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  #1  
Old 07-22-2011, 04:19 PM
doubled doubled is offline
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Why are some days hotter than others?

Yesterday in my city, the high reached 94. Today, the high was 103. Tomorrow, it's expected to "cool off" back in to the mid 90s.

Why? What changed? Not the distance of the earth to the sun, or the amount of sunshine we received. There was no noticeable wind or rain or anything like that over this time period. So what causes temperature to change so much from one day to the next?
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  #2  
Old 07-22-2011, 04:37 PM
barbitu8 barbitu8 is offline
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One cause can be different air masses. Or, if you live near a large body of water, the cooler day can be attributed to a wind shift off the water, which is cooler than the land. The atmosphere is not stable and is 3-dimensional. So downdrafts and updrafts can cause changes in the weather.

You don't near strong winds for a different air mass to arrive. What was the wind directions on the different days. That may give you a clue.
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Old 07-22-2011, 05:04 PM
Telemark Telemark is offline
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Why? What changed? Not the distance of the earth to the sun, or the amount of sunshine we received.
Actually, those change every day. Our orbit around the sun is not circular, and we are inclined so everyday the amount of sunlight we receive changes. Over time this causes patterns in the atmosphere that produce the weather.
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Old 07-22-2011, 05:15 PM
Finagle Finagle is offline
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Actually, those change every day. Our orbit around the sun is not circular, and we are inclined so everyday the amount of sunlight we receive changes. Over time this causes patterns in the atmosphere that produce the weather.
Well, yes...but not germane to the question asked. Technically, we're receiving less sunlight today than we did yesterday here in lovely New England, but the temperature is ten degrees hotter today.

It's mostly air masses. Air can hold and transport a surprising amount of heat and various dynamics in the atmosphere can push air masses upwards and downwards, forwards and back. You can even get odd effects where increases in air pressure cause sudden increases in temperature.

There are other influences as well -- amount of sunlight increasing the temperature during the day (clouds vs clear) and amount of heat radiating away at night (again, cloudy vs clear). Humidity probably plays a role -- I'm guessing that moist air can hold more heat.
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Old 07-22-2011, 06:18 PM
dracoi dracoi is offline
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One important issue is how air moves in relation to high and low pressure. Winds tend to blow clockwise around high pressure zones and counter-clockwise around low pressure. (The two hemispheres are different, therefore I'm talking about whichever one that statement is correct for.).

Anyway, this may mean that an east-moving zone of high or low pressure can sweep wind in from a northerly or southerly direction. Since the air definitely carries heat with it, the north/south issue clearly makes a difference.

Finally, it is a well established fact that planning a picnic causes rain.
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Old 07-22-2011, 06:28 PM
Rigamarole Rigamarole is offline
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Originally Posted by dracoi View Post
Winds tend to blow clockwise around high pressure zones and counter-clockwise around low pressure. (The two hemispheres are different, therefore I'm talking about whichever one that statement is correct for.).
I have to give you credit for the most boss statement I've read all day.

+1
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Old 07-22-2011, 06:34 PM
Shagnasty Shagnasty is offline
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Originally Posted by Finagle View Post
Well, yes...but not germane to the question asked. Technically, we're receiving less sunlight today than we did yesterday here in lovely New England, but the temperature is ten degrees hotter today.

It's mostly air masses. Air can hold and transport a surprising amount of heat and various dynamics in the atmosphere can push air masses upwards and downwards, forwards and back. You can even get odd effects where increases in air pressure cause sudden increases in temperature.

There are other influences as well -- amount of sunlight increasing the temperature during the day (clouds vs clear) and amount of heat radiating away at night (again, cloudy vs clear). Humidity probably plays a role -- I'm guessing that moist air can hold more heat.
That is the basic answer. Air, especially humid air, stores and transports heat otherwise June 21st would be the hottest day of the year in the northern latitudes but it isn't especially the further north you go. Things are just starting to heat up as the amount of sunlight is at its peak.

You mentioned odd atmospheric effects. The Boston area where I live is near the ocean and fairly far north so the weather is quite variable depending on the prevailing air currents. It is possible to have 60 degree days in January and 55 degree days in July under the right conditions even if both of those are far from the average. One of the weirdest weather related things I ever witnessed was during my commute on I-495 in late June a number of years ago. It was a fairly hot afternoon in the mid-80's but I noticed that the temperature on my instrument panel thermometer started dropping rapidly like a degree every few seconds. It went from 80F+ to about 50F in just a couple of minutes. Obviously I didn't believe it so I rolled down the window and it was downright chilly outside. Then, it heated up again just as fast as it started. I watched the news when I got home and, sure enough, there was this freak cold spot in that inland area just a few miles across with no obvious cause.

