Fal = Season  (plural:  fuuul)
ar-Rabii^c = Spring
a-Sayf = Summer
al-Khariif = Fall
ash-Shitaa' = Winter

a-Taqs = the weather  (also al-jaww; Egyptians say ig-gaww)
Maar = rain
Mumir = rainy
ash-Shams - The Sun
Mushmis = sunny
Baarid = cold
aarr = hot

[note that "aarr" and "baarid" are used with things, and "arraan" and "bardaan" are used with people]

Examples:

Nanu al-aan fii fal ar-rabii^c.
We are now in the season of spring.

Kaana haathaa ash-shitaa' baarid jiddan.
This winter was very cold.

Fii ar-rabii^c a-aqs jamiil jiddan fii Philadelphia.
In spring the weather is very beautiful in Philadelphia.

A-aqs al-yawm aarr wa mushmis.
The weather today is hot and sunny.

Ams kaana  a-aqs mushmis aydan.
Yesterday the weather was sunny too.

Yawm al-Jum^cah kaana  a-aqs mumir.
Friday the weather was rainy.

Fii ash-shitaa'  a-aqs baarid wa mumir fii Philadelphia.
In winter the weather is cold and rainy in Philadelphia.

Wa fii a-ayf  a-aqs aarr wa mushmis.
And in the summer the weather is hot and sunny.
 

Sun letters and Moon letters

These terms are used to describe the difference in the way letters are pronounced in Arabic.  There are 2 groups--the "Sun letters" and the "Moon letters."
With Sun letters, the "Al-" of the definite article ("the") gets assimilated to the following letter.  This means that the L is actually pronounced like the consonant
which follows it, as in "ash-Shams" The Sun.  With Moon letters, the L of the Al- is pronounced as normal, like an L, as in "al-Qamar" The Moon.  This is why we say AR-Rabii^c (Spring) but AL-Khariif (Fall.)  To pronounce the definite article AL- correctly, you need to know if the letter following it is a Sun letter or a
Moon letter.  It is easiest just to remember how words are pronounced, rather than memorizing a list of letters, but for reference the Sun letters are:  t, th, d,  th, r, z,
s, sh, , daad, , TH, and n.  (these are the letters pronounced at the front of your mouth, with your tongue against your teeth.)  All the other letters are Moon letters.

ash-Shams - The Sun
uruuf shamsiyyah = "Sun letters"
al-Qamar - The Moon
uruuf qamariyyah = "Moon letters"