Idioms are difficult to understand because their meaning is not what it appears to be at first sight. Some are easier to guess when they have some association with the original meaning of the individual words.

Thus, the meaning of once in a blue moon is not too difficult to guess. Blue moons do not appear in the sky very frequently so we may guess (correctly) that this means very rarely.

But what about on cloud nine as in:

• She was on cloud nine.

What does this mean? There are no clues here.

Sometimes the context will help us to understand the meaning:

• Charlotte had just been offered a job with a modelling agency when I saw her, so, of course, she was on cloud nine.

However, to be quite sure, we may need to check the meaning in a good dictionary or idioms reference book.

Here are some other informal expressions involving clouds or sky.

Over the moon / On cloud nine: if you are over the moon or on cloud nine, you are both happy and excited.

Blue skies: a very informal and rather unusual way of ending a letter. It means 'I hope all will go well for you in the future' or 'I hope the sun will shine for you'.

To have one's head in the clouds: to be out of touch with reality; to be a dreamer or an idealist; to be too concerned with your own hopes and ideals.

Pie in the sky: something which is promised or planned but is most unlikely to happen.

A cloud on the horizon: some sort of threat or bad thing which may happen sooner or later.

Under a cloud: if you are under a cloud, people have a poor or unfavourable opinion of you.

• When he heard that he had won the lottery, he was over the moon.
  
  
• Emma passed her exam last week so she is on cloud nine. I have never seen her so happy.
  
  
• With all my good wishes for a speedy recovery.
  Blue skies,
  Judith.
  
  
• He may be a idealist, but you couldn’t accuse him of having his head in the clouds all the time!
  
  
• When the government says that next year there will be no waiting lists for hospital treatment, we all know that is pie in the sky.
  
  
• My company is doing well at the moment, but the international economic recession is a real cloud on the horizon.
  
  
• After he caused the accident, Peter was under a cloud for several weeks.

If you want to practise using some of these words look at our Message Board in the You, Meand Us part of our website.