My answers are blue.
1. Ryan let out a whoop.
The whoop was of joy.
Snowflakes were falling.
The snowflakes were large and fluffy.
Ryan let out a joy whoop when the snowflakes were falling the large and fluffy.
Ryan let out a joy whoop when the large, fluffy snowflakes started to fall. or
...when he saw the large, fluffy snowflakes falling.
2. The aerobics class lasted only an hour.
The time seemed to go on forever.
The woman's lungs screamed.
They screamed for air.
Although the aerobics class lasted only an hour, the time seemed to go on forever as the woman's lungs screamed for air.
3. My cat ran quickly.
The running was toward the cherry tree.
She was a streak of white fur.
A collie was chasing her.
With the collie chasing her, my cat was just a streak of white fur as she ran toward the cherry tree.
4. Children shrieked with delight.
The roller coaster rushed down the incline.
The incline was steep.
The roller coaster flew aound a corner.
Children shrieked with delight as the roller coaster flew around a corner and rushed down the steep incline.
5. The driver noticed a gas station.
His car had sputtered and died.
The station was only a few blocks away.
The driver was relieved.
The driver's car had sputtered and died, so he was relieved when he noticed a gas station was only a few blocks away.
6. Students in the science class gasped.
They discovered that their teacher had an expectation.
They were expected to dissect a spider.
They spider was huge and hairy.
Students in the science class gasped when they discovered that their teacher expected them to dissect a huge, hairy spider.