Wharf Road Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wharf Road Observatory clear sky chart from 2005-11-06 to 2025-02-25.

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	18% 12% 4 10% 5% 4 4 3 5% 6% 29%	108670
	Night	19% 15% 5% 12% 6% 4 4 3 4 5% 23%	110073
Transparency	Day	57% 4 11% 18% 9% 1	112262
	Night	62% 5% 10% 16% 7% 1	114645
Seeing	Day	31% 19% 30% 15% 4 1	17013
	Night	31% 18% 28% 18% 5%	24945

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% 25% 8% 18% 10% 4 2 2 2 2 11%	3157
	11th to 20th	Day	15% 21% 11% 25% 7% 5% 3 2 2 2 7%	3745
	21st to end	Day	14% 17% 8% 20% 11% 6% 3 2 3 4 12%	4466
February	1st to 10th	Day	18% 18% 8% 15% 11% 5% 4 3 5% 4 10%	1800
	11th to 20th	Day	18% 14% 5% 16% 13% 5% 3 3 3 4 16%	1753
	21st to end	Day	11% 16% 7% 12% 9% 5% 4 3 4 5% 25%	1534
March	1st to 10th	Day	25% 18% 5% 10% 7% 4 3 2 3 4 21%	3363
	11th to 20th	Day	16% 15% 5% 11% 6% 5% 4 2 5% 5% 26%	3480
	21st to end	Day	28% 13% 6% 11% 5% 3 4 3 3 3 23%	3816
April	1st to 10th	Day	21% 15% 3 6% 4 3 3 3 4 6% 30%	2204
	11th to 20th	Day	20% 10% 3 5% 3 3 3 2 4 7% 39%	2418
	21st to end	Day	20% 11% 3 6% 4 4 3 3 4 6% 35%	2470
May	1st to 10th	Day	19% 11% 3 8% 5% 4 4 4 6% 6% 29%	4106
	11th to 20th	Day	16% 7% 2 7% 5% 5% 4 3 6% 8% 36%	4324
	21st to end	Day	16% 7% 2 6% 5% 4 4 3 6% 8% 37%	4836
June	1st to 10th	Day	23% 9% 2 6% 5% 4 4 4 5% 7% 32%	2771
	11th to 20th	Day	20% 8% 2 6% 4 3 4 4 6% 7% 37%	2850
	21st to end	Day	22% 9% 3 8% 5% 5% 4 3 5% 6% 31%	2850
July	1st to 10th	Day	20% 8% 3 6% 4 4 4 4 6% 8% 33%	4760
	11th to 20th	Day	22% 8% 2 6% 4 4 4 3 6% 8% 33%	4642
	21st to end	Day	15% 7% 3 6% 4 4 4 4 6% 8% 38%	4468
August	1st to 10th	Day	22% 11% 3 7% 4 4 4 4 5% 6% 31%	4105
	11th to 20th	Day	13% 7% 3 7% 5% 4 4 4 5% 9% 40%	4045
	21st to end	Day	10% 6% 2 5% 4 3 3 3 5% 6% 53%	4470
September	1st to 10th	Day	17% 6% 2 5% 3 3 3 3 4 7% 48%	2280
	11th to 20th	Day	15% 7% 3 6% 3 3 4 3 5% 6% 46%	2103
	21st to end	Day	18% 6% 3 6% 3 3 3 3 5% 5% 46%	2090
October	1st to 10th	Day	17% 9% 2 6% 4 3 4 3 5% 7% 41%	1905
	11th to 20th	Day	18% 10% 3 8% 3 4 3 3 5% 7% 35%	1900
	21st to end	Day	22% 12% 3 7% 4 4 4 3 4 6% 32%	2043
November	1st to 10th	Day	19% 13% 3 8% 5% 4 4 3 4 7% 29%	1680
	11th to 20th	Day	19% 19% 7% 8% 5% 4 4 3 4 6% 21%	1600
	21st to end	Day	21% 19% 7% 11% 6% 5% 3 3 4 4 17%	1600
December	1st to 10th	Day	20% 20% 9% 18% 5% 3 3 2 3 4 12%	2610
	11th to 20th	Day	24% 17% 6% 15% 7% 5% 3 2 3 4 14%	2950
	21st to end	Day	19% 18% 8% 17% 7% 4 3 2 3 4 15%	3476
January	1st to 10th	Night	15% 24% 9% 23% 7% 3 3 2 3 3 10%	6808
	11th to 20th	Night	16% 23% 11% 22% 5% 4 3 2 3 3 9%	6137
	21st to end	Night	14% 15% 7% 21% 13% 5% 4 3 4 5% 10%	6490
February	1st to 10th	Night	20% 19% 7% 18% 9% 4 3 3 3 4 10%	2600
