Observatory Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatory Park clear sky chart from 2005-11-06 to 2025-01-22.

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% 12% 4 9% 5% 4 4 3 5% 7% 30%	104870
	Night	15% 14% 5% 10% 5% 4 4 3 5% 6% 29%	104426
Transparency	Day	52% 6% 11% 19% 10% 1	108546
	Night	54% 6% 11% 18% 10% 1	108889
Seeing	Day	22% 19% 29% 18% 9% 3	14766
	Night	23% 17% 27% 23% 10% 1	24591

By Month

Percentage Cloud Cover SE (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	16% 24% 11% 15% 4 3 3 3 3 4 14%	2926
	11th to 20th	Day	12% 24% 8% 18% 7% 5% 3 2 3 5% 14%	2930
	21st to end	Day	23% 20% 12% 17% 7% 3 3 2 3 2 7%	3133
February	1st to 10th	Day	16% 21% 6% 15% 8% 4 3 3 4 4 18%	1710
	11th to 20th	Day	14% 15% 8% 16% 8% 6% 4 3 4 4 18%	1681
	21st to end	Day	16% 18% 4 13% 5% 4 5% 4 4 5% 21%	1570
March	1st to 10th	Day	20% 13% 8% 9% 4 4 4 3 5% 6% 25%	3391
	11th to 20th	Day	12% 16% 6% 11% 5% 4 3 2 4 5% 31%	3480
	21st to end	Day	20% 13% 5% 10% 5% 3 3 2 4 5% 29%	3815
April	1st to 10th	Day	14% 16% 4 8% 4 4 4 3 5% 7% 31%	2152
	11th to 20th	Day	19% 13% 3 7% 4 4 4 3 5% 6% 33%	2385
	21st to end	Day	20% 13% 3 7% 5% 3 3 3 4 5% 34%	2470
May	1st to 10th	Day	20% 9% 3 8% 5% 5% 5% 4 4 6% 31%	4082
	11th to 20th	Day	19% 9% 3 8% 4 4 4 4 5% 6% 34%	4081
	21st to end	Day	11% 5% 3 7% 5% 5% 5% 4 6% 7% 41%	4684
June	1st to 10th	Day	15% 6% 3 6% 5% 5% 5% 4 6% 8% 36%	2660
	11th to 20th	Day	13% 6% 2 6% 5% 4 5% 4 7% 8% 38%	2660
	21st to end	Day	13% 5% 2 6% 5% 4 5% 4 6% 10% 39%	2660
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 10% 40%	4260
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 7% 10% 43%	4120
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 7% 8% 41%	4466
August	1st to 10th	Day	14% 7% 3 7% 5% 5% 6% 4 6% 9% 36%	4104
	11th to 20th	Day	12% 7% 2 8% 5% 5% 6% 5% 8% 9% 34%	4044
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 4 7% 9% 41%	4420
September	1st to 10th	Day	15% 7% 2 6% 4 4 4 3 6% 7% 41%	2218
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 3 6% 8% 43%	2090
	21st to end	Day	20% 8% 3 7% 4 3 4 3 5% 7% 35%	2090
October	1st to 10th	Day	16% 7% 3 7% 4 4 5% 4 6% 8% 37%	1940
	11th to 20th	Day	22% 10% 3 7% 5% 4 5% 3 5% 6% 31%	1900
	21st to end	Day	23% 13% 3 9% 5% 4 5% 3 5% 7% 25%	2090
November	1st to 10th	Day	12% 14% 4 8% 5% 5% 4 3 5% 6% 34%	1776
	11th to 20th	Day	16% 21% 5% 9% 4 4 4 3 4 6% 24%	1704
