Summer Weather and Heat

Nick,
What is the hottest temperature we have ever seen in this country?
Ed
Kansas City, KS

Ed,

The hottest temperature ever recorded in the United States is 134° F (56.7 C) in Death Valley, CA. It occurred on July 10, 1913. For several years, this was a world record until Al Aziziyah, Libya in the Sahara Desert reached 136° F (58C) on September 13, 1922. But Death Valley, CA is one of the hottest places in the world. The summer of 1974 saw 134 days over 100 degrees. The summer of 1996 had 40 days over 120°F!

Can you please explain the difference between dew point & humidity?
Terry
Babbitt, MN

Dear Terry,

Dew point and humidity are closely related. Dew point refers to the temperature at which water vapor will condense into liquid water. You have seen this happen when water droplets condense onto the outside of a glass of ice water. When the glass gets colder than the dew point, water vapor in the air condenses into liquid on the glass. The higher the dew point temperature, the more moisture there is in the air. The closer the dew point temperature is to the actual air temperature, the higher the relative humidity will be. For example, if you notice in the morning that both the air temperature and the dew point temperature are 60 degrees, then relative humidity is 100%, and it is likely that fog is forming. However, if the dew point temperature is 70 and the air temperature is 85 degrees, there is actually more moisture in the air even though the relative humidity is only 61%. That’s why we call it “relative” humidity, because the humidity is relative to the air temperature. That’s why we talk a lot on the air about the dew point, because it gives you more of an idea of how much moisture is in the air, and how sticky or how dry the air will feel to you.

Please tell me what a dust devil is. Is it a small tornado?

Andrei
Belarus, Eastern Europe

Andrei,
A dust devil is a rotating column of wind that picks up dust or small debris. It is not a tornado, nor does it come from a thunderstorm. Dust devils can develop on clear, dry, hot days, especially in arid climates. When the ground heats up, the rising warm air can be deflected by a bump in the ground and begin to rotate. Passing cars or trucks or even tractors can start the hot air swirling. As the whirlwind picks up dust, it becomes visible. Dust devils are usually small, but have been known to have diameters as wide as 300 feet and heights up to one thousand feet. They often last only a few seconds, but can sometimes last for several minutes to even an hour or more in the deserts. They rarely do any damage, though some strong dust devils can have wind speeds of sixty miles an hour or more, which can damage small structures. The National Weather Service office in Flagstaff, Arizona has a web page devoted to dust devils. Go here for more information:
http://www.wrh.noaa.gov/fgz/science/dustdvl.php?wfo=fgz

Where has it been most consistently the coolest place during July, within the Continental U.S.A.?
Tom, Cape Cod

Tom,
The coolest place, on average, in July in the U.S. (at least out of those places that keep daily records) is Mount Washington, New Hampshire, with an average July high temperature of 54 degrees. But no one really lives way up there. The coolest metropolitan areas in July are downtown San Francisco (average July high of 66, tied with Whidbey Island, WA (also 66). Interestingly enough, San Francisco's average January high temperature is 56 degrees, so you can see what a mild climate they have there. That's not surprising, since the cool Pacific Ocean keeps most west coast locations cool in the summer, and since the ocean doesn't change temperature much even in winter,it's warmer here during January than in many other places in the country. Compare these July average highs with a few east coast locations: Caribou, ME-76, Boston-82, and Martha's Vineyard-79.

When is the longest day of the year?
Anithra McElroy
Chicago, IL

Traditionally, the first day of summer, or the summer solstice, is thought to be the longest day of the year. This year summer began on June 20. But look at the sunrise and sunset times for your city, Chicago (Go to weather.com and type in your zip code, look for the tab that says “Averages and Records,” click on June), and you will see that the longest days of the year (in terms of daylight) are the days just before the 20^th and a few days after, all of which have the same number of minutes of daylight. However, the longest day of the year as calculated by sunrise/sunset times may not necessarily be the day of the summer solstice. This is because the day of the summer solstice (as well as the winter solstice and the equinoxes) is not based on the sunrise and sunset times, but on the location of the center of the sun with respect to the earth. Sunrise and sunset times are based on the minute the top of the sun touches the horizon when it rises and sets. Another thing to keep in mind is that the sun is still actually below the horizon when you see the top edge of the sun at the horizon. This is because Earth's atmosphere bends light rays, so we see the sun “rise” before it actually does. These astronomical phenomena add a few minutes to the hours of daylight.

Why is the weather always so hot in Southern California, Nevada and Arizona?
Lynne
Wibaux, MT

Lynne,

Part of the reason why it is warm in southern California, southern Nevada and Arizona is simply the latitude...the closer you are to the equator the warmer the temperature, all other things being equal.

Also, this area is under a semi-permanent area of high air pressure. In high pressure, the air sinks, and clouds have trouble forming, so rain is scarce and the air is dry. Dry air heats up fast but also cools down fast at night.