	11th to 20th	Night	18% 17% 6% 16% 10% 6% 4 3 4 5% 13%	2533
	21st to end	Night	13% 16% 6% 14% 7% 4 5% 3 4 6% 22%	2014
March	1st to 10th	Night	22% 16% 7% 13% 9% 4 2 2 2 4 19%	3476
	11th to 20th	Night	17% 17% 6% 13% 7% 6% 5% 3 5% 5% 18%	3479
	21st to end	Night	28% 13% 6% 9% 5% 4 3 2 3 3 23%	3816
April	1st to 10th	Night	23% 15% 4 7% 4 4 3 2 5% 5% 27%	1976
	11th to 20th	Night	22% 12% 3 6% 4 3 4 4 6% 7% 29%	1708
	21st to end	Night	23% 11% 3 7% 4 4 4 3 4 7% 31%	1710
May	1st to 10th	Night	19% 16% 4 8% 4 4 3 3 5% 6% 26%	2834
	11th to 20th	Night	18% 10% 3 8% 5% 5% 4 4 6% 8% 31%	2577
	21st to end	Night	16% 10% 3 9% 5% 5% 5% 4 6% 7% 29%	2749
June	1st to 10th	Night	26% 8% 3 7% 4 4 4 4 7% 9% 26%	1423
	11th to 20th	Night	20% 8% 2 5% 4 3 5% 4 8% 8% 33%	1330
	21st to end	Night	20% 9% 4 9% 5% 7% 5% 4 5% 7% 25%	1330
July	1st to 10th	Night	24% 11% 3 8% 4 4 4 4 6% 7% 26%	2210
	11th to 20th	Night	24% 9% 3 6% 4 4 5% 4 6% 7% 27%	2293
	21st to end	Night	15% 8% 2 7% 5% 4 5% 4 6% 7% 36%	3082
August	1st to 10th	Night	23% 10% 3 7% 4 3 4 4 5% 7% 30%	2825
	11th to 20th	Night	16% 9% 4 7% 6% 5% 5% 4 6% 8% 30%	2782
	21st to end	Night	11% 7% 3 7% 4 4 4 3 6% 8% 44%	3184
September	1st to 10th	Night	15% 6% 3 5% 3 4 4 4 6% 7% 44%	1900
	11th to 20th	Night	17% 8% 3 5% 3 3 4 3 5% 7% 42%	2076
	21st to end	Night	17% 7% 2 6% 5% 5% 4 3 6% 6% 39%	2090
October	1st to 10th	Night	19% 7% 2 6% 4 4 4 3 5% 7% 39%	2275
	11th to 20th	Night	17% 9% 3 7% 4 4 4 3 5% 7% 37%	2280
	21st to end	Night	25% 14% 3 7% 4 3 4 2 4 5% 30%	2551
November	1st to 10th	Night	22% 12% 3 7% 5% 4 4 3 5% 6% 30%	2617
	11th to 20th	Night	21% 20% 6% 9% 4 4 4 3 4 5% 19%	2800
	21st to end	Night	22% 19% 6% 10% 4 4 3 3 4 5% 19%	2800
December	1st to 10th	Night	18% 22% 10% 17% 6% 3 3 3 4 5% 9%	4580
	11th to 20th	Night	17% 18% 6% 18% 7% 4 3 2 3 4 17%	5236
	21st to end	Night	22% 21% 7% 14% 6% 2 2 2 3 4 17%	7502

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	84% 2 10% 4 1	3158
	11th to 20th	Day	86% 4 5% 5% 1	3778
	21st to end	Day	76% 7% 8% 6% 3 1	4466
February	1st to 10th	Day	79% 4 8% 6% 3	1800
	11th to 20th	Day	75% 4 8% 9% 3	1761
	21st to end	Day	62% 5% 10% 15% 6% 1	1534
March	1st to 10th	Day	70% 3 7% 15% 4 1	3367
	11th to 20th	Day	63% 4 11% 15% 7% 1	3480
	21st to end	Day	71% 2 6% 12% 8% 1	3816
April	1st to 10th	Day	60% 3 8% 16% 12% 2	2204
	11th to 20th	Day	49% 3 9% 25% 13% 1	2418
	21st to end	Day	53% 2 10% 19% 13% 2	2470
May	1st to 10th	Day	57% 3 13% 20% 6% 1	4120
	11th to 20th	Day	49% 3 17% 21% 9% 1	4328
	21st to end	Day	45% 4 19% 18% 13%	4836
June	1st to 10th	Day	53% 4 11% 22% 10%	2771
	11th to 20th	Day	46% 4 13% 22% 13% 2	2850
	21st to end	Day	55% 5% 11% 18% 11% 1	2850
July	1st to 10th	Day	48% 4 13% 21% 12% 1	4685
	11th to 20th	Day	48% 5% 13% 25% 9%	4561
	21st to end	Day	43% 7% 15% 23% 11% 1	4325
