Yes. It is true that we can decide if an umbrella will be necessary or not, by just watching the sky and observe the clouds.They are in general 10 types of clouds, showing in the sky from high to low level.

※Latin root & translation:
Alto High
Cirrus Curl of hair
Cumulus Heap
Incus Anvil
Nimbus Rain
Stratus Layer

Cirrostratus(巻層雲)
Usually, we can see them in winter, thinly covering the whole sky. Sometimes you can observe a beautiful ring surround the sun or the moon. This type of the clouds tells us there is an upcoming rain or snow in the following 12-24 hours.

Cirrus(巻雲)
Commonly seen in autumn, the shape of Cirrus looks like a broom-swept mark. Seeing this cloud, means a day or days later, the clear weather will be going away.

Cirrocumulus(巻積雲)
This type of clouds usually covers the entire sky. Cirrocumulus looks like speckled fish scales. These “fish scales” are telling us, rainy days are coming soon.

Altostratus(高層雲)
These clouds look like a thick gray veil. Hardly you can find the shades on the ground since they could cover the whole sky. Seeing them in the air means the rain is on its way.

Altocumulus(高積雲)
People sometime just call them “sheep clouds” in Japan because these white round clouds look like a field of sheep. The point of observing this type of cloud lies in whether you could see the blue sky between the clouds or not. If yes, no worry about the rain.

Stratocumulus(層積雲)
Normally are floating in the low level of sky, indicates a fine day. Even if it rains, will probably be a short-visit shower.

Cumulus(積雲)
This is the clouds we can observe through the year on sunny days. They indicate a good weather in general. However, they could grow into Cumulonimbus and brings the rain.

Nimbostratus(乱層雲)
These clouds are the grown-up version of Altostratus, with bigger volume. These clouds are referred as “rain clouds”. Normally when you see them, it is about to rain or already is raining, and the rain will last for a while.

Status（層雲）
They are often seen in the mountain area on a chilly morning, at very low level. If the clouds are lower than the level of your eyes, they are to be called “fogs” but not “clouds” any more.

Cumulonimbus（積乱雲）
The most well-known giant clouds, some could even grow into 15km in height, usually can be observed during summer time. When you encounter them, be careful that a bad thunderstorm is coming!

Last Bonus knowledge: Airplane clouds(飛行機雲)
Airplane clouds which we see after airplane fly away. Actually, they are to be called Contrails, or Vapor trails, are artificial clouds produced by airplane’s engine. We cannot judge the weather by looking at the shape since they are all line-shaped. However, it is said the longer the clouds stay in the sky, the more likely to rain.