	21st to end	Day	17% 22% 7% 10% 6% 5% 4 3 5% 5% 16%	1600
December	1st to 10th	Day	16% 20% 10% 14% 6% 5% 4 3 4 5% 12%	2600
	11th to 20th	Day	21% 25% 7% 12% 7% 4 3 3 4 4 11%	2990
	21st to end	Day	22% 25% 7% 17% 5% 5% 4 3 3 3 8%	3988
January	1st to 10th	Night	14% 24% 12% 18% 5% 4 3 3 3 4 10%	4239
	11th to 20th	Night	13% 20% 10% 20% 6% 4 4 3 4 4 11%	4250
	21st to end	Night	23% 21% 9% 16% 7% 3 3 2 2 3 12%	4546
February	1st to 10th	Night	18% 17% 5% 14% 8% 5% 4 3 4 4 18%	2470
	11th to 20th	Night	15% 16% 7% 14% 7% 6% 5% 3 4 4 20%	2384
	21st to end	Night	19% 14% 6% 12% 6% 5% 4 3 4 5% 22%	1884
March	1st to 10th	Night	20% 16% 4 7% 5% 3 4 3 4 5% 31%	3462
	11th to 20th	Night	14% 12% 7% 10% 5% 4 4 3 4 5% 33%	3480
	21st to end	Night	17% 13% 6% 9% 4 3 4 3 4 4 33%	3817
April	1st to 10th	Night	16% 16% 3 7% 5% 4 4 3 5% 5% 31%	2028
	11th to 20th	Night	17% 11% 3 7% 4 4 5% 4 6% 6% 34%	1759
	21st to end	Night	15% 10% 3 8% 5% 4 5% 4 4 7% 35%	1710
May	1st to 10th	Night	23% 10% 3 6% 4 4 4 4 6% 6% 30%	2813
	11th to 20th	Night	14% 11% 3 7% 5% 4 4 3 6% 8% 36%	2812
	21st to end	Night	13% 4 1 5% 3 4 4 3 6% 10% 46%	2867
June	1st to 10th	Night	11% 7% 2 7% 5% 4 4 4 6% 7% 41%	1520
	11th to 20th	Night	9% 6% 3 6% 5% 5% 6% 4 5% 7% 43%	1520
	21st to end	Night	10% 6% 2 6% 4 4 5% 4 6% 9% 42%	1520
July	1st to 10th	Night	9% 6% 2 6% 4 3 4 3 5% 9% 47%	2695
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 6% 9% 48%	2840
	21st to end	Night	7% 6% 3 7% 5% 5% 5% 4 6% 9% 45%	3087
August	1st to 10th	Night	12% 8% 3 7% 5% 5% 5% 4 6% 7% 39%	2825
	11th to 20th	Night	14% 9% 2 7% 4 5% 4 3 5% 6% 41%	2785
	21st to end	Night	8% 5% 2 7% 5% 5% 5% 5% 7% 8% 43%	3203
September	1st to 10th	Night	13% 5% 2 5% 3 4 4 4 7% 7% 45%	1957
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 6% 52%	2090
	21st to end	Night	17% 7% 3 7% 4 5% 4 3 5% 6% 37%	2090
October	1st to 10th	Night	15% 8% 2 5% 4 4 4 3 6% 7% 42%	2239
	11th to 20th	Night	18% 9% 3 9% 4 5% 4 3 6% 6% 33%	2280
	21st to end	Night	24% 11% 3 6% 4 4 4 3 6% 6% 28%	2508
November	1st to 10th	Night	14% 11% 3 7% 5% 4 4 3 5% 6% 35%	2517
	11th to 20th	Night	16% 18% 5% 10% 5% 5% 4 3 5% 5% 24%	2694
	21st to end	Night	18% 20% 5% 9% 5% 4 4 3 5% 5% 20%	2800
December	1st to 10th	Night	13% 20% 8% 14% 7% 5% 5% 3 4 5% 15%	4580
	11th to 20th	Night	17% 24% 9% 12% 5% 4 3 3 4 5% 14%	5205
	21st to end	Night	20% 23% 7% 14% 5% 4 3 3 4 5% 11%	6950

