4 5% 5% 4 5% 7% 35%	1890
May	1st to 10th	Night	25% 11% 3 8% 5% 4 4 3 4 5% 27%	5257
	11th to 20th	Night	17% 8% 3 8% 5% 4 5% 4 6% 8% 32%	4683
	21st to end	Night	19% 8% 3 6% 3 4 4 4 5% 7% 36%	5114
June	1st to 10th	Night	11% 9% 3 6% 5% 5% 5% 5% 7% 7% 38%	1680
	11th to 20th	Night	12% 5% 2 6% 5% 5% 5% 5% 6% 8% 40%	1640
	21st to end	Night	15% 6% 2 5% 5% 4 4 5% 6% 8% 40%	1600
July	1st to 10th	Night	11% 5% 2 7% 4 4 4 4 6% 10% 42%	3600
	11th to 20th	Night	11% 6% 2 6% 5% 4 4 4 6% 8% 43%	3600
	21st to end	Night	11% 5% 2 6% 4 4 5% 4 6% 7% 48%	4379
August	1st to 10th	Night	11% 8% 3 7% 5% 4 4 4 6% 8% 41%	2915
	11th to 20th	Night	14% 8% 3 7% 5% 5% 4 4 6% 7% 37%	2875
	21st to end	Night	9% 5% 2 6% 5% 4 6% 4 6% 9% 45%	3441
September	1st to 10th	Night	14% 6% 2 5% 4 3 4 3 5% 8% 45%	2000
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 5% 8% 52%	2081
	21st to end	Night	17% 8% 3 7% 5% 4 4 5% 6% 6% 35%	2262
October	1st to 10th	Night	18% 6% 3 7% 5% 5% 4 3 5% 7% 40%	2400
	11th to 20th	Night	22% 10% 3 8% 4 5% 4 4 5% 6% 30%	2400
	21st to end	Night	26% 9% 3 7% 4 5% 5% 4 5% 6% 27%	2640
November	1st to 10th	Night	18% 10% 3 7% 5% 4 4 3 5% 6% 36%	2677
	11th to 20th	Night	16% 14% 4 10% 6% 5% 4 3 5% 6% 27%	2940
	21st to end	Night	20% 16% 5% 10% 6% 5% 4 4 5% 6% 20%	2940
December	1st to 10th	Night	19% 22% 7% 14% 6% 4 4 3 5% 5% 11%	4720
	11th to 20th	Night	21% 23% 5% 12% 7% 5% 4 3 4 4 14%	5348
	21st to end	Night	25% 20% 5% 11% 6% 5% 4 3 4 4 13%	7150

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	78% 4 6% 7% 4	4662
	11th to 20th	Day	86% 5% 4 3 1	5159
	21st to end	Day	81% 4 8% 5% 1	5780
February	1st to 10th	Day	75% 6% 10% 8% 1	1890
	11th to 20th	Day	71% 6% 9% 11% 3	1852
	21st to end	Day	68% 6% 11% 11% 4	1638
March	1st to 10th	Day	60% 4 13% 17% 6%	5045
	11th to 20th	Day	53% 5% 18% 16% 6% 1	5090
	21st to end	Day	59% 6% 7% 20% 7% 1	5586
April	1st to 10th	Day	57% 5% 10% 18% 9% 1	2499
	11th to 20th	Day	51% 5% 10% 20% 11% 2	2513
	21st to end	Day	52% 5% 11% 19% 12% 2	2730
May	1st to 10th	Day	54% 4 11% 20% 9% 3	7651
	11th to 20th	Day	49% 7% 10% 23% 10%	8166
	21st to end	Day	46% 4 11% 24% 12% 2	9061
June	1st to 10th	Day	42% 8% 12% 21% 16% 1	2940
	11th to 20th	Day	41% 8% 15% 23% 11% 1	2870
	21st to end	Day	39% 8% 14% 24% 14% 1	2800
July	1st to 10th	Day	37% 4 13% 29% 17% 1	6290
	11th to 20th	Day	41% 4 14% 25% 16% 1	6285
	21st to end	Day	36% 5% 15% 26% 17%	6271
August	1st to 10th	Day	43% 7% 14% 23% 13% 1	4221
	11th to 20th	Day	45% 6% 17% 25% 8%	4213
	21st to end	Day	37% 4 12% 35% 11% 1	4422
September	1st to 10th	Day	41% 7% 13% 26% 12% 1	2400
	11th to 20th	Day	35% 7% 13% 30% 13% 1	2315
	21st to end	Day	49% 6% 14% 20% 9% 1	2137
October	1st to 10th	Day	42% 5% 12% 27% 13% 1	3260
