The following list comes from my impressions of what I have tried and think works or not for keeping a house cool. None of these views are based on exacting research. This is just my casual observations and thus I can be incredibly wrong or not. These are some ideas for you to consider.
Local Geography
I live near an ocean coast and thus the water temperature most of the year is much cooler than inland. Further inland there is a desert and these two geographical bodies sort of come into conflict. When the sun rises in the East, where the desert is for us, the desert earth warms up causing an updraft because heat raises from barren soil, very little vegetation. The cooler air over the ocean moves across the land to make up for the rising air deeper inland but heats up the more it travels from ocean inland. This local air movement is called an ocean breeze. My house has south facing windows which face this breeze and this is a terrific solution for us to cool our house when the sun goes down.
Temperature Reading Location
I frequently check the outside temperature using thermometers mounted in shady spots, front and back of the house. The back or north side of the house is enclosed by high cement block walls which stagnates the air a bit from the ocean breeze. The North side of the house also has a pool is almost always 5 degrees hotter than the South side because the water stores the heat for a long time.
I have noticed that the thermometers mounted just outside our north and south windows really are in error. The pool and stagnate air around it due to the high walls makes for about a 5 degree increase in temperature.
The South side temperature gauge is also a bit wrong because the house and ground near the gauge radiates heat from soil and building. The whole house acts like a stored heat source for quite some time. We have a wide driveway on the South side of the house which really stores heat for a long time. The point I am trying to make is to be aware of your surroundings when placing a temperature gauge and always keep it out of direct sun light.
I have discovered that I must walk more than 20 feet south of the house to gain a more realistic feel for how hot or cold the ambient outside temperature is. The house and all the cement driveway, sidewalk, stone work planters and any standing water store heat. The outside temperature is definitely hotter closer to my house. This became apparent when I checked my cell phone to see if it was cool enough to open the South windows and allow the ocean breeze to flow into the house. The cell phone weather applications would show for example 76 degrees but the temperature gauge would be close to 80. When I walked away from my house, the ambient temperature seemed to more closely match the weather application on my cell phone.
Cell Phone and Computer Weather Applications
I am quite annoyed with my cell phone weather applications reporting accurate up to date temperature readings. I use Yahoo Weather, AccuWeather and The Weather Channel cell phone applications. Readings can differ between products by as much as 7 degrees. Readings can lag real time by as much as 15 minutes. This may depend upon how you set up your cell phone for how quickly it receives updates. Most applications allow you to force a weather update. I recommend that you use different methods and try to get a consensus. I recommend at least two weather applications, look at your outside temperature gauges and you might even walk outside away from your house to “feel” the temperature as a final check.
Air Flow
A very, very, important factor to consider is air flow, direction and intensity. Almost always our house has a flow from the southern direction and almost no air flow from the North. For this reason, we put fans into the North facing windows to bring the cool outside air into the house in rooms where we get no south to north air flow. I discovered that rooms with only North facing windows seem to never change inside temperature. I have to use window fans. We do not need to put fans in the other half of the house because we have South facing window rooms and if we open the bedroom windows on the South and North side of the hose, the air from the South rooms will flow right through the North bedrooms. If privacy is an issue and someone closes their bedroom door this whole airflow cycle stops and we resort to window fans.
A real annoying science fact that throws a lot of people off is that a breeze will feel cool but you should never use a breeze to decide when to open windows of a house. Always depend upon a temperature gauge and not how cool the air feels. Example: if you are outside and feel a cool breeze and then open the house windows you must realize that that cool air flow typically will not be felt inside the house. The cooling effect takes place when you receive the breeze upon your body directly. You may be letting in hot breezy air into a cooler house if you do not follow this advice. It is way better to look at the outside thermometer first to “see” what the temperature actually is. Try to use science. If the thermometer shows a higher temperature outside than inside your house, just park yourself outside the house, in the shade, in a comfortable chair, to enjoy the apparent cooler air flow experience.
A nifty trick is to put a fan in a room to get the internal air to circulate. We love to use the vertical fans that cycle back and forth. The room will feel less stuffy and the air flow across your body will provide a cooling effect.
Temperature Difference
Another important factor to consider is temperature difference. If the outside air is just a few degrees cooler than the inside of our house, I tend to not turn on fans to get that cooler air into the house. I am way more willing to turn on a fan if the temperature difference is more significant. By this, I mean that if the inside air temperature match, lets say at 77 degrees and I had the air conditioner on set for 77, I will turn the AC off but not open windows. The reason is outside air close to the coast is typically moist but air inside the house has had the moisture removed by the air conditioning process. You may reason that there is no reason not to open the windows and shut the AC off at 77 but I think you would be wrong. Instead, I coast what the AC provided by turning off the AC and keep the windows closed until the outside temperature drops a few degrees more, typically fifteen minutes to a half hour. Reason? Moist air will feel hotter. If I open the windows too soon we feel hotter due to the rise in moisture.
Updated: 3-6-19 because AccuWeather link time out thus removed.