Last edited by Shagnasty; 07-22-2011 at 06:36 PM..
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Old 07-23-2011, 05:16 AM
barbitu8 barbitu8 is offline
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Originally Posted by Shagnasty View Post
You mentioned odd atmospheric effects. The Boston area where I live is near the ocean and fairly far north so the weather is quite variable depending on the prevailing air currents. It is possible to have 60 degree days in January and 55 degree days in July under the right conditions even if both of those are far from the average. One of the weirdest weather related things I ever witnessed was during my commute on I-495 in late June a number of years ago. It was a fairly hot afternoon in the mid-80's but I noticed that the temperature on my instrument panel thermometer started dropping rapidly like a degree every few seconds. It went from 80F+ to about 50F in just a couple of minutes. Obviously I didn't believe it so I rolled down the window and it was downright chilly outside. Then, it heated up again just as fast as it started. I watched the news when I got home and, sure enough, there was this freak cold spot in that inland area just a few miles across with no obvious cause.
When the temps dropped off, were you passing near the ocean, bay, inlet, etc., at the time? In late June, the sea surface temps at Boston could still be quite chilly, and if the wind passed over them, it would've cooled the air.
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  #9  
Old 07-23-2011, 07:07 AM
Colophon Colophon is offline
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Here's a weather chart showing the different air masses over Europe currently. The colours denote the temperature of the air mass at approximately 1 mile above sea level (strictly speaking, the temperature at the level where pressure is 850mb).

You can see, based on the map at the time of writing, that temperatures of the airmass vary widely over short distances - at the moment there's a northerly airflow over the UK, which is bringing cool air (around 2C) over the country. Meanwhile, over Scandinavia, the air is coming up from the south around that low-pressure system and is some 10C warmer than over the UK. Obviously that doesn't mean that the temperature at ground level is that cool - today has a high of about 19C in London - but it does have a bearing on it.

Hence this morning, although the sun was shining, it felt noticeably chilly here in the UK. The forecast for later in the week has a change to a much warmer air mass, so we can expect it to feel warmer in the sunshine that it does today.

(Here is the map for the same time showing pressure patterns - the white lines are isobars, H means high pressure centres, T means low pressure centres. The colours on this version of the map are mostly related to pressure, rather than temperature as such, but it's a bit complex to explain). You can see the low pressure centred over Denmark which is giving northerly winds to the UK and southerlies over the Baltic.

Last edited by Colophon; 07-23-2011 at 07:10 AM..
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Old 07-23-2011, 01:33 PM
Leo Bloom Leo Bloom is offline
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So, just to recap for me: I live in NYC, and all the weathermen say (at times) "a high-pressure front should be coming our way..." and then he starts to smile. Meaning, high-pressure fronts do not push (carry with?) humid hot air in general? That can't be right.

It has to be something about being between landmass New Jersey and the Atlantic?

Sorry for being so obtuse, but I hear something like this ever damn day, and hate not knowing about it.

Last edited by Leo Bloom; 07-23-2011 at 01:34 PM..
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  #11  
Old 07-23-2011, 01:50 PM
Colophon Colophon is offline
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Originally Posted by Leo Bloom View Post
So, just to recap for me: I live in NYC, and all the weathermen say (at times) "a high-pressure front should be coming our way..." and then he starts to smile. Meaning, high-pressure fronts do not push (carry with?) humid hot air in general? That can't be right.

It has to be something about being between landmass New Jersey and the Atlantic?

Sorry for being so obtuse, but I hear something like this ever damn day, and hate not knowing about it.
There's no such thing as a "high pressure front". There are warm fronts and cold fronts (and occluded fronts), and they basically mark the division between different air masses. They circulate around high and low pressure systems, as seen on weather maps.

However, generally speaking, high pressure generally means drier and clearer weather, which in summer means warm and sunny, and in winter can mean cold and crisp. Low pressure usually brings more unsettled conditions.

That's not always true, of course - high pressure systems can often have a lot of cloud trapped in them, especially if they formed over water (we get a lot of these in the UK), and equally, low pressure can often bring plenty of sunshine in between heavy showers.

In the northern hemisphere, a high pressure cell will generally have
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  #12  
Old 07-23-2011, 04:10 PM
barbitu8 barbitu8 is offline
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A high pressure area (anticyclone) is associated with sinking air. A low pressure (cycone) is associated with rising air. Rains are generally associated with rising air because as the air rises it cools. Cool air cannot hold as much moisture as warmer air, and so the moisture condenses and can fall as rain. Although a high means sinking air, it can result in turbulence, producing cumulous and nimbo-cumulous clouds, and possible showers or thundershowers if it is warm enough, because, although the air mass as a whole is pressing down (high pressure), pockets of rising air can occur if there is instability (mixing of air layers). Again, as these pockets rise, the moisture is condensed out resulting in rain if the moisture content is sufficient. In the northern hemisphere, winds blow clockwise around a high and counterclockwise (anticlockwise to you across the pond) around a low. This is why mountains "squeeze out" the moisture, and it rains on the upslope, but is dry on the downslope (air rises as it goes up the mountain and the moisture content is decreased, but also it warms up on the downslope).