August	1st to 10th	Day	56% 4 10% 19% 10%	4092
	11th to 20th	Day	42% 6% 17% 25% 11%	4084
	21st to end	Day	32% 4 15% 28% 20% 1	4470
September	1st to 10th	Day	39% 3 12% 29% 15% 2	2280
	11th to 20th	Day	40% 4 12% 26% 17% 2	2103
	21st to end	Day	43% 3 9% 27% 15% 2	2090
October	1st to 10th	Day	45% 5% 11% 28% 11%	3165
	11th to 20th	Day	47% 4 9% 28% 13%	3140
	21st to end	Day	46% 3 13% 23% 14% 1	3345
November	1st to 10th	Day	58% 4 11% 19% 7% 1	1680
	11th to 20th	Day	67% 4 10% 15% 4 1	1600
	21st to end	Day	73% 4 10% 11% 2	1600
December	1st to 10th	Day	78% 4 6% 10% 1	2610
	11th to 20th	Day	75% 5% 7% 9% 3 1	2949
	21st to end	Day	75% 4 10% 6% 6%	3476
January	1st to 10th	Night	81% 4 7% 7% 1	6808
	11th to 20th	Night	82% 4 6% 6% 2	6179
	21st to end	Night	77% 5% 8% 8% 2	6490
February	1st to 10th	Night	80% 4 6% 7% 4	2600
	11th to 20th	Night	77% 3 7% 9% 4	2536
	21st to end	Night	64% 5% 10% 15% 6% 1	2014
March	1st to 10th	Night	69% 6% 6% 12% 6% 1	3485
	11th to 20th	Night	68% 5% 9% 11% 6% 1	3480
	21st to end	Night	68% 3 7% 13% 7% 1	3816
April	1st to 10th	Night	61% 4 9% 14% 11% 3	1976
	11th to 20th	Night	54% 4 9% 20% 12% 1	1708
	21st to end	Night	56% 4 10% 17% 13% 1	1710
May	1st to 10th	Night	60% 3 12% 19% 6% 1	2840
	11th to 20th	Night	51% 4 17% 21% 6% 1	2578
	21st to end	Night	51% 6% 18% 19% 6%	2750
June	1st to 10th	Night	53% 5% 13% 21% 8% 1	1423
	11th to 20th	Night	46% 5% 14% 23% 10% 1	1330
	21st to end	Night	54% 8% 11% 15% 8% 3	1330
July	1st to 10th	Night	55% 5% 12% 16% 11%	2175
	11th to 20th	Night	51% 8% 16% 16% 9%	2251
	21st to end	Night	45% 6% 14% 23% 12% 1	2985
August	1st to 10th	Night	54% 4 13% 23% 7%	2813
	11th to 20th	Night	46% 7% 18% 22% 7%	2809
	21st to end	Night	35% 7% 15% 26% 16% 1	3183
September	1st to 10th	Night	39% 5% 11% 29% 14% 1	1900
	11th to 20th	Night	41% 4 11% 28% 16% 1	2076
	21st to end	Night	45% 4 9% 25% 15% 2	2090
October	1st to 10th	Night	47% 4 13% 24% 11% 1	3795
	11th to 20th	Night	43% 4 12% 25% 15%	3779
	21st to end	Night	49% 3 9% 25% 13%	4199
November	1st to 10th	Night	55% 6% 12% 18% 8%	2617
	11th to 20th	Night	67% 5% 9% 13% 5%	2800
	21st to end	Night	68% 5% 13% 11% 3	2800
December	1st to 10th	Night	78% 5% 8% 8% 1	4580
	11th to 20th	Night	72% 5% 9% 13% 2	5238
	21st to end	Night	74% 3 7% 11% 4	7502

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	46% 25% 21% 9%	200
	11th to 20th	Day	41% 25% 24% 9% 2	302
	21st to end	Day	30% 27% 33% 9% 1	440
February	1st to 10th	Day	37% 26% 27% 9% 1	400
	11th to 20th	Day	41% 28% 21% 7% 2	396
	21st to end	Day	33% 25% 28% 12% 1	361
March	1st to 10th	Day	37% 23% 31% 7% 2	460
	11th to 20th	Day	36% 27% 25% 9% 2	460
	21st to end	Day	37% 26% 23% 10% 3 1	506
April	1st to 10th	Day	43% 25% 22% 8% 2 1	460
	11th to 20th	Day	34% 26% 26% 11% 2	452