	11th to 20th	Day	47% 4 16% 20% 12% 1	3240
	21st to end	Day	53% 3 14% 22% 8% 1	3538
November	1st to 10th	Day	56% 6% 11% 18% 8% 1	1834
	11th to 20th	Day	58% 7% 14% 14% 5% 2	1680
	21st to end	Day	70% 6% 11% 10% 3	1680
December	1st to 10th	Day	78% 5% 9% 7% 1	2680
	11th to 20th	Day	74% 5% 10% 9% 2	3066
	21st to end	Day	78% 5% 8% 7% 2	4048
January	1st to 10th	Night	76% 5% 6% 10% 3	8234
	11th to 20th	Night	84% 5% 4 4 2	7786
	21st to end	Night	81% 4 6% 6% 2	8401
February	1st to 10th	Night	74% 5% 9% 10% 2	2730
	11th to 20th	Night	69% 6% 10% 12% 3	2665
	21st to end	Night	69% 5% 9% 11% 5%	2165
March	1st to 10th	Night	61% 4 10% 17% 7% 1	5028
	11th to 20th	Night	52% 5% 15% 19% 8% 2	5090
	21st to end	Night	54% 4 10% 18% 12% 1	5588
April	1st to 10th	Night	59% 4 9% 18% 9% 1	2121
	11th to 20th	Night	51% 5% 11% 19% 13% 1	2074
	21st to end	Night	49% 6% 10% 20% 13% 2	1890
May	1st to 10th	Night	60% 5% 6% 16% 10% 4	5264
	11th to 20th	Night	49% 6% 13% 19% 12% 1	4684
	21st to end	Night	48% 4 9% 23% 15% 2	5115
June	1st to 10th	Night	43% 8% 9% 22% 16% 2	1680
	11th to 20th	Night	41% 7% 13% 21% 17% 1	1640
	21st to end	Night	42% 7% 14% 21% 14% 2	1600
July	1st to 10th	Night	40% 3 12% 26% 17% 2	3560
	11th to 20th	Night	40% 3 13% 27% 16% 1	3552
	21st to end	Night	36% 4 14% 23% 21% 2	4250
August	1st to 10th	Night	41% 6% 14% 26% 11% 1	2904
	11th to 20th	Night	47% 5% 15% 24% 8%	2902
	21st to end	Night	37% 4 13% 32% 13% 1	3441
September	1st to 10th	Night	38% 7% 13% 26% 14% 2	2000
	11th to 20th	Night	33% 5% 13% 30% 17% 1	2081
	21st to end	Night	47% 8% 12% 20% 11% 1	2262
October	1st to 10th	Night	44% 4 8% 25% 18% 1	3920
	11th to 20th	Night	51% 4 13% 19% 13%	3899
	21st to end	Night	48% 4 15% 24% 8% 1	4252
November	1st to 10th	Night	52% 6% 10% 21% 10% 2	2677
	11th to 20th	Night	60% 7% 12% 13% 6% 2	2940
	21st to end	Night	67% 8% 12% 11% 3	2940
December	1st to 10th	Night	78% 5% 11% 5% 1	4720
	11th to 20th	Night	77% 5% 7% 9% 2	5348
	21st to end	Night	76% 5% 7% 9% 2	7150

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	27% 30% 27% 10% 4 1	220
	11th to 20th	Day	30% 29% 27% 10% 4 1	308
	21st to end	Day	28% 26% 26% 16% 3	363
February	1st to 10th	Day	32% 25% 28% 12% 2	330
	11th to 20th	Day	32% 31% 28% 8% 1	330
	21st to end	Day	39% 21% 29% 10% 1	333
March	1st to 10th	Day	29% 21% 30% 15% 4 1	540
	11th to 20th	Day	34% 16% 31% 14% 4 1	540
	21st to end	Day	32% 20% 25% 16% 5% 1	594
April	1st to 10th	Day	28% 22% 28% 15% 5% 1	540
	11th to 20th	Day	34% 20% 22% 15% 6% 2	532
	21st to end	Day	30% 17% 29% 16% 7% 1	540
May	1st to 10th	Day	28% 16% 30% 15% 7% 3	543
	11th to 20th	Day	23% 14% 31% 19% 10% 3	567
	21st to end	Day	20% 14% 32% 21% 9% 4	617
