When the jar is sealed, air is trapped inside. The air pressure in the jar is the same as the pressure outside of it. The pressure inside the jar will not change because it is sealed. However, when the weather changes, the air pressure also changes: sunny, warm weather brings high pressure. This high pressure pushes down on the balloon and makes the straw tip rise upwards. When it is wet, windy weather, the air pressure is lower. This means the air pressure in the jar is now higher than outside and this causes the balloon cover to bulge outwards making the tip of the straw point downwards.
Y8C Geography teacher