Answer: Annual high temperatures in the summer have increased over many decades.
Weather describe the state of the atmosphere for a short time period of a given place on the basis of chief atmospheric components like amount of precipitation, sunshine availability, direction and speed of wind, dryness and temperature. A weather represent fluctuations or short term changes in the atmosphere. Example of weather conditions are thunderstorm in hot summer, foggy month in autumn or other weather conditions describing typical region or season. Climate describes the long term atmospheric condition of a given place. Examples of climatic conditions are cold-dry desert and tropical climate.
Annual high temperatures in the summer have increased over many decades describes climate rather than weather because it gives idea about the long term atmospheric conditions of a region instead of a change for a short duration.