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	76% 4 10% 9% 2	2927
	11th to 20th	Day	76% 3 9% 10% 3	2930
	21st to end	Day	85% 4 5% 5% 2	3133
February	1st to 10th	Day	71% 5% 8% 11% 5%	1710
	11th to 20th	Day	70% 5% 8% 11% 6%	1690
	21st to end	Day	63% 6% 10% 12% 6% 2	1570
March	1st to 10th	Day	58% 7% 9% 22% 4	3391
	11th to 20th	Day	56% 6% 14% 17% 6% 1	3480
	21st to end	Day	58% 4 9% 17% 11% 1	3815
April	1st to 10th	Day	52% 5% 12% 16% 13% 1	2152
	11th to 20th	Day	53% 5% 10% 17% 13% 2	2385
	21st to end	Day	52% 5% 10% 20% 10% 3	2470
May	1st to 10th	Day	53% 7% 8% 23% 9% 1	4120
	11th to 20th	Day	48% 8% 12% 21% 10% 1	4085
	21st to end	Day	38% 9% 12% 21% 15% 4	4698
June	1st to 10th	Day	44% 7% 11% 25% 12% 1	2660
	11th to 20th	Day	41% 8% 13% 21% 14% 2	2660
	21st to end	Day	40% 7% 12% 25% 15% 1	2660
July	1st to 10th	Day	37% 8% 12% 25% 15% 3	4195
	11th to 20th	Day	34% 8% 12% 27% 18% 1	4051
	21st to end	Day	39% 7% 14% 21% 18% 1	4324
August	1st to 10th	Day	42% 9% 14% 23% 11%	4091
	11th to 20th	Day	43% 9% 11% 29% 7% 1	4083
	21st to end	Day	37% 7% 14% 28% 13% 1	4420
September	1st to 10th	Day	41% 8% 13% 21% 14% 3	2218
	11th to 20th	Day	38% 8% 12% 24% 17% 1	2090
	21st to end	Day	49% 6% 10% 22% 13% 1	2090
October	1st to 10th	Day	35% 5% 15% 26% 19% 1	3226
	11th to 20th	Day	47% 5% 12% 18% 14% 3	3140
	21st to end	Day	45% 5% 14% 24% 11% 1	3428
November	1st to 10th	Day	49% 6% 13% 19% 12% 1	1776
	11th to 20th	Day	61% 5% 11% 13% 7% 2	1704
	21st to end	Day	69% 5% 9% 12% 5%	1600
December	1st to 10th	Day	75% 5% 10% 8% 2	2600
	11th to 20th	Day	75% 6% 8% 9% 2	2986
	21st to end	Day	82% 4 7% 6% 1	3988
January	1st to 10th	Night	79% 4 7% 8% 3	4239
	11th to 20th	Night	77% 6% 8% 8% 1	4250
	21st to end	Night	80% 3 8% 7% 2	4546
February	1st to 10th	Night	69% 5% 10% 12% 4	2470
	11th to 20th	Night	67% 5% 8% 14% 5%	2386
	21st to end	Night	65% 5% 8% 13% 7% 1	1884
March	1st to 10th	Night	55% 7% 11% 19% 8%	3462
	11th to 20th	Night	54% 5% 15% 16% 8% 1	3480
	21st to end	Night	53% 4 11% 19% 12% 1	3817
April	1st to 10th	Night	55% 5% 10% 17% 12% 2	2028
	11th to 20th	Night	49% 6% 11% 19% 13% 2	1759
	21st to end	Night	48% 6% 11% 20% 12% 3	1710
May	1st to 10th	Night	52% 7% 10% 18% 10% 3	2840
	11th to 20th	Night	45% 7% 13% 22% 12% 2	2813
	21st to end	Night	32% 8% 14% 26% 18% 3	2874
June	1st to 10th	Night	40% 7% 10% 25% 15% 2	1520
	11th to 20th	Night	38% 7% 13% 21% 18% 3	1520
	21st to end	Night	37% 8% 13% 24% 18% 1	1520
July	1st to 10th	Night	35% 5% 13% 24% 18% 4	2650
	11th to 20th	Night	31% 7% 13% 28% 18% 3	2786
	21st to end	Night	35% 8% 13% 19% 23% 2	2988
August	1st to 10th	Night	41% 7% 11% 26% 13% 1	2814
	11th to 20th	Night	43% 6% 12% 28% 10% 1	2812
	21st to end	Night	35% 6% 13% 29% 15% 2	3203
September	1st to 10th	Night	36% 8% 12% 26% 16% 2	1957
	11th to 20th	Night	32% 7% 14% 24% 21% 2	2090
	21st to end	Night	47% 6% 10% 21% 15% 1	2090
October	1st to 10th	Night	35% 6% 10% 26% 22% 1	3733
	11th to 20th	Night	46% 6% 10% 20% 16% 2	3779
	21st to end	Night	42% 4 14% 24% 14% 1	4120
November	1st to 10th	Night	48% 6% 13% 20% 11% 2	2517
	11th to 20th	Night	61% 5% 11% 15% 6% 3	2694
	21st to end	Night	65% 5% 11% 14% 4	2800
December	1st to 10th	Night	71% 6% 13% 8% 3	4580
	11th to 20th	Night	73% 4 10% 11% 1	5208
	21st to end	Night	75% 6% 10% 9% 1	6950