These desert areas are also on the lee sides of mountain ranges. When clouds run into the mountains, the moist air rises up the sides of mountains, and the moisture cools and condenses into clouds. The clouds bring precipitation over the windward slopes of the mountains. When the air descends on the lee side of the mountains, it sinks, compresses, and warms, bringing on the heat. The clouds evaporate in the sinking air and, having lost some of its moisture content on the windward slopes, the air reaches the lee side with lower relative humidity than before it reached the mountains. This leaves the area on the lee side of the mountains relatively dry.

You should note that not all of Southern California and not all of Arizona is hot. The Pacific Ocean keeps coastal areas of California cool, and the higher elevations in parts of Arizona keep places like Flagstaff, much cooler than the valleys.

Nick

In the desert, why is the temperature during the day so hot, but at night so cold?
Nathan
Huntsville, AL

Nathan,

The desert is a perfect example of how moisture in the air (or the lack of it) can affect temperature. Dry air tends to heat up and cool down quickly, whereas humid air takes longer to heat up than dry air, and it tends to hold onto the heat more than dry air does. So when the air is dry, temperatures can warm up quite a bit during the day, only to cool down at night.

There is also something else to consider. When the air is very dry, skies will most likely be clear. During the day, clear skies allow the sun to shine and heat up the ground. As the ground radiates that heat, we feel the warmer temperature. Then when the sun goes down, the ground radiates the heat out into space, and since there are no clouds to hold the heat close to Earth, temperatures cool. In some of the higher elevations of the Southwestern United States, temperatures in the dry summer air can range from the 40s and 50s at night to the 90s during the day.

With all the heat lately in California, I am wondering what the all-time hottest day was in Palm Springs, California.
Diana
La Quinta, California

Diana,
Palm Springs was one of at least six dozen cities that saw record highs this past weekend, with temperatures well above 110 degrees F. But twice on record, Palm Springs has hit a whopping 123 degrees! The first time was on August 1, 1993, and again two years later on July 28, 1995. But as hot as that is, that’s still a far cry from the temperature recorded just 300 miles away in Death Valley back in 1913. At 134 degrees F, it still holds the all-time record for the hottest temperature in the United States.

When ozone pollution is forecast to be at code red on a particular day, is there a time of day that it reaches dangerous levels, or is it dangerous all day?

B. Cicon
Arlington, TX

As you probably know, ozone forms when emissions from cars, power plants, refineries or other sources react chemically in sunlight. Ozone pollution is especially a concern in the summer when temperatures are warm and sunlight is plentiful. This is because sunlight facilitates the formation of ozone, and high temperature increases ground-level concentrations of ozone. The dangers are at their worst in the late afternoon, as temperatures rise to the warmest of the day. Ozone pollution is usually a problem in large cities, but winds can transport ozone to rural areas as well.

Here is what colorized ozone forecasts from the Environmental Protection Agency mean:

Green: Low ozone, good conditions

Yellow: Moderate levels of ozone, unusually sensitive people should consider limiting prolonged outdoor exertion.

Orange: Unhealthy of sensitive groups. Active children and adults, and people with respiratory disease, such as asthma, should limit prolonged outdoor exertion.

Red: Unhealthy. Active children and adults, and people with respiratory disease, such as asthma, should avoid prolonged outdoor exertion; everyone else, especially children, should limit prolonged outdoor exertion.

Violet: Very unhealthy. Active children and adults, and people with respiratory disease, such as asthma, should avoid all outdoor exertion; everyone else, especially children, should limit outdoor exertion.

I recently noticed a dew point temperature in Alice, Texas of 82 degrees with an air temperature of 98 degrees. That has to be so humid! What is the highest dew point temperature ever recorded, and where it was recorded? Zachary
Greenwood, SC

Zachary,
The official record of world climate extremes kept by the US Army Corps of Engineers lists a couple of interesting dew point temperatures. For example, the average afternoon dew point at Assab, Ethiopia in June is 84 degrees F. One of the highest dew point temperatures ever recorded is 93.2F at Sharjah, Saudi Arabia. As you know, dew point refers to the temperature at which water vapor will condense into liquid water. The higher the dew point temperature, the more moisture there is in the air. The closer the dew point temperature is to the actual air temperature, the higher the relative humidity will be. Those dew points in the 80s and 90s must have caused some unbearable humidity!

Nick,
I was told that the sun is stronger through the clouds and that is why we can get sun burned so much easier. Why would that be true?
Theresa
Milwaukee, WI

Theresa,
The sun's rays are not stronger through the clouds than through a clear sky, but it is true that we can get sunburned almost as easily on a cloudy day as on a sunny day, especially if those clouds are thin and high. People have a tendency not to pay as much attention to putting on sunscreen on a cloudy day because they think they
won't sunburn. But the sun's ultraviolet rays can shine through the clouds, and in the summertime, those rays can especially be harmful if we're out outside too long.
Here's a link to the Ultraviolet Index forecast for various cities so you can get an idea how easily you can be burned on any given day. http://www.atmos.washington.edu/data/uv_report.html

Summer Weather and Heat

©Copyright 2007 Small Gate Media/Nick Walker