Pressure differences create winds. The greater the gradient, the stronger the winds. Air, rising up in a warm area diverges out of that area. Since massis leaving the relatively warmer regon, there is less mass and the surface pressure drops. The warmer air moves toward the colder region, adding mass to that region and causing the surface pressure to rise there. This tilts the normal sea level pressure (of 1000 millibars), leaving us with relatively higher pressure where it is colder near the surface and relatively lower pressure where it is warmer near the surface. This creates the circulation.

This is the same phenomenon that causes the sea breeze effect. The ocean absorbs more solar radiation during the day than the sand does because it is darker. The ocean isn't hotter than the sand because it has an internal circulation and because liquid water has significant thermal inertia. Warm air rises over the heated land and cool air sinks over the cooler ocean. The surface sea breeze blows inland from the cooler sea to the warmer air. If the land surface becomes colder at night, the circulation will reverse, and a land breeze will develop.

On the beach, the land surface heats up during the day, and the thickness of the layer between two isobars increases. Air rises over the land and spreads toward the sea, creating a pressure difference between the land air and the sea air. The cool sea air starts moving inland. When the cooler air from the sea collides with the warmerair over the land, a front is formed. The sea air burrows beneath the less dense air over land, forcing the land air to rise over the sea breeze front. If the land air is sufficiently moist and if the frontal lifting is strong and deep enough, the land air can be lifted to its lifting condensation level. If the land air reaches its level of free convection (the level at which a rising air parcel is slightly warmer than its surrounding environment), and thus becomes buoyant. That's why thunderstorms may occur 10-15 miles landward from the sea during summer. Ultimately, the rising parcel will exhaust its supply of water vapor and cool down to the same temp as its surrounding.

Winds swirl counterclockwise around a cyclone, but most winds are angled across the isobars toward the low. This creates surface convergence, leading to ascent, clouds, and storms.

A cold front is the surface position of a dome of dense air, pushing against the warmer, less dense air ahead of it. The colder air subtends the warmer air, forcing the less dense air to rise and causing clouds and precipitation. The pressure is low all along the front, but a kink can sometimes develop along the front and launch a more localized circulation. Spin makesor enhances low pressure, so the lowest pressure along the front is in the center of the spin, producing a cyclone. The developing cyclone's counterclockwise circulation starts pushing he air masses more firmly against each other and the cold front starts to swing around the south end of the cyclone. As the fast moving cold front sweeps around the cyclone, it catches upwith the slow moving warm front, first near the cyclone's center. This forms an occluded front. As the occlusion advances, the fronts zip up and precipitation may be heaviest now. Heaviest rains can also occur ahead of the cold front in prefrontal squall lines.
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  #13  
Old 07-23-2011, 04:21 PM
Philster Philster is offline
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If you live in NY, any high pressure system set up in the right location will funnel hot, humid air from the south and the gulf stream in the area. These are called 'Bermuda Highs'.

Cloud cover at night will act as an insulator. Hot, clear day and cloudy night = chance of more total heat the next day. Wind direction, etc, all factor in.


Humidity will actually keep temps down. If you remove the humidity, the temp can actually go even higher.

Too many factors to list.
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  #14  
Old 07-23-2011, 04:31 PM
barbitu8 barbitu8 is offline
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"Bermuda highs" are so called because they are located near Bermuda. They are very good to have around if you live on the east coast come tropical cyclone season (which, officially, has begun but we need not worry too much about them until they start forming off Africa later in the season). These highs will block the tropical cyclones from reaching the east coast. They will be diverted into the Gulf (not good for the Gulf states) or into the middle of the Atlantic (good for the USA). This time of the year, most tropical storms develop in the Gulf or Caribbean areas and do not have enough time to become too strong before hitting land or the colder waters of the north Atlantic. (Sea surface temps must be at least 80 degrees F to sustain a hurricane.)
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  #15  
Old 07-23-2011, 05:16 PM
njtt njtt is online now
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Well, it would be weird if it was exactly the same temperature every day.
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  #16  
Old 07-23-2011, 10:57 PM
Leo Bloom Leo Bloom is offline
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Thanks. I'm formulating questions and waiting to get off my iPad in the one cool room and get to the desktop so i can type normally. That room is too fucking hot right now.
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