天気 and 気候: a lunch condition and regional climate
Compare 天気 in a conditional lunch and watching mountain weather with 気候 in a region's mild climate and a comparison using data. Separate the condition, observation, and completed comparison.
Weather as a condition for lunch
天気（てんき） means weather. 「天気がよければ公園で昼食を食べます。」（If the weather is good, we will eat lunch in the park.） よければ sets the condition for eating in the park. The sentence gives a conditional plan; it does not report that lunch has already taken place.
「山の天気は変わりやすいので、空をよく見ていました。」（Mountain weather changes quickly, so we watched the sky carefully.） The past action described is watching the sky because mountain weather changes readily. 空 is what was watched. Watching carefully does not add a successful forecast to the account.
Climate as a regional characteristic
気候（きこう） means climate. 「この地域は温暖な気候です。」（This region has a mild climate.） この地域 identifies the region, and 温暖な describes its mild climate. The focus differs from the weather condition governing a particular lunch.
「資料を使って各地の気候を比べた。」（We used data to compare the climates of various regions.） 比べた describes a completed comparison of regional climates using data. Read the climates as the objects compared, without supplying temperatures or a numerical definition of climate that the sentence does not give.
Reading the focus of each sentence
The lunch sentence links 天気がよければ to eating in the park. The climate sentences describe a regional characteristic or compare the climates of several regions. The location, object, and predicate show each sentence's focus.
Lunch and watching a mountain sky concern particular weather situations. A mild regional characteristic and a data-based comparison concern climate. These examples provide no fixed number of days that divides the two terms.
In 「天気がよければ公園で昼食を食べます。」 and 「資料を使って各地の気候を比べた。」, what is conditional, and what completed action is reported?
Show the answer and why
Eating lunch in the park depends on good weather. The completed action is comparing regional climates using data. The sentences do not say lunch has already happened or define climate by a fixed number of days.