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	31% 32% 26% 8% 2	300
	11th to 20th	Day	33% 35% 24% 6% 2	300
	21st to end	Day	28% 35% 29% 6% 1 1	300
February	1st to 10th	Day	33% 26% 33% 7% 1	270
	11th to 20th	Day	31% 38% 24% 6% 1	270
	21st to end	Day	38% 32% 25% 4	225
March	1st to 10th	Day	29% 27% 29% 12% 3	427
	11th to 20th	Day	26% 27% 31% 12% 3 1	460
	21st to end	Day	36% 22% 27% 11% 4	506
April	1st to 10th	Day	27% 28% 27% 13% 5% 1	460
	11th to 20th	Day	32% 21% 25% 14% 7% 1	452
	21st to end	Day	28% 23% 30% 13% 4 2	460
May	1st to 10th	Day	27% 23% 30% 15% 3 2	463
	11th to 20th	Day	21% 15% 31% 23% 7% 2	487
	21st to end	Day	15% 11% 33% 26% 12% 4	529
June	1st to 10th	Day	18% 15% 35% 20% 11% 1	490
	11th to 20th	Day	16% 14% 31% 27% 11% 1	490
	21st to end	Day	15% 12% 36% 26% 8% 2	490
July	1st to 10th	Day	9% 8% 28% 28% 19% 7%	485
	11th to 20th	Day	13% 12% 29% 26% 15% 5%	484
	21st to end	Day	11% 8% 33% 28% 14% 6%	532
August	1st to 10th	Day	11% 10% 27% 28% 17% 8%	486
	11th to 20th	Day	13% 9% 29% 27% 14% 7%	479
	21st to end	Day	12% 11% 25% 27% 17% 7%	539
September	1st to 10th	Day	17% 10% 25% 25% 19% 5%	490
	11th to 20th	Day	11% 7% 24% 29% 20% 9%	490
	21st to end	Day	22% 15% 28% 20% 11% 4	490
October	1st to 10th	Day	17% 14% 29% 21% 16% 3	490
	11th to 20th	Day	25% 22% 32% 14% 5% 2	490
	21st to end	Day	34% 20% 28% 15% 2	539
November	1st to 10th	Day	23% 26% 30% 13% 6% 1	314
	11th to 20th	Day	35% 23% 27% 11% 4	252
	21st to end	Day	31% 30% 34% 7%	200
December	1st to 10th	Day	27% 36% 27% 8% 3 1	200
	11th to 20th	Day	29% 36% 28% 8% 1	200
	21st to end	Day	31% 32% 29% 7% 1	227
January	1st to 10th	Night	29% 28% 31% 9% 3	800
	11th to 20th	Night	33% 26% 27% 13% 2	800
	21st to end	Night	27% 25% 32% 14% 2	841
February	1st to 10th	Night	30% 25% 27% 16% 2	760
	11th to 20th	Night	29% 25% 31% 14% 2	743
	21st to end	Night	32% 24% 29% 13% 1	628
March	1st to 10th	Night	25% 24% 31% 17% 3	760
	11th to 20th	Night	27% 23% 27% 18% 5% 1	760
	21st to end	Night	25% 19% 30% 19% 6%	836
April	1st to 10th	Night	26% 18% 30% 21% 5%	760
	11th to 20th	Night	25% 16% 34% 17% 6% 1	635
	21st to end	Night	19% 14% 34% 26% 6% 1	570
May	1st to 10th	Night	22% 13% 27% 28% 10% 1	570
	11th to 20th	Night	17% 10% 28% 34% 9% 2	561
	21st to end	Night	12% 6% 28% 35% 18% 2	495
June	1st to 10th	Night	12% 7% 33% 34% 13% 1	380
	11th to 20th	Night	12% 7% 29% 37% 15% 1	380
	21st to end	Night	10% 9% 28% 39% 14%	380
July	1st to 10th	Night	7% 5% 20% 41% 22% 5%	462
	11th to 20th	Night	10% 6% 24% 33% 24% 3	552
	21st to end	Night	9% 6% 20% 40% 23% 2	624
August	1st to 10th	Night	11% 5% 18% 39% 23% 4	567
	11th to 20th	Night	12% 4 17% 37% 28% 3	564
	21st to end	Night	12% 6% 22% 30% 27% 3	667
September	1st to 10th	Night	16% 7% 19% 31% 24% 4	760
	11th to 20th	Night	13% 4 19% 35% 24% 5%	760
	21st to end	Night	21% 10% 24% 26% 15% 4	760
October	1st to 10th	Night	20% 9% 28% 24% 17% 2	760
	11th to 20th	Night	24% 19% 26% 23% 7%	760
	21st to end	Night	31% 19% 29% 18% 3	836
November	1st to 10th	Night	23% 19% 28% 23% 6% 1	780
	11th to 20th	Night	29% 24% 27% 14% 5% 1	800
	21st to end	Night	34% 24% 26% 13% 3	800
December	1st to 10th	Night	33% 24% 28% 11% 4	800
	11th to 20th	Night	30% 29% 28% 13% 1	800
	21st to end	Night	31% 25% 30% 12% 2	880

Page generated at 2025-01-19 03:06:50. Version ac07

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