June	1st to 10th	Day	22% 14% 34% 21% 7% 1	570
	11th to 20th	Day	16% 17% 31% 23% 10% 3	550
	21st to end	Day	20% 15% 31% 25% 8% 2	530
July	1st to 10th	Day	11% 10% 27% 31% 17% 4	525
	11th to 20th	Day	16% 11% 30% 26% 13% 5%	524
	21st to end	Day	11% 9% 35% 28% 12% 5%	576
August	1st to 10th	Day	15% 8% 23% 30% 16% 8%	526
	11th to 20th	Day	13% 8% 28% 26% 16% 8%	519
	21st to end	Day	15% 8% 32% 23% 15% 7%	583
September	1st to 10th	Day	16% 13% 23% 23% 17% 8%	530
	11th to 20th	Day	11% 7% 21% 25% 24% 13%	530
	21st to end	Day	18% 15% 28% 22% 12% 6%	530
October	1st to 10th	Day	24% 11% 26% 20% 14% 5%	530
	11th to 20th	Day	26% 20% 29% 18% 5% 2	530
	21st to end	Day	35% 19% 24% 15% 7%	583
November	1st to 10th	Day	27% 21% 29% 16% 6% 1	372
	11th to 20th	Day	34% 24% 27% 10% 3 2	220
	21st to end	Day	35% 28% 25% 10% 2	220
December	1st to 10th	Day	33% 24% 29% 10% 4	220
	11th to 20th	Day	35% 26% 25% 13% 2	220
	21st to end	Day	36% 29% 21% 10% 2 1	242
January	1st to 10th	Night	31% 25% 28% 13% 4	840
	11th to 20th	Night	33% 24% 25% 15% 3	840
	21st to end	Night	26% 23% 31% 16% 5%	924
February	1st to 10th	Night	27% 25% 28% 16% 2	840
	11th to 20th	Night	27% 23% 30% 17% 3	823
	21st to end	Night	29% 22% 29% 17% 2	692
March	1st to 10th	Night	25% 19% 31% 21% 5%	840
	11th to 20th	Night	29% 17% 30% 19% 5%	840
	21st to end	Night	26% 14% 31% 21% 7% 1	924
April	1st to 10th	Night	27% 16% 32% 21% 3	840
	11th to 20th	Night	25% 16% 28% 23% 7% 1	830
	21st to end	Night	23% 9% 31% 29% 8%	630
May	1st to 10th	Night	23% 10% 23% 30% 11% 2	630
	11th to 20th	Night	16% 8% 32% 36% 8%	433
	21st to end	Night	16% 10% 25% 33% 14% 1	455
June	1st to 10th	Night	14% 8% 31% 35% 12%	420
	11th to 20th	Night	15% 10% 29% 34% 12%	410
	21st to end	Night	17% 7% 33% 32% 11%	400
July	1st to 10th	Night	10% 5% 26% 42% 16% 1	390
	11th to 20th	Night	12% 7% 28% 33% 19% 1	388
	21st to end	Night	13% 4 25% 38% 18% 2	634
August	1st to 10th	Night	12% 4 20% 40% 23% 2	597
	11th to 20th	Night	11% 6% 20% 34% 26% 3	594
	21st to end	Night	14% 5% 24% 32% 23% 2	787
September	1st to 10th	Night	17% 8% 22% 33% 18% 3	800
	11th to 20th	Night	14% 5% 21% 30% 27% 5%	800
	21st to end	Night	20% 10% 24% 28% 16% 3	800
October	1st to 10th	Night	19% 10% 27% 26% 16% 2	800
	11th to 20th	Night	28% 15% 25% 23% 8% 1	800
	21st to end	Night	32% 14% 27% 23% 4	880
November	1st to 10th	Night	25% 16% 27% 23% 9% 1	820
	11th to 20th	Night	25% 21% 28% 17% 8% 1	840
	21st to end	Night	28% 22% 28% 16% 5%	840
December	1st to 10th	Night	34% 23% 23% 15% 4	840
	11th to 20th	Night	30% 27% 24% 15% 3	840
	21st to end	Night	35% 20% 27% 16% 2	924

Page generated at 2026-06-13 02:31:44. Version ac07

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