	21st to end	Day	35% 18% 28% 13% 5%	460
May	1st to 10th	Day	34% 22% 30% 11% 3	463
	11th to 20th	Day	27% 20% 29% 19% 5%	626
	21st to end	Day	30% 19% 30% 18% 3 1	735
June	1st to 10th	Day	35% 16% 25% 19% 4	680
	11th to 20th	Day	24% 16% 35% 19% 6% 1	680
	21st to end	Day	31% 13% 33% 16% 5% 1	680
July	1st to 10th	Day	23% 12% 38% 20% 7% 1	670
	11th to 20th	Day	22% 9% 38% 21% 8% 1	668
	21st to end	Day	21% 11% 41% 23% 4	533
August	1st to 10th	Day	19% 12% 42% 22% 5% 1	486
	11th to 20th	Day	21% 12% 35% 22% 10% 1	479
	21st to end	Day	18% 14% 31% 27% 8% 2	539
September	1st to 10th	Day	20% 14% 39% 21% 6% 1	490
	11th to 20th	Day	23% 16% 31% 20% 7% 3	490
	21st to end	Day	24% 17% 30% 20% 8% 2	490
October	1st to 10th	Day	28% 16% 25% 20% 9% 3	393
	11th to 20th	Day	31% 18% 29% 15% 5% 2	390
	21st to end	Day	34% 21% 28% 12% 4 1	429
November	1st to 10th	Day	33% 23% 28% 11% 4 1	395
	11th to 20th	Day	44% 22% 20% 11% 4	400
	21st to end	Day	41% 25% 25% 7% 3 1	400
December	1st to 10th	Day	39% 27% 23% 10% 1	400
	11th to 20th	Day	49% 26% 18% 5% 1	380
	21st to end	Day	40% 26% 25% 6% 1	220
January	1st to 10th	Night	38% 25% 23% 13% 1	900
	11th to 20th	Night	38% 23% 23% 12% 3	814
	21st to end	Night	31% 28% 28% 11% 3	880
February	1st to 10th	Night	37% 24% 26% 11% 2	800
	11th to 20th	Night	40% 23% 25% 10% 2	783
	21st to end	Night	26% 26% 27% 17% 3	647
March	1st to 10th	Night	33% 24% 28% 13% 1	760
	11th to 20th	Night	33% 22% 26% 17% 3	760
	21st to end	Night	31% 21% 27% 16% 4	836
April	1st to 10th	Night	39% 18% 26% 16% 2	646
	11th to 20th	Night	30% 21% 30% 17% 3	564
	21st to end	Night	33% 16% 27% 18% 6%	570
May	1st to 10th	Night	29% 15% 33% 18% 4 1	570
	11th to 20th	Night	28% 14% 29% 24% 5%	562
	21st to end	Night	21% 15% 39% 20% 4 1	616
June	1st to 10th	Night	29% 12% 31% 23% 5%	473
	11th to 20th	Night	23% 12% 32% 26% 6% 1	380
	21st to end	Night	23% 9% 39% 20% 9% 1	380
July	1st to 10th	Night	19% 9% 39% 26% 6%	370
	11th to 20th	Night	24% 7% 34% 23% 12%	412
	21st to end	Night	19% 8% 35% 33% 5%	623
August	1st to 10th	Night	19% 8% 35% 29% 8% 1	568
	11th to 20th	Night	20% 9% 30% 29% 10% 1	562
	21st to end	Night	19% 8% 33% 30% 10%	627
September	1st to 10th	Night	18% 11% 34% 27% 9% 1	570
	11th to 20th	Night	22% 10% 27% 28% 12% 1	746
	21st to end	Night	23% 12% 30% 23% 10% 1	760
October	1st to 10th	Night	23% 10% 27% 29% 10% 1	760
	11th to 20th	Night	24% 19% 28% 19% 8% 1	760
	21st to end	Night	37% 20% 22% 16% 4	836
November	1st to 10th	Night	33% 18% 26% 17% 5% 1	820
	11th to 20th	Night	36% 23% 21% 15% 5%	900
	21st to end	Night	38% 21% 23% 15% 3	900
December	1st to 10th	Night	40% 21% 24% 12% 2	900
	11th to 20th	Night	39% 27% 23% 9% 2	900
	21st to end	Night	41% 23% 21% 12% 2	990

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